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Cards (356)

Section 1

(50 cards)

betas

Front

Electron emitted from a nucleus

Back

Affects average energy

Front

kVs mS and mA DON'T effect ave (mAs)

Back

Mechanism and probability of Compton scatter

Front

Compton Scattering (bad): straight out a compton X-ray hits an outer shell electron→ outer shell electron is ejected→ incoming x-ray/photon (now with less energy) changers direction and flies off Produces 3 things: Free electron Ionized atom (missing an electron) Photon of energy Probability of Compton scatter: Does NOT depend on the Z of the atom because the energy of these outer shell electrons is low Depends on the density of the material (more tightly packed atoms = more electrons to crash into) Above 25-30 k contributes to dose and fogs up the image (Dominant) major source of occupation exposure

Back

Clinical exposure times

Front

Chest - 5 ms Abd - 50 ms Head CT 500 ms Fluoroscopy - minutes

Back

mA vs kVp

Front

both mA and kVP =less QM/noise but Compton limits the benefit of increase kVp at does above 30 so mA is better at reducing noise Not enough mAs=QM Excessive kVp= excessive scatter and lost contrast Noise increased ^2 the distance of the tube to detector

Back

What is the K-shell binding energy of tungsten?

Front

-69.5 keV No resultant photon can have more energy than the incident electon K shell binding energy is proportiona to Z^2 - lower Zs give lower energy xrays

Back

Auger electrons

Front

Energy released form the filling of an inner shell vacancy by an outer shell electron is imparted to another electron (instead of being emitted as a photon), which is then ejected- the 2nd electron = Auger electron No x-rays emitted Heavy elements more likely to emit x-rays Lighter elements more likely to emit Auger electrons.

Back

Actual focal spot vs. apparent focal spot, and

Front

Actual focal spot: where the x-rays land on the target anode Apparent focal spot: where the x-rays land on the patient Defines the amount of blur Changes in angle: goal of smallest possible focal spot (best resolution) as allowed by heat tolerance. Larger anode angle→ larger surface area, bigger apparent focal spot Smaller angle→ smaller surface area (less heat tolerance), smaller apparent focal spot

Back

"line-focus principle"

Front

Line-Focus Principle: method of angling the anode to give a smaller apparent focal spot (and less blur) small (steep) angle = small effective focal spot

Back

DEXA Dual Energy X-ray absorption

Front

need two different photon energies filter with the k-edge right in the medial of the spectrum you get 40 and 70 kVp Switch tube voltage 70-140 and get 45 and 100kVp Dose = 0.001 mSv - spine xr 1.5 mSv

Back

Method to reduce scatter

Front

Grid→ more strips = higher grid ratio = more scatter reduction Linear grid: needs to be aligned parallel with anode-cathode to avoid grid cut-off. Bucky grid = moving grid→ grid lines if grid motion fails Bucky factor describes the increased mAs required when a particular grid is used compared to a study using no grid Air gap: only done with mag view on MM → small amount of distance between patient and detector will reduce the scattered photons being registered. Grid results in increased dose (because ABC increases output to compensate)

Back

What factors have an effect on geometric unsharpness?

Front

Small focal spot → less blur Smaller SOD → more blur Smaller ODD → less blur More magnification→ more blur

Back

Secondary Ionization

Front

Delta ray ejected photon knocks out another electron

Back

Nyquist frequency-

Front

how little I can sample something and still be able to tell what it is. it is harder and harder to keep signal the smaller and smaller something ets

Back

Radiation that escapes the housing

Front

leakage

Back

Scatter depends on:

Front

Scatter depends on: Collimated field: ↓ FOV → ↓ scatter Thickness of imaged part: thinner part→ ↓ scatter Beam energy chosen→ compton dominates above 26 kVp in soft tissue, and above 35 kVp in bone

Back

Focal spot sizes

Front

mammo .3 to .1 mm small xray: .6 -1.2 mm large portables have a stationary anode - limits tube rating

Back

Grid ratio

Front

height x width increase GR = increased contrast, decreased scatter AABC increased dose is the trade-off Grid cut off - too much grid to the point of causing QM

Back

Coherent scattering (Rayleigh scattering):

Front

Coherent scattering (Rayleigh scattering): Photon excites the entire atom → Eventually de-excites→ photon emitted with same energy but different direction from original photon IF these photons reach image receptor they can cause some loss in contrast. Coherent scattering does NOT result in ionization No net transfer of energy No dose to patient No generation of x-rays Seen primarily at low energies (i.e. mammo) → wastes about 15% of photon interaction below 30 keV

Back

how to improve the heel effect

Front

larger anode angle larger source to image distance smaller film size (FOV)

Back

Heel effect, and what increases or decreases it; effect on FOV

Front

Because x-rays at anode side have to pass through a greater thickness of the anode, there is a reduction in the intensity of these x-rays Heel effect can be ↓ by ↑ the anode angle or ↓ the size of the x-ray field Used in MM where the thicker part of the breast/chest wall are aligned with the cathode Heel effect worse with: Small anode angle Decreased SID Increased FOV/field size Heel effect on FOV: Because the energy ↓ on the anode side, this also ↓ the FOV on that side (the side of the nipple) To compensate for this, the entire tube is angled up to 20 degrees Effective anode angle = sum of anode and tube angles

Back

which part of the x-ray tube is aligned with the thickest part imaged

Front

Chest - cathode

Back

gamma vs x-ray

Front

originate from the nuclei of an atom from interactions between electrons and atoms

Back

K-shell binding energy is proportional to...

Front

Z squared- so lower Z gives lower energy x-rays

Back

X-ray factors

Front

kVp -voltage, kinetic energy, QUALITY - increase 15% - double the intensity ^2 mA-Currant, # of thermionic emissions, QUANTITY - linear double mAs double dose mS - time mS X mA = mAs entrance skin dose will change as the square of the change in kVp - voltage

Back

Noise

Front

Quantum Noise= background Quantum mottle = cause by few photons (rain drops) more photons = less noise Scatter Post processing can improve noise Increased field size(less collimation) decreases QN but the dose and scatter are increased - usu increase mAs to overcome this Assume ABC - so collimation decrease noise

Back

alpha particle

Front

Helium atom 2+ big can't travel or penetrate HIGHEST LET

Back

Linear attenuation vs. mass attenuation

Front

Linear attenuation: actual fraction of photons interacting per unit thickness of an absorber, or the fraction of photons removed from the x-ray beam in a certain distance factoring in effects of compton scatter, PLE, and coherent scatter. -In contrast to mass attenuation, linear of ice, water, and water vapor is different (different lengths for the same # of molecules). -More attenuation occurs with denser object, higher Z material, and at k-edge. -Lower attenuation occurs with higher kVp Mass attenuation: fraction of photons interacting, scaled per gram of tissue. Supposed to reflect the attenuation. -Mass attenuation of ice, water, and water vapor is the same.

Back

Average x-ray energy

Front

1/2 kV

Back

Differences in technique for newborn x-ray

Front

Technique for peds (newborn) x-ray Do NOT use a grid Lower the kVp: good technique ~66 kVp (adult CXR ~120-140 kVp) Use the same or lower mAs (~2-4 mAs)

Back

Unit of energy deposited per second in a patient

Front

watts

Back

Interactions

Front

Dx energy levels coherent compton PLE way above dx levels pair production photodisintegration

Back

Mechanism and probability of photoelectric effect (PLE)

Front

Photoelectric Effect (PLE) (good) CONTRAST X-ray hits an inner shell electron → if energy great enough to overcome the k-shell binding, the inner shell electron will be ejected (photoelectron) All or nothing reaction Creates inner electron vacancy→ downward cascade and release of characteristic x-ray OR production of an Auger electron Since Auger electron production tends to dominate in tissues (unlike Tungsten) → some biologic damage PLE contributes to image contrast (Compton only makes noise) Probability of PLE: Directly proportional to the atomic # Z3 Probability ↑ with object density Probability inversely proportional to the energy cubed Chance of PLE sharply ↑ when photon energy and electron binding energy are the same Dominates at lower energy relative to compton auger electrons predominate in soft tissue -this doesn't really effect the image but makes free radicals

Back

What accounts for most x-ray production?

Front

Bremsstrahlung (radiative losses): account for 80% of x-rays produced - Amount of Bremsstrahlung interaction is proportional to energy of the incoming charged particle and the atomic # Z of the absorber Higher Z = more Bremsstrahlung NM: low-Z materials (plastic) are used to shield beta-emitters (e.g. Y-90) in order to minimize bremsstrahlung production (would increase with lead) its a spectrum up to Kvs - braking/ changes electrons coarse

Back

Focal spot in general X-ray, MM, and portal x-ray devices

Front

General XR: focal spot of 0.6 and 1.1 mm MM: focal spot of 0.3 and 0.1 mm Portable x-ray devices: often use a stationary anode (doesn't rotate to dissipate heat): limits their tube rating.

Back

X-ray spectrum: effect of changing mA, kVp, changing x-ray targets. What causes loss of characteristic x-rays?

Front

Changing mA changes area under the curve Changing kVp changes average energy and max energy Changing x-ray targets: whole curve shifts and the characteristic peaks change Loss of characteristic x-rays: if kVp dropped below threshold for k-shell electrons

Back

Clinical tube currents

Front

Radiography ~200-1000 mA CT ~ 200-1000 mA Fluoroscopy ~ 1-5 mA

Back

Signal to Noise CT

Front

although increased kVp increases noise from scatter the overall SNR is improved

Back

What factors affect the HVL?

Front

HVL = amount of material required to attenuate 1/2 beam intensity. Higher the average photon energy, more penetrating it will be, and higher HVL Beam Filtration→ ↑ average photon energy (which determines penetration capacity), and decrease the area under the intensity curve More filtration→ ↑ HVL Less filtration→ lower HVL With each HVL, the average photon energy goes up 3rd HVL > 2nd HVL > 1st HVL HVL of an x-ray beam does NOT depend on mAs HVL does depend on beam filtration and on anode material Monoenergetic beam has higher HVL than polyenergetic beam at same kVp 10th HVL: thickness of material that can attenuate an x-ray to 90% → used for shielding calculations

Back

X-ray interactions

Front

coherent - forget it - <5% no energy transfer or ionization Photoelectric -chance increase with atomic number Z^3, increases at the K-edge chance Decreases with KeV 1/E^3 (peaks over a Bremsstrahlung continuum) compton

Back

Effects of mA and kVp on focal spot

Front

Actual focal spot enlarges with an ↑ in mA because the ↑ # of electrons start to repel each other→ "blooming" or widening of the beam" Repulsion is most significant at low kV With ↑ kV, you can get a slight ↓ or "thinning" of the focal spot High mA, low kVp = wider spot "blooming" High kVp = smaller spot "thinning"

Back

Bremsstrahlung

Front

radiative losses - braking hits nucleus - gives max energy to xrays comes close and changes direction and gives off modest energy close enough to get veer coarse and give off low energy Proportional energy of the photon 1/E^3 and the atomic number Z^3 Max energy = max kVp

Back

How should you set kVp for a contrasted study?

Front

Contrasted study: want kVp to be at least 2x the binding energy of the contrast agent being used → maximizes contrast Iodine: k-edge = 33 keV → set to at least 66 kVp Barium: k-edge = 37 keV→ set to at least 74 kVp

Back

Magnification increases with...

Front

Mag = (SOD + ODD) / SOD or SID / SOD Magnification increases with: Patient far away from detector = greater ODD Source closer to patient = smaller SOD

Back

Misc xray vocab

Front

off-focal radiation: scatter from the anode outside the focal area - increased patient exposure and image blurring Xray tube insert: Vacuum with a port to allow xrays to exit in one direction Unwanted rads: Leakage - escapes through the housing Secodary - characteristic rads from interaction with the tube Scattered - deflected xrays Stray= leakage+scatter collimation - process of restricting the size and shape of the xray beam emerging from the port - improves quality - increased collimation decreases the field size Grid control: -cuts off electron flow, its a cup over the filiament"line-focus principle"

Back

Excitation results in?

Front

heat NO xrays

Back

15eV

Front

and above is ionizing radiation

Back

Clinical voltages

Front

extremity ~ 60 kV ABD ~ 80 kV Chest ~ 120 kV

Back

spatial resolution

Front

resolve things separately ~ spatial frequency line pairs per mm = 1/2 Nyquist frequency- how little I can sample something and still be able to tell what it is unsharpness = lost special resolution - motion - reduce with shorter mS - system; detector fault - grain size w/ film - CR laser reader size - DR size of the thermoluminescent transistor - Geographic: focal spot, SOD, ODD, mag

Back

increased PLE

Front

increased absorption and decreases transmission kvp, near k edge, and higher z and more density

Back

Section 2

(50 cards)

Beryllium

Front

mammo window

Back

Brightness gain, and the type of gain that worsens with age of II

Front

Brightness gain: describes the increase in emitted light from the II compared to that without an II Due to the combined effects of flux gain and minification gain BG = flux gain x minification gain Conversion gain (Gx): describes the efficiency of an II in changing incident x-rays into light at the output surface The older the II, the worse the conversion gain.

Back

MM viewboxes and reading room light

Front

MM = 3000 cd/m2 General XR = 1500 cd/m2 Reading room light: should not exceed 50 lux

Back

Modulation transfer function

Front

Modulation transfer function (MTF) Buzzword = Contrast Buzzword = "function of spatial resolution (frequency)" MTF changes as a function of spatial frequency (resolution) → 100% at low spatial resolution, 0 at high spatial resolution.

Back

Mechanism of CR and factors that affect spatial resolution

Front

CR = indirect digital Storage phosphor (CR) → uses special photostimulable phosphor (Barium fluorohalide) Storage phosphor doesn't emit the absorbed energy as light after interaction with an x-ray→ instead holds a latent x-ray image (phtotstimulable luminescence) Latent image read using a red laser to scan detector→ records how much blue-green light is emitted Amount of BLUE GREEN light detected proportional to the intensity of the incident x-ray Plate reset by exposure to bright white light to erase it→ ghosting artifacts occur if not performed.

Back

Purpose and effects of collimator

Front

Collimator: defines the shape of the beam Decreases scatter, improves image contrast, and reduces dose Changes to smaller input surface, but using the same output surface → actually minifies the image less → decrease in minification gain (less light from output phosphor)

Back

Digital MM: resolution, dose, noise

Front

Smaller pixels = better spatial resolution Can lose some spatial resolution from spread of electronic or light signal inside digital or computed detectors. Most digital systems have lower spatial resolution (~5-11 lp/mm) relative to analog (11-13 lp/mm) MQSA does NOT have specific line-pair requirements for digital→ linked to manufacturer's specifications. Digital machines have ~15% less dose than analog due to better beam quality and fewer repeat exams. Dose is not fixed (fixed with screen-film) Digital images have variable contrast, which can be altered using window and level. The noise is fixed after exposure is taken. Dark-noise: from electronic fluctuations within the detector elements → effect is proportional to the temperature of the detector → seen more with underexposed regions. Flat-field test→ imaging large piece of acrylic to improve image quality and calibrate the digital detectors

Back

Film artifacts: fogging, double exposure, quantum mottle, incomplete erasure, ghosting

Front

Fogging: adding charge to the detector (blackening the film) Can occur if cassette left in the room with scattered x-rays Big black blob on film Double exposure: cassette used twice without changing film or erasing the receptor Looks like it has 2 images on it Quantum mottle: noise from lack of photons→ looks underexposed Incomplete erasure→ looks like double exposure Ghosting: result of prior exposure, leading to difference in x-ray sensitivity of different parts of the detector Looks like a dark object that doesn't belong on the image Occurs in DR more than in CR

Back

2 types of magnification in fluoro

Front

Magnification→ both types increase dose, but geometric mag increases it more Geometric magnification→ accomplished by bringing the object closer to the x-ray source (Mag = SID/SOD) Associated increase in dose based on inverse-square law Electronic magnification (Zoom) If you ↓ the FOV by half→ only ¼ of the input phosphor will be irradiated → ↓ brightness by ¼ (if other parameters held constant) Corresponding increase in dose as AEC compensates to maintain brightness at the output phosphor. Each level of mag increases dose by 1.4-2X

Back

Bit depth

Front

# of bits determines the # shades of grey more bits = more contrast bigger matrix - more pixels= better special resolution pixel - smallest component of a matrix - smaller pixel equals better spatial resolution

Back

Spatial resolution: screen-film MM, digital MM, CT, MRI, digital XR

Front

Screen-film MM: 15 lp/mm Digital MM: 7 lp/mm Digital XR: 3 lp/mm CT: 0.7 lp/mm MRI: 0.3 lp/mm MQSA does not have specific line-par requirements for digital MM- linked to manufacturer specifications.

Back

CR vs. DR

Front

Digital detectors: Storage phosphor - (CR): indirect -FPD (DR): - direct - indirect CR: uses photostimulable phosphor plate enclosed in a cassette. -2 stage process for image capture and readout. DR: uses detector that can both capture and read-out information. -Direct and indirect methods. indirect are scintillators xr->light->charge Direct photoconductors xr->charge

Back

Grid use in MM

Front

Since breast is placed in compression and a lower kVp is used (both of which intrinsically decrease scatter), a smaller grid ratio is used MM uses 4-5 grid ratio (6-16 for general XR) Use of grid requires change in technique to prevent underexposure→ dose is ↑ with grid. Bucky Factor: describes the increase in dose required for a higher grid ratio 2x dose = 2x Bucky Factor 2 for MM; 5 for general XR No grid used in Mag mode→ Air gap used to prevent scatter Increasing air gap will increase magnification and increase dose (because of AEC compensation)

Back

Mammo mag

Front

air gap No grid 0.1 focal spot smaller paddle less mA ~25 increased mS - 3 seconds

Back

Quantum mottle in fluoro

Front

Quantum mottle: due to fluctuation of quanta of photons per unit size absorbed by the screen Increasing the speed (kVp) increases the mottle Increasing the # of photons decreases the mottle

Back

Modulation Transfer function

Front

MTF - measures the ability of a system to maintain the signal contrast as a function of spatial resolution

Back

Use of silver and other anode/filter combinations in MM

Front

Silver (can be used as a filter for tungsten target, as can Rh): K = 47 K-edge = 25.5 keV Highest effective energy beam and ↑ heating capacity → allowing shorter exposure times, increased penetration, and lower dose. Contrast is lost, but can be accommodated by post-processing. Mo anode with Rhodium filter→ used for intermediate energy spectrum between Mo/Mo and Rh/Rh Never use Rh target (21 keV) with Mo filter (20 keV k-edge) Mo anode can be combined with aluminum filter→ harder beam to penetrate denser breasts.

Back

Spatial resolution in CR/DR

Front

↑ sampling frequency → ↑ resolution More pixels → ↑ spatial resolution Images with high resolution require large file sizes ↑ x-rays will NOT improve maximum spatial resolution Spatial resolution for selenium-based (direct) DR is higher than for indirect detectors (newer systems pretty close) Structured scintillators better than unstructured (less lateral dispersion) CR- use smallest plate size size matrix size is fixed to improve spatial resolution - smaller pixels

Back

contrast resolution

Front

tell one thing from another low kVp = high contrast High kVp= low contrast less scatter = better contrast Radiographic contrast - both subject and image receptor contrast. larger difference in Z improves subject contrast more BITS - data points improves contrast Window width: narrow widow increased contrast - black and white vs shades of grey Level - determines brightness -level up to look at dark stuff lungs - level down to look at light stuff bones

Back

Effects of compression:

Front

↓ thickness → ↓ scatter→ lower kVp can be used Lower kVp and ↓ scatter → ↑ contrast ↓ thickness→ ↓ mAs needed → ↓ dose ↓ movement→ ↓ motion artifact Breast smashed closer to Bucky→ ↓ geometric magnification ↓ motion and ↓ geometric magnification→ ↑ spatial resolution ↓ tissue overlap

Back

CR vs DR Spacial resolution

Front

CR has better spatial resolution pixel size and spacing increased pixel density= better SR Decreased pixel pitch(spacing)= better SR

Back

Flux gain and minification gain in II

Front

Flux gain: describes the increase in magnitude of light coming from output phosphor relative to input. Accomplished by using a high voltage difference between the photocathode and the output phosphor (25-35 kV) Uses focusing electrodes. The voltage causes electrons to accelerate → yields gain of energy Minification gain: electrons from the large photocathode surface are concentrated on a small output phosphor. This ↑ the # of electrons per unit area→ ↑ energy per unit area Mag mode: decreases minification.

Back

Indirect FPD mechanism and lateral dispersion.

Front

Indirect FPD: X-ray activates thallium doped Cesium iodide (CsI) → emits light→ photodiode turns the light into an electric signal that is read-out. Lateral dispersion→ light diffuses laterally→ decreases spatial resolution, with worsened effects at increased thickness of crystal Using too thin a crystal decreases sensitivity for x-ray absorption Gadolinium oxysulfide has more lateral dispersion than CsI Columnar structure→ permits thick crystal with decreased lateral dispersion

Back

ABC vs AEC

Front

fluoro used ABC which can crank up both the KVP and mA AEC CR - mainly does mA

Back

Dose compensation for geometric magnification

Front

1. You can attempt to collimate 2. Could possibly get rid of the grid, if you kept the receptor stationary, and move the patient closer to the source This would introduce an air gap, which naturally ↓ scatter → won't occur if the tube is brought closer, only if the patient is brought closer to the tube and the receptor remains stationary.

Back

MQSA: recall rate, cancers/1000 screened, and PPV for biopsy recs

Front

Appropriate target range for medical audit: PPV1 Recall rate: 5-7% Cancers/1000 screened: 3-8 PPV2 for biopsy recommendations: 25.4% PPV3 for biopsy yield of malignancy 31% FDA runs MQSA BI-RAds is ACR

Back

What has better dose efficiency

Front

DR: you can half mA and increase KVP 15% and decrease pt dose

Back

Solutions to account for an aging image-intensifier

Front

Solution 1: use an aperture with a large hole in it Downside: increases image noise Solution 2: allow the ABC to compensate Downside: increases dose Solution 3: replace the II II usually replaced when the conversion gain falls to 50%

Back

LUTs

Front

Look-up-table DR histogram of known input intensities and corresponding grayscale - greatest influence of contrast on digital system - KVP with flim - digital has wider dynamic range - can adjust for higher kvp

Back

AEC

Front

ionization chamber its like a timer controls quantity of radiation no effect on kVp - ABCs will mess with kVp

Back

Digital radiography pixels

Front

Most systems use pixels with 8 bits- 2^8 or 256 possible brightness levels

Back

Fill-factor in DR/CR

Front

Fill-Factor: the area of the detector that is sensitive to x-rays in relation to the entire detector area Higher fill factor→ more efficient detection DR systems→ the electric field shaping allows for a fill factor of nearly 100% This is not seen with indirect (CR) systems → lower fill factor and thus lower detection efficiency

Back

MM film/screen combination

Front

MM uses single-emulsion film matched to a single intensifying screen in the cassette Advantages: Less parallax Less crossover Better spatial resolution Disadvantage: increased dose Screen: basically a scintillator that turns x-rays into light Allows less dose (lower exposure time) Lateral light diffusion within screen will reduce spatial resolution No effect on scatter Thicker screen: Reduces dose Worsens lateral light diffusion→ worse spatial resolution The film should be positioned on top of the intensifying screen→ limits halation, and improved spatial resolution (less blur).

Back

CR spatial resolution depends on...

Front

Spatial resolution depends on: Laser spot size Phosphor plate density and thickness Rate of light sampling (sampling pitch)

Back

MQSA mean glandular dose

Front

"Average breast" = 4.2 cm of compressed tissue that is 50% adipose and 50% glandular Measured dose is using a grid Dose under 300 millirads (3 mGy) → only for phantom, not a real human breast

Back

Image-intensifier components:

Front

Image Intensifier: converts x-rays to electrons, accelerates them, and then converts them to a visible image. Along the course of conversion from x-rays to light and then back to electrons, the electron flux and energy are amplified by the II → results in a greater amount of light emerging from output phosphor. Input phosphor = CsI → captures x-rays and converts them to light Photocathode: converts light to electrons Output phosphor: converts electrons to light. Smaller than input phosphor.

Back

Fluoro technique vs XR: mA, kVp, exposure time, focal spot

Front

mA: 0-5 (versus 200-800 in regular XR) kVp: 50-120 (same as XR) Exposure times: much longer than in XR Focal spot: 0.3-0.6 (1-1.2 XR) in order to limit geometrical blurring Larger focal spot used for the spot image (same as conventional XR) because greater tube current is needed. ' Fluoro frame shot has more quantum mottle than spot image (fewer photons) Grid used (same as regular x-ray, ~10:1)

Back

Direct FPD mechanism

Front

Direct FPD: converts x-ray directly to charge using amorphous selenium X-rays absorbed by selenium→ electrons released→ electron-hole pairs travel to surface of selenium and neutralize a portion of the applied "bias" charge, done in proportion to x-ray intensity. NO LATERAL DISPERSION Electrical charges are drawn in along the electric field lines to the charge storage capacitor electrodes connected to the TFT The pattern of charges is scanned and converted to a digital signal stored by each TFT.

Back

Focal spot size in MM versus XR

Front

MM: 0.3 mm, 0.1 mm (mag) XR: 0.6 mm and 1.2 mm Requires lower mA (due to heat tolerance of small focal spot) → limited to 50 mA for 0.1 mm, 100 for 0.3 mm Also longer exposure times (try to keep below 2 seconds) Cathode side aligned with chest wall Loss of energy on anode side (nipple side) compensated for by angling the tube up to about 20 degrees Effective anode angle = anode angle + tube angle

Back

Molybdenum versus Rhodium

Front

Molybdenum: low kVp and low Z→ gives Moly high characteristic x-ray production and low Bremsstrahlung. 18 keV vs 70kev W Z = 42 K-edge = 20.0 keV Characteristic x-rays→ 17.9 and 19.5 keV K-edge filter: placed on outside of tube with goal of creating a nearly mono-energetic beam in target range of 16-23 keV. Rhodium: better for thicker/denser breasts than Mo due to higher energy spectrum (Rh/Rh) Z = 45 K-edge = 23.2 keV Char. x-rays = 20.2 and 22.7 (more penetrating)

Back

Privilege to read a mammogram

Front

240 mammography examinations in a 6 month period 3 months of formal training 60 hours of education

Back

MQSA spatial resolution

Front

Spatial resolution using line-pair phantom: RSNA: 13 lp/mm in anode-cathode direction 11 lp/mm in left-right direction Huda: 12 lp/mm for screen-film, and manufacturer specs for digital (~7 lp/mm)

Back

DR basics, scintillators vs. phosphors

Front

Flat-Panel Detectors (DR) Faster than conventional film development or CR plate reading Composed of amorphous (not crystalline) selenium Photon from x-ray stored as an electric change within a square array of pixels Information read out by scanning one row at a time with columns read in parallel Indirect (scintillators) = x-rays → light → charge Direct (photoconductors) = x-rays → charge Scintillators vs phosphor Scintillators have 2-3X more efficiency in x-ray absorption than phosphor at same thickness Scintillators produce more visible light per x-ray Scintillators emit a wavelength of light that is a better match for the TFT detector. thin film transistor

Back

Digital artifacts

Front

Ghosting: caused by a residual image from the prior exposure burned into the detector Seen when highly attenuating objects are placed in the beam This is why lead is not allowed on flat-panel digital systems Pixels gone bad→ can manifest as a square or streak

Back

Detector Quantum Efficiency

Front

DQE- estimates the required exposure necessary to create an optimal image predicts dose compares ideal detector noise to measure effectiveness of a detector compares SNR in to SNR out as a function of spatial frequency Perfect is one High DQE=Low dose LOW DQE=High dose DQE is proportional to MTF inversely to SNR better at low spatial resolution DR ~ 0.45 CR ~0.25

Back

Advantages and disadvantages of film-screen and DR

Front

Film-screen limitations: Intolerant to errors in exposure → leads to loss of contrast Noise→ compton scatter increases with increasing film size Quality of films breaks down over time No post-processing Film-screen advantage: slightly better spatial resolution than CR (5 lp/mm versus 2.5 lp/mm) DR advantages: Easily stored and manipulated with post-processing Higher dose efficiency Wider dynamic range of detection Superior contrast resolution

Back

Ideal energy for MM and typical voltage used

Front

Ideal energy for MM is between 16 and 23 keV → requires using voltage of 25-30 kVp (50-120 for general XR)

Back

MQSA tasks:

Front

Processor QC: daily Darkroom cleanliness: daily Viewbox conditions: weekly Phantom evaluation: weekly Repeat analysis: quarterly Compression test: semi-annually Darkroom fog: semi-annually Screen-film contrast: semi-annually

Back

digital range vs response to exposure

Front

digital = larger DR with a LINEAR response to exposure Film = Narrow dynamic range and a curvilinear response - does poor and low and high exposure

Back

DQE in CR/DR

Front

Detector quantum efficiency (DQE): efficiency of detector in converting x-ray energy into an image signal → ideal is 1.0 The better the DQE, the less radiation you need to maintain your signal Like MTF, DQE is better at low spatial resolution DQE of DR is ~0.45, versus lower DQE for CR or XR at 0.25 Factors that affect DQE: Radiation exposure Spatial frequency (resolution) MTF Detector material kVp mAs

Back

Section 3

(50 cards)

Kv for contrasted studies

Front

Iodine- 70Kv Barium- >100 Kv

Back

Skin Dose Levels

Front

< 2 Gy→ no action needed 2-5 Gy→ advise patient to watch for burns, especially 10 days post-procedure > 5 Gy→ procedure and dose should be reviewed by physics 2 Gy: early transient erythema 3 Gy: temporary epilation (hair loss) 6 Gy: chronic erythema 7 Gy: permanent epilation 10 Gy: telangiectasia 13 Gy: dry desquamation 18 Gy: moist desquamation/ulceration 24 Gy: secondary ulceration

Back

kVp and dose

Front

less skin dose more organ dose CT increased kVp increased skin dose

Back

FPD system components

Front

Entrance surface of FPD: carbon fiber to protect components CsI needles: act as a phosphor→ convert photons to light Photodiode array: absorbs light and converts to electrons Readout elements (transistors and gates): Determine charges of elements in photodiode array and construct images No TV camera needed in FPD systems I I I CF __________ CSI needles---------- photodioade--------- read out elecment - ---

Back

Dose in IR

Front

50% of dose delivered in the superficial 3-5 cm of skin/fat The depth of this 50% depends on kVp and filtration (higher kVp + copper filtration = more penetration) For body (or part) measuring < 10 cm, grid should be off Thicker patient gets higher skin dose due to ABC compensation (less penetration → higher kVp) Lateral view doubles dose to patient and operator Typical dose of ~0.3-0.5 mGy per frame at the entrance skin position (10-20x higher per image than regular fluoro) Total dose = (dose per frame) x (frame rate) x (duration x # of runs)

Back

Spatial resolution limited by what in FPD and II systems?

Front

FPD: limited by detector element size (around 2.5-3.0 lp/mm) II limited by TV systems (1.0-2.0 lp/mm for GI, 2.0-4.0 lp/mm for angio)

Back

HU in CT

Front

Attenuation of tissues is a "relative attenuation" based on comparison to water, which is always 0. HU = 1000 x (attenuation of material - attenuation of water) / attenuation of water When HU ↑ by 10, x-ray attenuation ↑ by 1/% HU changes with KV cupping artifact - things are less dense in the center

Back

Fluoro in IR: focal spot, kVp, filter, grid

Front

Focal spot: small focal spot + large # of x-rays Anode angle usually smaller than that with conventional XR, but heel effect remains zero due to small FOV and small image detector Focal spots can be exchanged (large, small, micro) depending on need for resolution kVp: best kVp to use with contrast is between 60 and 80 kVp (average beams hit the k-edge nicely) Higher kVp loses iodine contrast Equalization filter or soft filter: reduce intensity to taper the radiation profile Often used when imaging the leg, arm, or peds patients. Grids: NOT USED IN EXTREMITIES OR PEDS

Back

Scatter reduction in CT

Front

Collimation: used both at the x-ray tube and at the detector to shape the beam Defines the section thickness on a single slice Also reduces some of the scatter Additional scatter reduction via anti-scatter septa (type of grid)

Back

What improves and limits spatial resolution in an I.I. system?

Front

Spatial resolution with II: Improved by magnification (less minification) Limited by the quality of the display TV

Back

KAP

Front

Derma-area prouct - kerma multiplied by the cross sectional area of the x ray beam essentially the total radiation potentially incident on the patient Dose x Xsectional area electronic Mag increases the Air Kerma but NOT KAP

Back

Factors affecting contrast resolution: dose, noise, SNR, slice-thickness

Front

Contrast Resolution: the ability to discriminate small differences in object density from its surroundings for a specific target size and radiation dose (noise) Dose: As the # of x-ray photons ↑, the signal detected ↑ → ↑ contrast resolution But radiation dose ↑ in proportion to photon flux Quantum Noise: directly dependent on the # of x-ray photons As the # of x-ray photons doubles → signal ↑ by a factor of 2, and noise ↑ by a factor of √2 SNR: contrast resolution improves with SNR If the # of x-ray photons is doubled, the signal doubles, and the noise ↑ by a factor of √2 → SNR ↑ by 2/√2 Thus, as signal (and proportionately the dose) ↑, the contrast resolution also ↑, but at a lesser rate. Slice thickness: Small slices = fewer photons per slice → less contrast resolution (image is noisier) But smaller slices = less partial volume averaging→ improved spatial resolution

Back

When is pulse fluoro less dose than regular fluoro

Front

30 pulse /second 50% lower pulse rate gives you 30% less dose

Back

CT technique: kVp, mA, focal spot, anode-cathode axis, variable mA

Front

3rd generation CT: x-ray tube and detectors rotate in synchrony -X-ray tubes use tungsten alloy targets placed on high-speed rotating anodes -Reasonable voltage (80-140 kV, usually ~120). Peds and skinny patients kVp ~80 (reduces dose, increases image contrast). Higher for large patients. -Very high tube currents (up to 1000 mA) -Typical focal spot is large (0.6-1.2 mm) in order to handle the large amount of power (~100 kW) -Filtration Al performed to remove low-energy x-rays that would only increase dose -X-rays are highly-filtered, high kV (average energy 75 keV) -X-ray tube anode-cathode axis is positioned perpendicular to the imaging plane to reduce the heel effect. Variable mA: adjusted via scout image or continuous tube modulation. Reduces radiation and gives more uniform SNR

Back

deterministic effects

Front

Gy 2: early transient erythema 3: temp epilation 6: Chronic epilation 7: Permanent epilation 10: telangiectasia 13: dry desquamation 18: moist dequamation/ulceration 24: secondary ulceration

Back

Pitch and fill-factor in FPD system

Front

Pitch: essentially the linear dimension of a detector element Fill factor: The system isn't 100% efficient; only a portion is actually sensitive to light. The ratio of the sensitive area over the total area is the fill factor FF = sensitive area / Pitch2 As the detector element gets smaller, you get better spatial resolution, but the fill-factor ↓ → smaller detector element will have superior resolution but require more radiation Matrix = the # of detector elements (or pixels) on the surface of the FPD in each dimension (horizontal and vertical) Pixel (detector element size) = FOV/matrix

Back

DSA

Front

DSA: takes a single frame mask image and subtracts it from another single frame contrast image Goal to remove anything that isn't moving, leaving moving objects (e.g. blood)

Back

Frame-averaging pros and cons

Front

Frame-Averaging (Recursive Filtration): an image-processing feature that adds several images together with different weighting factors Pros: ↓ quantum mottle and ↑ signal → SNR improves Cons: ↑ susceptibility to motion artifact and ghosting

Back

QC fluoro

Front

spatial resolution - lead bar pattern distortion - mesh screen or plate

Back

Kerma

Front

kinetic energy release per unit mass total total amount of energy deposited from the ionizing radiation this includes heat

Back

Artifact associated with change from area of heavy attenuation to area of less attenuation

Front

Flair or Glare artifact: with transition from heavy attenuation to minimal attenuation Can see bright white "glare" at the periphery near the area of decreased attenuation. From overproduction of x-rays in the thin area to compensate from the nearby thick area.

Back

Spatial resolution in fluoro systems: vertical, horizontal, diagonal

Front

I.I. system: spatial resolution limited by TV display system Resolution of the TV depends on raster (scan) lines, the bandwidth, and the FOV. FPD system spatial resolution not limited by the display (usually have displays with same matrix as the image receptor) The pure spatial resolution of II systems is better than for FPD, and change with FOV Vertical resolution: limited by the # of raster lines that the display monitor uses Max # of line pairs in the vertical direction is half the # of actual lines used Bars in the line-pair pattern may not totally line up with the raster lines→ the resolution usually less than predicted→ correct with Kell factor (0.7) Vertical resolution = (raster lines x Kell factor) / (2 x FOV in mm) Thus, ↓ FOV (mag mode) improves spatial resolution Horizontal resolution: The # of dots per line must be calculated indirectly from bandwidth The # of line pairs in the horizontal direction is ½ the dots in a raster line Measuring the spatial resolution at a diagonal (45 degrees to the raster lines) Improves spatial resolution by a factor of √2 over the vertical or horizontal resolution

Back

Adjusting kVp and mA and dose

Front

If mA is ↑ before kVp, the dose gets higher If kVp is ↑ first, the dose goes up less

Back

Vignetting artifact

Front

Vignetting artifact: edges look darker than the center Because distances from the focusing point to the outer phosphor tend to vary, with the closest path in the center and the farthest path at the edge → can get dark periphery and light center

Back

Factors Affecting Spatial Resolution in fluoro

Front

Factors Affecting Spatial Resolution FOV: smaller FOV→ better resolution Focal spot size: usually not an issue unless you get the anatomy away from the image receptor Image Receptor: limited by detector elements in FPD, TV for II Motion and temporal factors→ motion creates ghosting Dynamic range: only an issue for II systems→ variability in very dense or very transparent objects Pixel binning: binning increases pixel size→ reduced spatial resolution (but improved SNR) Frame averaging: increases SNR, but more susceptible to blur Pulsed fluoro: less motion artifact (better spatial resolution in moving objects) and overall less dose if <

Back

Effects of pixel size on spatial resolution and contrast resolution

Front

Holding matrix size constant and ↓ FOV will ↓ pixel size → this ↑ spatial resolution but ↓ contrast resolution (less photons per box) Holding matrix size constant and ↑ FOV will ↑ pixel size → this ↓ spatial resolution but ↑ contrast resolution (more photons per box)

Back

Artifacts in I.I. system associated with larger FOV

Front

Pincushion distortion: With large FOV may see appearance of bent lines at the periphery (should be straight). Inward bowing pattern may resemble a pincushion S-Shaped Distortion: similar to pincushion, and also with larger FOV Due to interference of the earth's magnetic field with the flow of electrons heading toward the I.I. Adding "mu metal" can supposedly deflect the magnetic field and improve this.

Back

Techniques to reduce patient dose

Front

Techniques to reduce patient dose: Positioning patient away from the source Use smallest FOV by collimating (also improves resolution) Avoiding magnification Xray filtration increased kVp with decrease patient does within 6 feet - lead up 1 mm stops 90%

Back

Pitch in CT

Front

Pitch: increasing pitch decreases dose (less overlap) Pitch = table movement per tube rotation (D) / beam width (W) Pitch = 1 Beam = the distance the table turns in 1 rotation (no gap) Pitch > 1 Gap between the slices Pitch < 1 Overlap→ increases dose - overscanning

Back

Skin doses

Front

<2 gy - ok 2-5: advise pt - 10 days for effect >5 Gy - physics review

Back

what determines the minimal slice thickness in CT

Front

the detector element aperture width

Back

Operator Dose Limits:

Front

Operator Doses: You get ~ 0.1% of what patient gets at 1 meter 1 year→ typically get ~ 5 mSv Regulatory dose limit: 50 mSv per year Conceptus dose limit: 0.5 mSv per month Eye dose limit: 20 mSv/year (recently changed from 150 mSv/year) Extremity dose limit: 500 mSv/year

Back

Effects of smaller FOV

Front

Smaller field of view: II: increases dose FPD: also increases it usually, but doesn't have to

Back

High level contral

Front

Air kerma limit of 76 mGy/min 10 roentges per/min up toe0 176 mGy/min or 20 roentgens/min audible visual indicators

Back

Ideal positioning of x-ray tube and II

Front

Positioning: want x-ray tube far away, and II as close as possible to the patient Placing II as close as possible to the patient has several effects: Decreases patient dose Decreases scatter to the operator Increases image sharpness Decreases focal spot blur and magnification Raising the image receptor → Increases the dose to compensate for the source to image receptor distance.

Back

Artifacts in FPD fluoro systems

Front

Bad pixel: appear as white or black spots One method to correct this (often built into system) is to interpolate in order to fill in the data Lag artifact (ghosting): tends to occur if the exposure uses very high radiation FPD systems don't have pincushion or S-distortion, vignette, glare, or saturation artifacts.

Back

Effect of increasing the beam width via the collimator

Front

↓ scan time (larger coverage with 1 turn) ↓ motion artifact (less scan time) ↑ partial volume (more divergent beam) DOES NOT change radiation dose (mAs unchanged→ larger area is scanned, but decreased scan time)

Back

Interventional reference point (IRP)

Front

Interventional Reference Point (IRP): Describes the use of an ionization chamber with a set reference point (15 cm closer to the source than the isocenter of the IR system) to measure radiation emitted from the source Skin dose can be above or below this point Ignores geometry, table attenuation, and back-scatter→ probably underestimates the patient's skin dose The dose (outside lead) standing 1 meter from the patient is ~ 1/1000 the dose received by the patient

Back

Dose area product (kerma area product)

Front

Dose-area product (Kerma area product): Measures the radiation dose to air in mGy multiplied by the collimator area→ mGy/cm Measurement is independent of beam location As beam moved away the intensity ↓, but it spreads out more. Low dose to large skin area = high dose to small skin area Magnification will ↑ air kerma, but not KAP Gives an estimate of total energy deposited in the patient, effective dose, and cancer risk. If something ↓ the DAP/KAP, it probably also ↓ the scatter and patient dose

Back

Air Kerma

Front

how many photons are in a unit of air prior to the energy striking the skin - essentially estimating the peak skin dose

Back

Filtration and reconstruction effects on resolution

Front

Reconstruction filter: Bone algorithm→ higher spatial resolution Soft-tissue algorithm→ better contrast resolution Oversampling→ better spatial resolution Filter back projection - mathematical filter is applied prior to back projjection Iterative reconstruction→ better contrast resolution than FBP - can correct for noise and can use lower dose

Back

MDCT: # of thickness of slices

Front

The # of detectors in the axial direction determines the # of slices that can be simultaneously acquired MDCT can acquire images with "isotropic resolution" → can do non-axial reconstructions without stretching the pixels. Minimal slice thickness determined by the detector element aperture width.

Back

CT vocab

Front

ray - one detector to tube projection - line of rays sonogram - graphic s shaped depiction of the data

Back

Artifact associated with imaging very dense objects

Front

Saturation artifact: associated with very dense material Dose is cranked up to try to penetrate a very dense object (classically metal) → end up with regions around the metal that appear very bright

Back

Factors affecting spatial resolution in CT

Front

Focal spot size: Smaller focal spot→ less blur, better spatial resolution Magnification: ↑ magnification blurs the image → ↓ spatial resolution Detector aperture size: As detector size is ↓, the craniocaudal resolution ↑ In-plane ("x-y axis") resolution is not affected by aperture size Projections: More projections = more data = better resolution Reconstruction slice thickness: The thinner the detector element aperture→ better the spatial resolution in the Z-direction Pixel size and display FOV: Display FOV is always < the scan FOV Pixel size = DFOV / matrix size ↓ DFOV and ↑ matrix size → smaller pixels→ better spatial resolution Pitch: As pitch ↑, the width of the slice sensitivity profile (SSP) also ↑ As SSP widens, slice thickness ↑ → ↓ spatial resolution Motion → blurring → ↓ image resolution

Back

Filtration in CT

Front

Copper or aluminum (6 mm) Heavily-filtered beam can have HVL of up to 10 mm Al Bow-tie filters: compensate for uneven attenuation of beam by the patient Attenuate less in center and more on the edges Made of low-Z materials (like Teflon) to reduce hardening differences Reduce scatter and dose

Back

Fluoro dose limits

Front

Patient entrance dose limited to maximum value of 87 mGy per minute (10 R/minute) in normal mode of operation High-level mode: requires audible or visual alarms Maximum patient entrance dose limited to 174 mGy per minute (20 R/minute) 5-10 digital spot films = dose of 1 minute of fluoro (assuming same FOV)

Back

Axial versus helical modes in CT

Front

Axial: table stationary Better spatial resolution in the z-direction since full image-sets are taken. No partial volume effect along the long axis Helical: table moves at constant speed, with tube on continuously. Much faster than axial CT Post-acquisition flexibility in the selection of slice location and lower probability of anatomic discontinuities between adjacent slices containing moving anatomy in chest/abdomen. Artifacts of partial volume with helical CT are more pronounced along a curved surface (e.g. skull)

Back

CT matrix size, pixels, voxels

Front

Matrix size for CT: 512 x 512 Each pixel represents 4096 possible shades of gray (12 bits, 212 = 4096) Pixel size = FOV / matrix Pixel w x h = voxel w x h Voxel has a 3D (depth) component that represents the slice thickness

Back

Binning in FPD systemsOISE

Front

Binning: takes several detector elements (DELs) to make a large DEL (only with FPD, not II) Concept: ↓ the amount of data → ↓ variation in x-ray photons from pixel to pixel and ↓ quantum mottle With ↓ mottle, you can ↓ radiation and keep the same Noise Larger DEL does ↓ spatial resolution Binning is especially useful with large FOV (where there are too many pixels in the image) Binning and spatial resolution: -Binning: large FOV will have lower spatial resolution (but can use less radiation to maintain the noise level) combining 4 DEL you can cut radiation /2 -No binning: spatial resolution doesn't change with different fields of view. Small FOV: dose is increased to reduce quantum mottle

Back

Section 4

(50 cards)

Zebra artifact:

Front

Zebra Artifact: also a reformat artifact Can occur from helical data secondary to the helical interpolation process (increases noise along the Z-axis) Effect manifests as stripes (like a zebra) most pronounced on a 3D image Effect is most significant away from the axis of rotation (noise is worst off-axis).

Back

CT fluoro

Front

Near real-time imaging→ CT image constantly updated (6 per second) Low tube currents (20-50 mA) used to minimize dose

Back

Artifact classically associated with scanning the shoulders

Front

Photon Starvation: High-attenuating areas (classically the shoulders) → can result in photon starvation → manifests as streaking Seen when the beam travels horizontally through the area of greatest attenuation Fixing photon starvation: Automated tube current modulation→ increases the dose through the area of greater attenuation Adaptive filtration→ corrects the attenuation profile by "smoothing" the data in the high attenuation portions.

Back

Artifact seen in the bladder with artifactual peripheral echoes

Front

Beam-width artifact→ due to divergence of the beam in the far-field beyond the width of the transducer Example: bladder with peripheral echoes How to improve: Adjust the focal zone to the level of interest Place the transducer at the center of the image

Back

Transducer crystal thickness

Front

The thickness of the transducer crystal = ½ the wavelength Lower frequency: seen with thicker crystals Higher frequency: seen with thinner crystals

Back

Stair-Step Artifact

Front

Seen as a "stair-step" on the edges of MPR images, when you have a wide collimation of non-overlapping intervals. Less severe with a helical scanner, where you are getting some overlap Fixed by using thin slices

Back

Attenuation in US: what affects it, and how is it described?

Front

Attenuation = 0.5 dB/cm/Hz for soft-tissue Proportional to frequency: 2-MHz US beam will have 2X the attenuation of a 1-MHz beam A 10-MHz beam will have 10X the attenuation per unit distance Attenuation is logarithmic: beam intensity is exponentially attenuated with distance Half-value thickness (HVT): the thickness of tissue necessary to attenuate the incident intensity by 50% (which is equal to a 3-dB reduction in intensity). As frequency increases, the HVT decreases

Back

Axial resolution in US

Front

Axial resolution: the ability to distinguish 2 closely-spaced objects in the direction of the beam → depends on spatial pulse length Minimum required separation between 2 reflectors in order to distinguish them = ½ the spatial pulse length (SPL) Objects closer than ½ the SPL will not be resolved due to overlapping echoes SPL = # of cycles emitted per pulse by transducer x wavelength Axial resolution is independent of depth Improves with shorter pulses, greater damping (shorter pulse), and higher frequency (shorter wavelength) Gain doesn't help - it widens the beam

Back

Relationship between pitch and dose

Front

Doses in helical scanning with pitch of 1.0 are similar to those from axial scanning Pitch < 1.0 → dose increases because slices overlap PItch > 1.0→ dose decreases because energy is more spread out Relationship is proportional: pitch of 2 halves the dose; pitch of 0.5 doubles dose.

Back

Artifacts associated with beam hardening in CT

Front

Beam hardening: as x-ray beam passes through an object the lower energy photons are removed preferentially → leaves a harder beam with increased average energy Cupping: x-rays passing through the middle of a uniform shape (e.g. head) are hardened more than those at periphery (shorter path) Center of image appears darker than periphery Dark bands/streak: occurs in the setting of 2 dense objects X-rays that pass through one are less attenuated than objects that pass through both Result in dark bands and streaks between those objects Classic location in bone or where dense contrast was used

Back

What causes non-uniformity of the US beam?

Front

Beam intensity (power, measured in watts) is not uniform Beam spread: intensity will decrease from the center to the edges Maximum sound pressure is always found along the acoustic axis (center-point) Divergence of the beam in the far-field causes the power to be spread over a large area Interference occurs from numerous point sources interacting in the near field (less with broadband transducer)

Back

Who needs individual dose monitoring?

Front

Individual dose monitoring is mandated if occupational dose is favored to be > 10% of the annual dose limit (500 mrem)

Back

Wave interference patterns

Front

constructive effects destructive effects

Back

Sound wave matte interactions

Front

reflection refraction scattering absorption

Back

Metal artifact and correction

Front

Metal Artifact: causes streak Via beam hardening, partial volume, aliasing, and having density ranges higher than can be handled by the computer Metals with high Z (iron, platinum) tend to have more artifacts than those with lower Z (titanium) Fixing metal artifact: Remove the object Increase kVp (sometimes works) Use thinner slices Certain interpolation software techniques

Back

2 distinct components of the US beam

Front

Near-Field (Fresnel Zone): converging The length of the near-field (converging portion) depends on the transducer frequency and diameter Higher transducer frequency → longer near field Larger diameter element → longer near field Far-Field (Fraunhofer Zone): diverging As the beam diverges, the ability to distinguish 2 close-by objects is reduced. Higher frequency→ less divergence Larger diameter element→ less divergence US intensity in the far-field gradually decreases with distance

Back

Risk of Radiation-Induced Cancer per Dose

Front

Risk of Radiation-Induced Cancer per Dose Adult→ 5% per Sv Child→ up to 15% per Sv About 1/10th that of someone over 50

Back

Smooth vs sharp Kernels

Front

low noise lower spatial special resolution - brain higher spatial resolution more noise - bone

Back

CT radiation dose: variations within the scan plane, and z-axis variation

Front

Variations within Scan Plane: Head CT: central and surface doses are very similar Body CT: surface is about twice the central dose Z-axis variation: "tails" of radiation along the edge of the area being scanned The profile of radiation is not limited to the primary area being imaged Add up on multiple scans

Back

Dampening block: thick versus thin

Front

Dampening block: dampens the transducer vibration to create a pulse with a short spatial pulse length → needed to preserve detail along the beam axis (axial resolution) Thin block = "low-damping" = high Q Creates a narrow bandwidth à used in Doppler to preserve velocity information. Long spatial pulse length - doppler Thick block = "heavy damping" = low Q Broad bandwidth Short spatial pulse length → high spatial (axial) resolution (fewer interference effects and thus more uniformity)

Back

Reverberation vs comet tail vs ring-down artifact

Front

Reverberation artifact: looks like multiple equidistantly-spaced linear reflections Sound wave encounters 2 parallel highly-reflective surfaces→ echoes generated from a primary US beam are repeatedly reflected back and forth Recorded and displayed as multiple echoes Comet-tail artifact = a form of reverberation 2 parallel highly-reflective surfaces are closer together, so the sequential echoes are closely spaced. If the distance between them is less than ½ the SPL (required for axial resolution) → displayed echoes look like a triangle Ring-down artifact: line or series of parallel bands extending posterior to a collection of gas Sound wave encounters fluid trapped between a tetrahedron of air bubbles → the vibrations create a nearly continuous sound wave transmitted back to the probe

Back

Under-Sampling Artifact in CT

Front

Under-Sampling: insufficient # of projections used to reconstruct the CT can diminish quality and create misregistration artifacts. View aliasing: under-sampling between projections See fine stripes radiation from the edge (but at a distance from) a dense object Fixed by acquiring the largest possible # of projections per rotation- slowing the rotation speed. Ray aliasing: under-sampling within a projection See stripes appearing close to the structure. Fixed by using specialized high-resolution techniques.

Back

Signal to noise

Front

twice the rays twice the signal twice the rays 2root 2 or 1.4 the noise INCREASE SNR higher mA longer rotation time higher kVp larger slice thickness Large pixel size Decreased pitch edge enhancement filter will improve spatial resolution at the cost of increase noise

Back

Types of scattering (reflectors)

Front

Specular (smooth): strength of reflection is highly dependent on the angle of incidence. -Relatively independent of frequency Non-specular (diffuse): angle has no effect on strength High frequency = small wavelength = surfaces appear more rough = more scatter -Absorption also increases with frequency Hyperechoic = high scatter amplitude relative to average background Hypoechoic = low scatter amplitude relative to background

Back

Helical Artifact in Multi-Section

Front

Helical Artifact in Multi-Section Distortion is more complicated, with a classic "windmill" appearance where several rows of detectors intersect Worsens with increased helical pitch Z-filter used to reduce severity of windmill artifacts

Back

Impedance

Front

density X speed of sound Rayl units larger difference of impedance between tissues results in larger reflection

Back

What is the difference between linear, curved, and phased-array transducers?

Front

Linear (includes curved) Simultaneous firing of small group of adjacent elements → width of the transducer is equal to the width of the individual elements Rectangular FOV used for small /superficial things Curved array: trapezoidal FOV. Scan lines diverge deeper into the image → wider FOV for deeper structures (e.g. abdominal or pelvic) Phased array: fires elements at different times Individual wave firing times can be adjusted to cause constructive and destructive wave summationsà steers and focuses the beam without having to move the probe.

Back

CT window width and level

Front

CT Window Width and Level Level = midpoint of the gray-scale display Want level at the attenuation of your target (e.g. for bone, high level) Width: selected based on what you are comparing For things with very different densities, you want a wide width For similar densities (e.g. white and gray matter), you want a narrow window width Things above the upper width level will be solid white, and below lower level will be solid black.

Back

Methods to fix or prevent beam hardening

Front

Filtration: pre-hardening the beam to remove lower energy components before they hit the patient, and/or using bowtie filter. Calibration correction: use phantom to allow the detector to compensate for the hardening effect Correct software→ can use an iterative correction algorithm Avoidance: can tilt the gantry or position the patient to avoid areas that cause beam hardening

Back

Average dose: CT head (adult), CT abd (peds and adult)

Front

Average Dose (CTDI- in mGy) → reference doses (set by ACR at 75th percentile) Adult head CT = 58 mGy (effective dose 1-2 mSv) Adult Abd CT = 18 mGy (effective dose 8-11 mSv) Peds Abd CT = 15 mGy

Back

Relative intensity in US: what represents a 50% decrease in intensity

Front

Relative intensity: recorded in decibels (dB) → based on a log10 scale A change of 10 in dB scale corresponds to 2 orders of magnitude (100 times) Reducing the sound intensity to 10% is -10 dB Reducing to 1% is -20 dB Reducing to 0.1% is -30 dB A loss of 3 dB (-3 dB) represents a 50% loss of signal intensity (power) The tissue thickness that reduces the US intensity by 3 dB is the half-value thickness

Back

Techniques to reduce dose to peds patients and breast

Front

Peds: recommended to reduce mAs Reduced techniques possible due to greater x-ray penetration Dose reduction in head CT more modest than in abdomen Breast: Reduced mA → images look noisy mA modulation (preferred) → adjust technique based on density Bismuth shield→ artifact and degraded image Beam hardening may falsely elevate HU directly deep to shield

Back

Embryo dose from CT AP

Front

~30 mGy

Back

CTDI measurements, DLP, effective dose

Front

CTDI: dose normalized to beam width -Weighted CTDA: ⅓ the central CTDI + ⅔ the peripheral CTDI (given in mGy) -Volume CTDI: CTDIweighted / pitch DLP = CTDIvol x length of scan in cm Effective Dose for CT: given in Sv Effective dose = k x DLP (k is body-part specific) if the patient is larger than the phantom the dose is overestimated

Back

ACR Established Diagnostic CT Reference Values: CT head, CT abdomen (adult and peds)

Front

ACR Established Diagnostic CT Reference Values: CTDIvol: CT head = 75 mGy CT abdomen (adult) = 25 mGy CT abdomen (peds) = 20 mGy

Back

Dual energy

Front

USE TWO levels ussu 80 and 140 kVp

Back

Focal zone, and the effect of multiple focal zones

Front

Focal Zone: narrowest portion of the beam → point of maximum intensity and lateral resolution. Multiple focal zones: by repeatedly acquiring data over the same volume, but with changes in the phase timing of the array elements Each focal zone requires an independent set of pulses → increasing the # of focal zones decreases frame rate and temporal resolution both mechanical and electronic focusing improves lateral resolution goal of a standoff pad is to help lateral resolution in the near field

Back

Partial volume and methods to correct it

Front

Partial Volume: can occur in 2 main ways Pattern 1: Partial volume effect A dense object protrudes partially into the width of an x-ray beam Results in divergence of the beam → manifests as shading artifacts adjacent to the object. Pattern 2: CT voxels are 3D cubes. If you have a dense thing taking up ½ the cube and a low-attenuating object in the other half, the machine will average them together→ intermediate density Classic location is skull base averaging with CSF or brain→ looks a little like blood Fixing partial volume: Make slices thinner If noise is a problem, acquire thin slices then generate thicker slices by adding them together

Back

Helical Artifact in Axial Plane: Single Section

Front

Helical Artifact in Axial Plane: Single Section Seen most frequently around the top of the skull→ from anatomy changing rapidly in the Z-direction The higher the pitch, the worse the effect Minimizing helical artifact: Reduce variation in the z-direction Use lower pitch Use 180 degree instead of 360 when possible Use thin sections instead of thick This is why head CT is commonly done with axial scanning instead of helical

Back

Lateral resolution in US

Front

Lateral resolution: ability to resolve objects in a distance perpendicular to the beam direction → depends on transducer element width Lateral resolution improves with a thinner beam→ thinnest at the end of the near-field (at the focal zone) Lateral resolution is highest at the focal zone Worst in areas close to and far from the transducer surface (away from the focal zone) Depth-dependent → lateral resolution worsens in the deeper field Increasing gain worsens lateral resolution, because it widens the beam. Improved by: - narrowing the beam in the proximal field (adding an acoustic lens) -minimizing gain -using phased array with multiple focal zones -increasing the line density (lines/cm).

Back

Techniques for cardiac CTA

Front

Cardiac Imaging→ best performed during diastole Prospective ECG-triggering: AKA "Step and Shoot" Based on R-R interval Axial imaging, not helical ↓ dose because tube isn't on the whole time No functional imaging Retrospective EKG-Gating: scans the entire time, then back-calculates Can do functional imaging Lower pitch→ higher radiation dose Helical mode scanning

Back

Ring artifact in CT

Front

Calibration error or defective detector on 3rd generation scanner will cause errors in angular positioning → circular artifact Fix it by recalibrating the detector or replacing the broken part

Back

What beam-related artifact is more common with linear-array transducers?

Front

Side-lobe artifact → More common with linear array transducers Side-lobes are created from radial expansion of the piezoelectric crystals→ creates off-axis low-energy beams on the side Received echoes are incorrectly attributed to the main beam, and incorrectly placed. Example: incorrectly placed echoes overlap an anechoic structure (bladder, gallbladder)

Back

Methods to reduce motion artifact

Front

Patient Motion→ can cause misregistration, manifesting as shading or streaking (especially on reconstructions) Fixing motion artifact: Align the scanner in the primary direction of motion (e.g. vertically above or below a chest scan for breathing) Overscan an extra 10% with the repeated portion averaged Gating

Back

Contrast resolution

Front

better with CT: minimal scatte reaching the detectors - tight collimation and tight windowing what improves resolution - decreased noise

Back

Matching layer: purpose, and optimal layer

Front

Matching layer: creates an interface between the transducer and patient. Minimizes the acoustic impedance differences between the transducer and patient Optimal matching layer = ¼ the wavelength

Back

Snell's law

Front

Has to do with refraction (change in direction of transmitted US energy at tissue boundary when beam is not perpendicular to the boundary). Refraction is influenced by speed change (based on tissue compression) and the angle of incidence. Sin angle1 / Sin angle2 = Speed 2 / Speed 1 No refraction occurs if the sound is the same in the 2 media or with perpendicular incidence. Refraction is the cause of shadows seen at the edges of a fluid-filled structure (e.g. GB) as sound passes from tissue to fluid.

Back

Pixel size

Front

matrix constant Pixel size = FOV / matrix decreasing the FOV decreases the pixel size increasing the FOV increases the pixel size

Back

What changes in a sound wave

Front

The frequency is constant WAVELENGTH not speed 1540 m/s

Back

Elevational resolution in US

Front

Elevational resolution (slice-thickness): same as lateral resolution but measured in a plane orthogonal to the image plane → Depends on transducer element height Associated with volume averaging in regions close to the transducer and in the far-field Improved by: -Using a fixed focal length across the entire surface of the array (downside is partial volume effects). -Minimizing slice thickness- done by phase excitation of the outer to inner arrays.

Back

Section 5

(50 cards)

Sodium iodide well counter

Front

Basically a small gamma camera with just one PMT Sample is placed into a hole in the block of NaI crystal Gives great efficiency at detection, but can be overwhelmed (dead counts) Good for in-vitro blood or urine samples Good for wipe test samples. Thyroid probe is a modified NaI well counter

Back

Safety issues with OB US

Front

Temperature rises are most likely to occur at bone surfaces and adjacent soft tissues- baby with increasing mineralization (2nd and 3rd trimester) has theoretical risk of heating sensitive tissues such as brain or SC. 1st trimester recommendations: -Pulsed dopper (spectral, color, and power) should not be routinely used. -Keep TI under 0.7-1.0 TI: Below 0.7 for OB imaging Between 1.0-1.5 should not exceed 30 minutes Between 2.5-3 should not exceed 1 min > 3.0 should not be used

Back

What parameters are used as indicators of potential biologic effect of US?

Front

Thermal index (TI): max temperature rise in tissue secondary to energy absorption Mechanical index (MI): how likely cavitation is to occur considering peak rarefaction pressure and frequency -Indicator of mechanical bioeffects (streaming and cavitation).- Matter most with contrast-enhanced US. -Involves stable and transient cavitation.

Back

What doppler artifact is secondary to turbulent blood flow?

Front

Tissue vibration: secondary to turbulent blood flow Doppler shows a mixture of red and blue colors Classic example is an AVF after kidney biopsy

Back

Geiger Muller counter

Front

Used to detect small amounts of radioactive contamination Very sensitive Great for low-level radioactive survey Terrible for very high radiation fields (dead time) Has a gas-filled chamber. Dead time: vulnerable to being overloaded by a large dose of radiation. With dead time, the ionization must dissipate before it can respond again. Max dose it can handle is about 100 mR/h

Back

Aliasing in Doppler

Front

Super high velocities are displayed as low (negative) On spectral, it looks like the spectrum is cut off and wrapped around the baseline, appearing on opposite end. Occurs when the doppler shift is > than the Nyquist frequency Nyquist limit (kHz) = 1/2 x pulse repetition frequency (PRF) Reduced or eliminated by: -Decreasing Doppler shift- via using lower frequency transducer or using doppler angle closer to 90. -Increasing the PRF (increasing Nyquist) or selecting a sample volume at a lesser depth or increasing the scale.

Back

Mechanism of harmonic imaging

Front

US beam is progressively distorted, primarily in the central portion where the intensity is high. A distorted pulse gives rise to distorted echoes → these have significant energy at harmonic frequencies Tissue harmonics: made using the 2nd harmonic component of the echo signal, with the fundamental frequency excluded. Undistorted echoes coming from lower intensity areas (fringes of the beam, side-lobes, superficial tissues) are not seen.

Back

Color bleed

Front

Looks like color extending beyond the vessel wall Can decrease sensitivity to thrombus or stenosis Improved by decreasing the color gain

Back

What decay is seen with neutron excess?

Front

Beta minus decay: seen with neutron excess Neutron: converted to proton, then emits and electron (beta particle) + antineutrino Worthless of imaging Can harm DNA: basis of radionucleotide therapy with P-32, Sr-89, Y-90, I-131, and Sm-153. Isobaric - no mass is changed use plastic to shield

Back

Alpha decay: types of atoms, particles produced

Front

Alpha tends to occur in heavier, unstable atoms Alpha particles: basically helium nuclei (2 protons, 2 neutrons) -Slow and fat, worthless for imaging, but lots of tissue damage

Back

dose calibrator

Front

look for the dipper

Back

dosimeter

Front

Back

Activity

Front

Curie - 3.7 X10^10 disintegrations/s Becquerel Bq, I /S Specific activiey= activity/unit mass

Back

Distance

Front

Distance has NO effect on sensitivity it effects Resolution

Back

Power Doppler

Front

Gives a very sensitive look for the presence of flow without information on direction. -Still get color, but instead each pixel registers the total # of frequency shifts, and the amplitude. -Does NOT exhibit aliasing (seen on both spectral and color Doppler) -NO dependence on Doppler angle (can image totally perpendicular to vessel) -Extremely sensitive to flow, even if very slow.

Back

Pinhole collimator and magnification

Front

Pinhole collimator: magnifies and inverts the image -Usually cone-shaped -Used for thyroids and other small parts Magnification occurs at a ratio of: -pinhole to detector "f" / pinhole to patient "b" -If F = B there is no magnification -If F > B there is magnification -If F < B the object gets smaller Magnification at the front is greater than at the back- large objects get distorted. Pinhole cameras have poor sensitivity Want collimator and detector as close as possible to the patient for best spatial resolution (affected by distance). Sensitivity is not affected by distance.

Back

Isotopes vs. isobars vs. isotones vs. isomers

Front

Element is defined by the # of protons (Z). There may be multiple isotopes of a given element varying in the # of neutrons (N). The particles in the nucleus (Z + N) are referred to as nucleons. The atomic mass # (A) is given by A = 2 + N Isotopes = same # of protons (Z) -Examples- I-131 (53 protons) and I-123 (53 protons) Isobars = same atomic mass (A) -Examples: I-131 (53 protons) and Xe-131 (54 protons) Isotones = same # of neutrons (N) -Examples: N13 (7 protons) and O14 (8 protons) Isomers = different energy state -Examples: Tc99 and Tc99m

Back

Technical factors and limits related to mechanical and thermal index

Front

Rate of energy absorption increases with frequency. Eventually the rise in temperature slows secondary to conduction and perfusion with an eventual steady state. Thermal-induced damage is a threshold phenomenon (no damage until a certain temp is reached). Cavitation is most likely to occur with low frequency and high pressure. Spectral doppler deposits more heat than gray-scale Per NRCP, risk-benefit decision needed when TI exceeds a value of 1.0 and MI exceeds 0.5.

Back

Star artifact

Front

In you have very focal intense energy, you can sometimes see star artifact, caused by septal penetration of the hexagonal collimator holes. Typically seen in thyroid bed after high therapeutic dose (using medium energy collimator instead of high)

Back

How is field uniformity tested, and what are the acceptable values?

Front

2-5% non-uniformity allowed (1% if SPECT) Tested via a FLOOD: checks if camera can produce a uniform image along the entire crystal surface Can use either Na-Tc-99m-O4 or Co-57 Extrinsic (with collimator): done daily, testing both collimator and crystals Intrinsic (no collimator): weekly Recommended counts for both range between 5 and 10 million Bull's eye appearance of tubes indicates a problem

Back

Converging hole versus diverging collimator

Front

Converging hole collimator (cone beam): holes are close together on the object side and far apart on the crystal side -Magnifies WITHOUT inverting the image Diverging collimator: opposite of converging, with holes far apart on the object side, and close together on the camera side. -Takes a large object and minimizes it- results in image of a large part of the body on a small crystal. -Increased area, decreased sensitivity and resolution.

Back

Production of radiotracers

Front

Bombardment - reatctor: Neutrons --> taget + Z Mo99 Fission Cyclotron is carrier free I- 131, Xenon 133, strontium 99m, Mo 99, cesium 137

Back

Ionizing chamber

Front

Lower sensitivity Stable across a wider voltage range: excellent for accurate estimates (or exposure). Used when higher doses are expected. Don't have dead time Used in most dose callibrators Can measure dose rates.

Back

Testing energy window

Front

Correct window needs to be used prior to each study- thus it must be tested daily Most common approach is to use a symmetric window centered at the peak energy used in the imaging test. For Tc, you would use a 20% window, centered at 140 keV.

Back

A-mode, B-mode, and M-mode US

Front

A-mode → A for amplitude Currently only used in ophthalmology Generates a signal of amplitude given as a function of time B-mode→ B for brightness Basically the conversion of A-line information to brightness-modulated dots on a display Proportional relationship of brightness to the echo signal amplitude M-mode→ M for motion (standard US) Used to display the echoes from a moving organ (e.g. heart valves) from a fixed transducer and beam position on the patient 4X the energy of B mode

Back

Speed displacement artifact

Front

Speed Displacement Artifact: appearance of a discontinuous, focally displaced liver border The speed of sound slows down in fat, relative to liver → beam takes longer to return → perceived as being further away

Back

Compound imaging

Front

Compound imaging: commonly used in breast imaging Uses the electronic steering of the US beams to image an object in multiple different locations → sharpens the edges and decreases posterior shadowing (which could make a cyst look solid)

Back

Pseudoflow artifact

Front

ureteral jet

Back

Advantages and disadvantages of harmonics:

Front

Advantages: Improved lateral spatial resolution Reduced side-lobe artifact Removal of multiple reverberation artifact (from adjacent anatomy) Cysts look clearer Disadvantages: 2nd harmonic is at a higher frequency than fundamental frequency→ attenuates far more rapidly → reduced depth of penetration The required processing generally requires 2 transmitted pulses for each line of sight in the image → reduces frame-rate by a factor of 2 Hypoechoic masses look like cysts

Back

Refraction artifact

Front

Refraction is due to speed differences in tissues → causes the shadows seen at the edges of a fluid-filled structure (classically gallbladder) as sound passes from tissue to fluid Causes a change in direction of the beam Appearance: Object can appear wider than it actually is Object can appear misplaced to the side of the returning echo Object can appear duplicated Example: classically shown with duplicated SMA when imaging deep to rectus muscles and midline fat Will appear as a normal single vessel if you move the transducer to the side

Back

What types of dosimeters are used with film badge and ring badge?

Front

Optically stimulated dosimeter: actually replaced the traditional film badge, and includes strips placed under a filter Ring badge: uses a thermoluminescent dosimeter. dominant hand, index finger, label in (palm), under the gloves

Back

Parallel hole factors: septal length, hole diameter, and septal thickness

Front

Long septa: -Low sensitivity (noisy) -High spatial resolution Short septa: -High sensitivity, low spatial resolution Wider hole diameter: high sensitivity, low resolution Narrow hole: low sensitivity, high resolution Thick septa: less penetration, less available space for holes (less sensitivity) Thin septa: more penetration (blur), more available space for holes (more sensitivity) Generally want long, thick septa + wide holes for high energy and short, thin septa + narrow holes for lower energy

Back

Downscatter

Front

High energy photons can spill into the window of a low-energy emitter, mainly resulting from Compton scatter effects. For a study using more than one tracer, the one with the LOWER photon energy must be used FIRST. -E.g. V/Q scan, need to do Xenon portion (81 keV) before the Tc-99m (140 keV)`

Back

Dynamic study collimator

Front

High sensitivity collimator

Back

Radioactive decay

Front

physical half life biologic half life effective half life = P+B 1/E=1/P + 1/B

Back

Harmonics

Front

Transmit one frequency and receive another to exclude artifacts - namely reveberation works with center and far field beam this improves lateral resolution this uses a higher frequency beam so there is lost deep penetration careful this can make a solid mass look cystic

Back

What should the angle be for Doppler imaging?

Front

Doppler angle should be 30-60 based on formula for calculating it using cosine of the angle Cos 90 = 0; Cos 0 = 1 Why > 30 rather than 0 degree angle? -Angles < 20 degrees can cause refraction and loss of signal -Aliasing also becomes an issue Why not perpendicular? -If you placed the probe perpendicular to the vessel, it can give false impression of now flow (occlusion) -I can also create a mirror image.

Back

Pulsed wave (spectral) Doppler vs. color Doppler

Front

Pulsed wave (spectral): blood flow velocity varies, yielding a spectrum of doppler shifts rather than a single frequency. -Can obtain direction of flow (plotted above and below baseline) and velocity. Color doppler: uses the grayscale image with a superimposed color blood flow image -Gives direction of flow with colors -Color intensity varies depending on flow intensity (things that aren't moving are gray). -Obtains samples of each pixel multiple times, then displays the average shift. -Spatial resolution less than that of grayscale imaging (though smaller vessels are better seen). -Doppler angle not as important since info is semi-quantitative.

Back

Image linearity and spatial resolution

Front

Lead bar phantoms with parallel lines placed between the collimator and a Co-57 sheet are used to test resolution and linearity. Performed weekly Resolution: defined as ability to differentiate between 2 distinct points (tell the bars are separate) Linearity: tested by looking to see if all the bars are straight

Back

Gamma camera

Front

Back

Shielding with particles

Front

Particle emissions cause more problems than photon emissions Even though beta - and beta-+ don't travel far, they can damage DNA Plastic should be used to shield against them instead of lead, which would create Brehmmstrahlung x-rays

Back

Artifact commonly seen at the liver/lung interface

Front

Mirror-image artifact: US beam passes through a highly-reflective surface, then gets repeatedly reflected between the back side of the reflector and the adjacent structure. Appears as a duplication equidistant from but deep to the strongly reflective surface. Classic location along liver/lung interface → see liver parenchyma where you should see lung.

Back

Scintillation crystal and thickness

Front

Once photon emerges from collimator, it impacts on the crystal Crystal = thallium-doped sodium iodide -When struck with a photon, it produces a pulse of light. Thicker crystals = better sensitivity (less photons simply pass through), but worse spatial resolution Thinner crystals: better spatial resolution, but worse sensitivity

Back

What types of decay occur with proton excess?

Front

Proton excess (neutron deficiency): decay by beta-+ decay or electron capture. Beta-plus: proton transformed into neutron -Need 1.02 MeV for this to occur. -Anti-neutrono and positron (beta particle) emitted- travels a short distance before colliding with an electron -Mutual destruction of positron and electron emits two 5ll keV photons that travel 180 degrees apart. Electron capture: also seen with proton excess (neutron deficiency) -Occurs in setting of insufficient energy (beta-+ needs 1.02 MeV) -Proton captures an electron and turns into a neutron -The neutron then emits characteristic radiation -Leads to gamma emission and sometimes characteristic radiation, which can be used in imaging. isobaric and with isometric tansition

Back

Center of rotation

Front

Gamma cameras that are used for SPECT need routine testing for alignment offset at the COR Done with 5 small Tc-99m point sources along the axis of rotation. Axis should be straight, with minimal deviation Performed WEEKLY

Back

What radiotracers are used for the different types of parallel hole collimators?

Front

Gamma camera: radiotracer exits patient, then goes through collimator, then crystal, then PMT, then computer. Low energy (1-200 keV): Tc-99m, I-123, Xe-133, Tl-201 -Thinner plates Medium energy (200-400 keV): Ga-67, In-111 High energy (>400): I-131 (technically most energy peaks are medium). -Thicker plates

Back

output power vs receiver Gain

Front

output power (transmit gain) increase energy sent in to the body - degrades lateral resolution TGC- time gain compensatin adjust the amplification applied with depth

Back

Isomeric transition vs. internal conversion

Front

Isomeric transition (good): any process that gives off gamma radiation but doesn't change # of protons or neutrons. Mo99 (B-) -> Tc99m (IT)-----> Tc99 m= metastable Internal conversion (bad): excess energy exceeds binding energy, creating an ejected electron + characteristic radiation (or auger). These compete with each other - ratio termed "alpha" -Lower alpha = more useful radiation, and less harmful radiation produced.

Back

Twinkle artifact

Front

Occurs behind strongly reflecting surfaces such as calcification, manifesting as a noisy spectrum with rapidly fluctuating red and blue colors Has higher sensitivity than shadowing for detection of small stones Highly depending on machine settings and how round the reflecting surface is (more rough = more twinkle).

Back

SPECT and matrix with gamma camera imaging

Front

When improved spatial resolution is needed, SPECT can be performed by rotating a camera 180 or 360 degrees around the patient -The longer you image on SPECT, the better it looks, but higher risk of motion Matrix size: 128 x 128 has superior resolution than 64 x 64. -Larger matrix size also means LONGER acquisition and reduced count density per pixel (which affects image contrast).

Back

Section 6

(50 cards)

Signal to Noise

Front

Bigger voxel equals better signal Thicker slices - increased transmit RF pulse - decreased SSG Larger field of view Smaller matrix Short TE and long TR - limits number of echos/slices increase proportionally with field strength RF coils increased NEX (longer scan) Narrow receiver bandwidth will have less noise

Back

Radiation area vs high radiation area vs very high radiation area

Front

Radiation area: could get 0.005 rem (0.05 mSv) in 1 hour at 30 cm High rad area: could get 0.1 rem (1 mSv) in 1 hour at 30 cm Very high rad area: could get 500 rads (5 gray) in 1 hour at 1 meter

Back

Annual dose limit to public and unrestricted area

Front

Limit of 100 mrem to public (1mSv) Unrestricted area: no more than 2 mrem per hour (0.02mSv)

Back

Better MRI spatial resolution

Front

Small Voxel Small Field of View Larger Matrix Thinner Slices steep(large) SSG Thin transmit Bandwidth

Back

Radionuclidic purity of Tc-99m

Front

Radionuclidic purity: tested after washing off Tc from aluminum column using saline -Evaluating for residual molybdenum (T1/2 67 hours) -Sample placed behind lead shield and Mo assayed for first. High energy photons of Mo (~740 keV) will NOT be attenuated by the shield, but the 140 keV Tc photons will. -NRC allows no more than 0.15 microCi of Mo per 1 milliCi of Tc at the time of administration.

Back

Normalization scan vs blank scan in PET

Front

PET QA tests Normalization scan: corrects for discrepancies in the thousands of detector elements -Scan a calibrated position source placed in the FOV -Scan serves to normalize the detection lines -Should be done monthly (N-M = normal month) Blank scan: done to help keep the AC data accurate -Done with nothing in the FOV -Essentially zeroing the scanner -Done daily (start each day with blank slate) Dark block on the sonogram - bucket set up

Back

Mechanism of FSE sequence

Front

FSE: idea is to reduce the TR, which is a major contributor to the duration of the study - 90 pulse followed by multiple 180s -Done by applying multiple 180 degree RF pulses, each resulting in an echo after the initial 90RF -Echo train length = # of echoes in same TR -Normal phenomenon called J coupling, which occurs between nuclei of lipid molecules, causes intrinsic shortening of T2 signal. The fast repetition of 180 degree pulses messes up the J couples and causes T2 of fat to lengthen. Thus T2 fat signal is longer with FSE. -T2 blurring: with each progressive echo train the transverse signal gradually decreases. -Acquisition time: proportional to 1/ETL - echo train length

Back

2D vs. 3D PET, and time-of-flight (TOF) PET

Front

Modern scanners are 3D: use incident time to exclude scatter Older 2D scanners use septa to deal with scatter TOF PET: by measuring the difference in arrival times between the 2 photons from an annihilation event, a TOF image can be created -Can enhance spatial resolution and image contrast.

Back

Larmor Equation

Front

precessional frequency = the static magnetic field (Bo) x the gyromagnetic ratio(constant) w=Bo x y

Back

NEX

Front

double NEX with double the scan time improved signal by the square root of two

Back

Radiochemical purity of Tc99

Front

Tc comes out of the generator as Na-Tc99m-O4, which needs to be reduced before use by adding stannous ions (SnCl2). Checking for free pertechnetate Tested with thin-layer chromatography Limits for free Tc: -95% for Na99mTcO4 -92% for 99mTc sulfur colloid (MAA) -91% for all other Tc radiopharmaceuticals. Free Tc (pertechnetate) can occur from lack of stannous ions (reducing agent) or accidental air injection into the vial or syringe (which oxidizes) Shown on Tc scan with gastric, salivary gland, and thyroid uptake.

Back

T2 vs. T2*

Front

Signal of T2 decays faster than predicted by tissue-spin interactions alone, because the field is not entirely homogeneous, creating additional interaction that further speeds decay. Thus, T2* decay is ALWAYS faster than T2. T2* = tissue-spin interaction + field inhomogeneity T2 = tissue spin interaction only Fixing T2* and making it T2: at 1/2 the time of TE, a 180 degree pulse can be given to refocus the signal, helping to remove the fixed field inhomogeneity.

Back

Spatial resolution

Front

determined by the voxel Voxel volume = slice thickness X phase(FOV / Matrix size) X read(FOV/Matrix Size )

Back

What are the components of K-space information?

Front

inverse 2 dimensional Fourier transform = mathematical technique to convert data from time domain to frequency domain. Center of K-space: information about gross form and tissue CONTRAST. Periphery of K-space: information about SPATIAL RESOLUTION.

Back

Major advantage of SPECT over planar

Front

Improved contrast from overlapping structures SPECT is depth dependent (PET is not) iterative reconstruction

Back

Identify attenuation-corrected vs. uncorrected PET image

Front

Look at the skin: hot on uncorrected Look at lungs: also hot on uncorrected Corrected: light skin and lungs. metal will look hot on PET attenuation in PET is depth INDEPENDENT

Back

Diet considerations for PET

Front

Should fast for 4 hours before study: sooner can increase insulin and drive FDG into muscles Minimizing cardiac activity (e.g. thoracic tumor) can be done with 12 hour fast and low-carb/high-protein diet for 24 hours drink water to void bladder insulin is avoided but long acting is better is necessary. Metformin is ok Brown fat - keep the room warm or give propranolol or valium

Back

What is T2?

Front

RF pulse causes protons to synch up and precess in phase, establishing transverse magnetization. With time these gradually fall out of synch = T2 transverse relaxation T2 = decay of transverse magnetization- exponential curve with negative slope. -T2 shorter than T1 (takes less time to go down a hill). T2 = time at which 37% of original transverse magnetization has decayed. AKA spin-spin relaxation Free induction of decay

Back

Package labels and dose limits

Front

White 1: no special handling -Surface dose rate < 0.5 mrem/hr, 1 meter 0 mrem/hr Yellow 2: special handling -Surface rate < 50 mrem/hr, 1 meter < 1 mrem/hr Yellow 3: special handling -Surface dose rate < 200 mrem/hr, 1 meter < 10 mrem/hr

Back

Types of coincidence in PET

Front

True: TWO 5ll keV photons from an annihilation reaction detected in the same coincident window Scatter coincidence: one of the photons has a Compton interaction and is deflected, but still hits the detector within the coincident time window (just not in the calculated location). Random coincidence: 2 photons from different annihilation reactions just happen to land within the same coincident window- creates the false calculation that they occurred from the same event.

Back

How are STIR and FLAIR obtained?

Front

Inversion recovery: instead of initial 90 pulse, you start with a 180 degree preparation pulse. You then wait for relaxation of the thing you want to saturate to hit its null point, then hit it with 90 degree pulse, giving that tissue no signal. TI = time between the 180 and 90 pulses. STIR: uses short T1 (120-160 ms) to suppress fat FLAIR: uses long TI to suppress water (around 2000 ms) Relative to other fat-sat techniques, STIR is much less susceptible to magnetic susceptibility and field inhomogeneity. STIR cannot be used with Gad. Gd enhanced tissues have similar TI to that of fat, and may get nulled out. Longer exam= adding the TI makes the TR longer

Back

What spatial encoding direction takes longest?

Front

Phase encoding: encodes spatial information in the vertical direction. Much longer than frequency encoding: done on the thinner portion (side to side in head, front to back in abdomen), with the exception of in the breast. # of phase encoding steps contributes to the duration of a 2D imaging sequence Duration = TR x Npy x Nex TR = repetition time Npy = # of phase-encoding steps Nex = # of excitations.

Back

Code of federal regulations: parts 19, 20, and 35 refer to what?

Front

CFR 19: inspections CFR 20: radiation protection CFR 35: human use of radioisotopes Agreement states can be more strict, but not less strict than the NRC

Back

spatial encoding

Front

Slice select gradient - RF pulse gives slice selection and FOV - perpendicular - knock um down Phase Encoding: causes the protons in the same row perpendicular to the gradient to have the same phase - lines them up - takes much longer - all protons have the same frequency at this time -vertical Frequency Encoding: - perpendicular to the PEC - this modifies the Larmor freqs - you get colums with the same freq - applied at readout

Back

Effect of gradient and slice thickness on resolution

Front

Gradient with higher amplitude (more intense) or applied for a longer period of time results in better spatial resolution Thinner slices (decrease transmit RF pulse/slice-selection pulse or increase SSG) leads to better spatial resolution

Back

What level of activity is considered a major spill for Tc-99m, Tl-201, In-111, Ga-67, and I-131

Front

Tc-99m: major spill > 100 mCi Tl-201: major spill > 100 mCi In-111: major spill > 10 mCi Ga-67: major spill > 10 mCi I-131: major spill > 1 mCi Major spills require a call to radiation safety officer Clear area, cover spill with absorbent paper (don't clean), indicate boundaries, shield source, call officer, decontaminate persons Xenon leak: leave room quickly, close door -Wipe test doesn't work

Back

weighting

Front

short TE= T1W long TR = T2W Proton density= LTE and STR

Back

SUV calculation

Front

SUV = (tissue radioactivity concentration at time point 1 x patient weight) / injected dose activity

Back

Potential disadvantages of 3D pet

Front

Dead time - can get overwhelmed - LSO or more PMTs can help more random events - noise Scatter - get lots of it

Back

Tuning fork artifact

Front

Seen in cardiac SPECT, which uses a 180 degree orbit When a point source is imaged, it should look like a point source If there is an error with the COR (misregistration error), it will look like a tuning fork (2 lines in 1 direction, 1 line in the other)- same appearance can be seen with motion

Back

Spatial resolution and sensitivity in PET

Front

Depends on detector resolution and positron range/angulation Range: determined by photon energy, and is characteristic of the source (F18 vs C11) Scatter and random coincidences result in degradation of image quality Sensitivity: PET/CT much higher than that of SPECT because PET uses multiple pairs of detectors

Back

Spin echo

Front

90RF followed by an 180 RF Duration = TR X phase matrix X Nex Contrast is determined by the TR and the TE TR = one line in K space = duration of the scan

Back

Chemical purity testing of Tc-99

Front

The column in the generator is made of aluminum oxide, which can washout off, clump with Tc, and show up as liver activity, sulfur colloid aggregation, or show up in lungs on liver spleen scan Tested with pH paper less red of a dot than the standard Limit is < 10 microgram Al per 1 ml

Back

Total body dose per year, ocular lens per year, total organ equivalent organ dose per hear, total equivalent extremity dose per year, and total dose to embryo in rem and mSv

Front

Total body dose: 5 rem (50 mSv) per year Lens: 2 rem (20 mSv) Organ dose: 50 rem (500 mSv) per year Extremity: 50 rem (500 mSv) Total dose to embryo over 9 months: 0.5 rem (5 mSv)

Back

Effect of field strength on SNR and spatial resolution

Front

Stronger field increases signal and SNR, but also increases T1 times and thus acquisition time. No effect on spatial resolution Using surface coils also improves signal over that of coil within scanner

Back

Transport index and labeling

Front

TI = measured max dose at 1 meter Radioactive label 1: White 1 -No TI because the rate at 1 meter will be so low Radioactive label 2: yellow 2 -TI < 1 mR/hr Radioactive label 3: yellow 3 -TI > 1 mR/hr carriers- < 10 mR/hour multiple <50 mR/hour

Back

Prepare for f'ery

Front

1 rad = 1 rem 100 rad = 1 Gy 1 mSv= 100 mrem or 0.1rem 1 Gray =1 Sievert (biological effect) 10mSv=1 rem

Back

What determines slice thickness in MRI?

Front

Slice thickness: manipulated by adjusting the bandwidth of the selective pulse and the amplitude of the slice-selection gradient (SSG, applied at same time as initial 90 degree RF pulse). Slice thickness = transmitted RF bandwidth (SSG x constant). Increasing transmit RF bandwidth and decreasing SSG gives thicker slices. Using narrower RF pulse bandwidth takes longer to excite, so thinner slices = longer study

Back

Inherent Limitation of PET

Front

Crystal thickness- primary limiting factor that determines special resolution Positron range - it travels before annihilation - about 1mm Angulation: electron wobbles - 180.5 degrees is more accurate Scatter: different coincidence events

Back

Effect of changing receiver bandwidth on SNR and spatial resolution

Front

Increased (broad) bandwidth: rapid sampling of data -Fat bandwidth picks up more noise -Broad bandwidth = decreased SNR, but faster imaging time and decreased mismatch artifacts like chemical shift and magnetic susceptibility. -Narrow bandwidth = increased SNR No effect on spatial resolution

Back

Factors affecting SUV values

Front

Weight: fat has low FDG uptake, so SUV values in fat people are overestimated (more sugar around for tumor) -More accurate to use lean body mass than weight -Super fat people can have falsely low SUV due to truncation artifact from tissue outside FOV (no data for AC). -if only half of a lesion is see with AC the portion will be over corrected. Timing: longer uptake time = more FDG uptake Longer time to scan = increased SUV values Glucose: high glucose = lower SUV Size: smaller than 1 cm = lower SUV Dose extravasation = lower SUV Reconstruction: more iterations = higher SUV

Back

Rules for receiving radioactive material

Front

Within 3 working hours must survey packages GM counter test at surface and 1 meter Wipes of all surfaces of package Keep package in controlled area next working day within 3 hours

Back

QA on dose calibrator

Front

Dose calibrator uses an ionization chamber to measure pharmaceuticals, with readout in mCi. -Range of most devices is 30 microCi - 2 Ci -Dose should be within 5% of computed activity, and this should be checked daily Constancy: within 5% of computed activity DAILY using reference sources Linearity: checked with large activity of Tc QUARTERLY Accuracy: measures standard measurements of radiotracers at installation and then ANNUALLY Geometry: performed at installation and any time you move the device.

Back

Effect of matrix size on SNR and spatial resolution

Front

Larger matrix: decreased SNR, increased spatial resolution, but increased duration of exam.

Back

Recordable and reportable events

Front

Limit for difference is prescribed and administered dose is 20% from NRC 10% by some agreement states Medical event (reportable): 2 criteria 1. F it up: wrong dose (>20%), route, patient -Or patient receives dose to part of body other than intended site that exceeds 50% the dose expected. 2. Harm: whole body dose > 5 rem or single organ dose > 50 rem. -Must call NRC within 24 hours, letter to NRC in 15 days, and doctor within 24 hours (you or doc calls patient.). -Keep recorded for 3 years Recordable event: < 5 rem whole body dose or single organ dose < 50 rem Requires institutional review and recorded locally

Back

TR and TE

Front

TR (repetition time) = time between initiation of 2 successive RF pulses TE: time between the middle of the 90 degree RF pulse and the peak of the detected echo

Back

Which sequence has the best SNR

Front

PD

Back

Critical organs of radiopharmaceuticals: liver, spleen, stomach, gallbladder,kidneys, bladder, colon

Front

Heart- tagged red blood cells Liver: indium, I-131 MIBG, IV sulfur colloid Spleen: octreotide, damaged RBCs Gallbladder wall: HIDA Renal cortex: thallium, DMSA Bladder: MAG-3, DTPA, I-123 MIBG, MDP Stomach: pertechnetate (and thyroid) Proximal colon: PO sulfur colloid, sestamibi Distal colon: gallium

Back

What is spin-lattice relaxation

Front

Spin-lattice relaxation = longitudinal relaxation = T1 = recovering of longitudinal magnetization Return to longitudinal relaxation follows an exponential curve with positive slope approaching 100% over time T1 = time at which longitudinal magnetization has reached 63% of final value Each tissue has different T1 Greater field strength = longer T1 (greater net magnetization in stronger field, and take longer to hand energy over to lattice).

Back

Effect of number of excitations per slice (# of averages) on spatial resolution and SNR

Front

More excitations = increased signal and SNR -Tradeoff is longer imaging time No effect on spatial resolution

Back

Section 7

(50 cards)

Hemochromatosis

Front

The liver will get darker and darker due to signal loss but if you scan in before out you get faked out

Back

What are the threshold doses and latent periods for the acute radiation syndromes?

Front

Bone marrow: > 2 Gy -Latent period = 1-6 weeks -Possible to survive GI: > 8 Gy -Latent period = 5-7 days -Death within 2 weeks CNS: > 20-50 Gy -Latent period = 4-6 hours -Death within 3 days Total body dose of 0.75-1.25 Gy causes nausea ~30% of the time

Back

What is the LD 50/30 and LD 50/60?

Front

LD 50/30: ~3-4 Gy without treatment -May be able to tolerate up to 8.5 Gy with treatment LD 50/60: ~3-4 Gy -Used for bone marrow failure LD 50/4: GI failure -~10 Gy

Back

Selective pulse

Front

preparatory pulse: exploits resonance differences of tissues - ie knocks out fat spoiler gradient: dephases the selected tissue proceed with the scan as normal knock um down and kickm in the head Needs a strong homogeneous field

Back

Chromosome abnormalities and where they occur

Front

2 types have been described at metaphase Chromosome aberrations: damage occurs early in interphase (before DNA synthesis) -Both chromatids are broken so each daughter will get a broken copy. Chromatid aberrations: damage occurs later in interphase (after DNA synthesis) -Only 1 chromatid will have a break, while the other is fine.

Back

What things effect specific absorption rate (SAR)? and what are the limits

Front

SAR (watts/kg) estimates the amount of energy deposited in a patient - measures rate the RF pulse dissipates in tissue SAR = Bo^2 x alpha^2 x duty cycle -Doubling Bo quadruples SAR -Doubling flip angle quadruples SAR -Doubling duty cycle (making TR 1/2) doubles SAR -SE has higher SAR than GRE due to higher flip angles (especially inversions) Limits: -No tissue shall endure temp increase of > 1 deg C --FDA limits of 4 W/kg over 15 minutes, and 3 W/kg over 10 minutes.

Back

MRI blood flow

Front

Bright on GRE Dark on SE due to ghosting in the phase encode direction slow or stopped blood with have signal

Back

Order of sensitivity for cell cycles

Front

M > G2 > G1 > S G1 is the part of cycle that is most variable in length (shorter in cells with high turnover). Surviving cell synchronization: if you irradiate cells, the ones that survive will have synchronized cell cycles. -This is because they are most resistant in late synthesis, so most surviving cells are in this stage.

Back

What syndrome gives highest sensitivity to x-rays

Front

Ataxia telangiectasia Distractors include Bloom syndrome and fanconi anemia- genetic instability without real sensitivity to x-rays Xeroderma pigmentosia: more sensitive to UV radiation

Back

MRA sequences

Front

2D-TOF MRA: uses GRE sequence with saturation pulse employed to null venous or arterial blood flow -Small voxel size 3D-TOF MRA: Collected as 3D volume instead of slices, and allows for smaller voxels than 2D. Well-suited for high-flow arterial systems like COW. -Benefits include higher SNR than 2D, shorter imaging time, smoother vessel contours, and better saturation. Phase-contrast MRA: uses bipolar gradients to create contrast from flow. High velocity encoding time (VENC) is needed for arterial imaging, lower VENC for veins and sinuses. -Phase contrast MRA is a quantitative image: can measure mean blood flow velocity and direction. VENC In general, TOF MRA is faster and less sensitive to signal loss from turbulent vessels than phase-contrast. Phase contrast has advantages of better background suppression and decreased sensitivity to intravoxel dephasing. FLOW

Back

Dose with hair abnormalities and timing

Front

Temporary epilation: 3 Gy -Onset 21 days Permanent epilation: 7 Gy -Onset 21 days

Back

SSFP: type of sequence and use in cardiac imaging

Front

SSFP: type of GRE sequence- primary sequence used for wall motion and volume analysis. -Cine MRI GRE = bright blood- majority of cardiac imaging SE = dark blood- less susceptibility artifact, so useful in imaging patient with sternotomy wires and anatomy double inversion recovery

Back

What artifact occurs in the FREQUENCY encoding direction?

Front

Type 1 chemical shift artifact: from macroscopic fat -Bright rim on one side, dark on the other at SE or GRE -Chemical shift increases with field strength -Chemical shift decreases with increased gradient strength and wider read-out bandwidth. On the other hand, Type 2 chemical shift artifact (black boundary, or india ink) shows a black line in all directions of the fat-water interface. -Occurs on GRE sequences with voxels containing about 50% fat and 50% water. -Using SE sequences will get rid of india ink, but NOT chemical shift

Back

Mechanism of fMRI

Front

Increased blood flow to local vasculature accompanying neural activity results in local reduction of deoxyhemoglobin. Deoxyhemoglobin acts as a contrast agents because it is paramagnetic (alters T2* MR signal). BOLD imaging done before and after task.

Back

Risk models

Front

Determinisitc - has a threshold severity is dose related does not include cancer risk air kerma Stochastic - chance no threshold, no dose related probability increases with dose linear quadratic Kap

Back

Pareto chart

Front

Purpose: to highlight the most important factor among a set of contributing factors. -Many things contribute, but may be able to fix the problem by addressing a single issue. Chart contains both a line graph and a bar graph. Bars are ranked in descending order of contributing problems

Back

WHat type of contrast is most and least likely to cause NSF, and what is the GFR limit

Front

No gad for GFR < 30 NSF risk related to chelation structure, not the actual Gadolinium. Most likely to cause = linear non-ionic -Omniscan, optimark Intermediate risk = linear ionic -Multihance, magnevist Lowest risk = cyclic structure -Gadovist

Back

Imaging silicon

Front

no contrast sat fat and water

Back

What artifact occurs secondary to excitation of neighboring slices and how do you improve it?

Front

Cross-talk: RF and FT pulses placed close enough together can cause excitation of neighboring sections more than once in a single repetition. -Leads to partial saturation and lower signal -All sections (except on the ends) will be subjected to this -3D images are not susceptible to this because the entire volume undergoes excitation with sections within the volume acquired with gradients. Improved by: -Increasing the gap between sections. -Interleave slices (all odds, then all evens).

Back

Threshold for acute exposure and cataracts

Front

Acute exposure threshold: 2.5 Gy -Latent period inverse to exposure amount Annual dose limit: 0.15 Gy/year (or 150 mSv, though some say 20 mSv)

Back

Radiation effects on fetus

Front

2 Gy in 1st 2 weeks results in abortion (all or nothin) 2 Gy gets you congenital abnormalities during weeks 2-6 (organogenesis). Between weeks 8-15 you can get reduced head diameter and MR, with 40% risk of MR at 1 Sv. Takes a very low dose (just a few XRs) to fetus to increase risk of childhood leukemia Fetal thyroid doesn't take up iodine prior to week 8 -Mom gets I-131 prior to week 8 no hypothyroidism.

Back

How are GRE sequences different from SE?

Front

GRE uses flip angle < 90 and does NOT have a 180 degree pulse. Advantage to low flip angle is faster recovery, shorter TR/TE times, and faster scan. Because you aren't using a 180 degree pulse, you are dealing with T2* and not T2- these are therefore more susceptible to susceptibility artifacts. GRE has lower specific absorption rate (less heating) Steady state - residual transverse mag 2 main methods for dealing with residual transverse magnetization in GRE -Spoiled (incoherent) GRE: uses gradients and/or RF pulses = basically T1 -Refocused (coherent) GRE: uses rewind gradient (e.g. SSFP). Basically T2 or T2*.

Back

What are the annual MRI QC tests?

Front

Performed by physicist or MR scientist Magnetic field homogeneity Slice position accuracy Slice thickness accuracy RF coil check Display monitor check

Back

Doses for skin changes and timing

Front

Early transient erythema: 2 Gy skin dose -Onset = hours Severe "robust" erythema: 6 Gy skin dose -Onset = 1 week Telangiectasia: 10 Gy skin dose -Onset = 52 weeks Dry desquamation: 13 Gy skin dose -Onset = 4 weeks Moist desquamation/ulceration: 18 Gy skin dose -Onset = 4 weeks Secondary ulceration: 24 Gy skin dose -Onset: > 6 weeks

Back

What sequence is most likely to trigger peripheral nerve stimulation?

Front

EPI sequences due to greatest strain on gradients Peripheral nerve and muscular stimulation is triggered by the rapid switching of magnetic field gradients reduce readout bandwidth increase TR to fix

Back

Perfusion imaging

Front

quantitates cerebral micro-vascularization parameters - regional blood flow, volume and mean transit time

Back

What artifact worsens with ascites?

Front

Dielectric/Standing wave effects: due to local eddy currents in imaged tissues -Effects are worse with stronger magnet (because RF waves are shorter at 3T) -Seen with large bellies and ascites -Dark signal in central abdomen (usually over left lobe of liver) Improved by: -Applying dielectric pads -Parallel RF transmission (SENSE): gives a longer pulse by sending independent RF pulses from a set of coils.

Back

Air kerma vs. entrance air kerma vs. air kerma product (KAP)

Front

Air kerma: the sum of kinetic energy of all charged particles made when an x-ray or gamma ray passes through a unit mass of air. Entrance air kerma: this is the air where the x-ray beam would enter the patient (measured without the patient). KAP: supposed to account for the total amount of radiation on the patient. -Calculated by multiplying the entrance air kerma and the cross-sectional area. -KAP IS INDEPENDENT OF THE SOURCE DISTANCE. This is because changes in beam intensity (from inverse square law) are matched exactly by changes in cross-sectional area.

Back

High and low LET molecules

Front

LET = average amount of energy deposited per unit path length of the incident radiation- important for assessing the potential tissue and organ damage High LET: neutrons, protons, alpha particles, and heavy ions. MUCH MORE DAMAGING (higher quality factor) -More likely to cause direct damage (on DNA) and DS-DNA breaks. Low LET: photons, gamma rays, electrons, positrons -More common indirect damage and SS-DNA breaks

Back

GADs

Front

positive - T1 prolongation negative - mag field is jacked t2 shortening DTPA chelator Gad is 3+ the electrons augment the local mag field Pseudolayer in the bladder - layering gad it hydrophilic renal elimination

Back

Inhomogeneous fat suppression and way to improve

Front

Inhomogeneous fat suppression: -Local field inhomogeneities cause fat protons to precess at different frequencies- allows certain areas of fat to resist suppression -Can mimic edema Improved by using inversion recovery (STIR), especially in the setting of metal.

Back

Zone 2

Front

waiting and dressing room screen patients

Back

What is kerma and what are its units?

Front

KERMA = kinetic energy released per unit mass. Energy transfer involves 2 steps: -Energy transfer to charged particle (via Compton, PLE) -Newly charged particle transfers energy to a medium via excitation and ionization = kerma involves the first step only -If you are dealing with a low energy photon, then kerma will be the same as absorbed dose -If a high energy photon, kerma will be MORE than absorbed dose because some of these secondary electrons will escape the area of interest before depositing their energy. They will be counted in kerma, but not absorbed dose. In general, tissue does are ~10% higher than air kerma Kerma: SI unit is the Gy

Back

What MRI artifact is commonly seen at high-contrast interfaces and how do you improve it?

Front

Truncation/Gibbs: ripples in k-space data, especially at abrupt tissue intensity changes- produce appearance of lines, classically seen at high-contrast interfaces (skull-brain, cord-CSF, meniscus/fluid). -CSF-cord location is classic- mimics syrinx. -Due to limited sampling of free induction decay. -Can be in frequency or phase encoding direction, but more commonly phase-encoding Improve it: more matrix -Decrease the bandwidth or decrease the pixel size (more PE steps, less FOV, more matrix) -Penalty: increased acquisition time and reduced per-pixel SNR.

Back

Magnetic susceptibility, and how to improve it

Front

Susceptibility = ability of substance to be magnetized by the external field. -Worse with GRE than SE (because of 180 degree refocusing pulse to lose T2* effects) -Worse on in phase than out of phase. This is because in phase is performed later, and the longer the TE, the more susceptibility. Making it better: use SE and FSE instead of GRE -Swap phase and frequency -Use wider receiver bandwidth -Align the longitudinal axis of the metal implant with the axis of the main field -Use STIR for fat-suppression

Back

Oxygen enhancement ratio

Front

OER: relative effectiveness of radiation to produce damage at different oxygen levels -Idea is that tissue is more sensitive to radiation in an oxygenated state. -OER ONLY REALLY MATTERS WITH LOW LET RADIATION -With high LET radiation, the OER is often 1 (biologic damage without oxygen = damage with oxygen).

Back

What type of sequence is used with delayed enhancement cardiac imaging?

Front

IR: used to null myocardium -Phase-sensitive IR (PSIR) or TI scout series used to choose correct time to use for inversion. -Choose the one with the darkest myocardium.

Back

DWI

Front

based on the movement of molecules - Brownian motion B0- little diffusion more t2W- poor mans T2 B000 lots of diffusion weighting - if it has signal it is because it has low apparent coefficient ADC:compares B0 with B1000 and maps it DARK LOW signal is true restriction the bright stuff is shine through

Back

Public exposure limits: infrequent, continuous, embryo/fetus, controlled areas, uncontrolled areas, genetically significant dose

Front

Infrequent = 5 mSv/year Continuous = 1 mSv/year Embryo/fetus: 5 mSv/year Embryo/fetus (post-declared pregnancy) = 0.5 mSv/month. Controlled areas = 50 mSv/year Uncontrolled areas = 5 mSv /year Genetically significant dose = 0.25 mSv or 1 Gy

Back

What is that noise

Front

gradient coils echo planar 140 db - 99db for patients with hearing protection - fetal osscicles +/-

Back

Timing with in/out of phase imaging

Front

Fat and water protons precess at different rates. A spoiled GRE is performed when protons are spinning with each other (~4.4 msec at 1.5T) and directly out of phase with each other (~2.2 msec at 1.5 T). Microscopic fat will drop on out of phase. Out of phase imaging at 2.2 msec MUST be done BEFORE in phase at 4.4 seconds If you compare the 6.6 second out of phase you will not be able to distinguish fatty liver from iron-filled liver 2.2 msec: fat is dark, iron is bright on out of phase 6.6 msec: both fat and iron are dark on out of phase.

Back

Relative biologic efficiency and relationship with LET

Front

RBE: relative capability of radiation with differing LETs to produce a different biologic reaction. RBE = dose of 250 kV x-rays / Dose in Gy of test radiation -Use dose of 250 kV x-rays required to produce the same effect. As LET increases, RBE will increase to a point -Above 100 keV/micrometer, RBE decreases with increasing LET because the maximum potential damage has already been done, and any additional increase in LET is wasted dose.

Back

Motion correction with breast MRI

Front

Breathing and cardiac motion would normally degrade the image on breast MRI -Corrected by running the phase-encoding direction side-to-side rather than front to back (like in body imaging). -Axial: phase encoding direction is left to right -Sagittal: breast is top to bottom for phase-encoding. fix chemical shift by increasing bandwidth

Back

What imaging sequence is the technique of choice for DWI?

Front

Echo planar imaging (EPI): Can be done with SE (90+180) or GRE (90 + a bunch of gradients) Fastest MRI acquisition method- 1 RF pulse used to acquire data for an image (AKA single shot) Works by rapidly turning the phase and frequency encoding gradients on and off- fast filling of k-space. Compared to GRE, EPI is more susceptible to magnetic susceptibility, gives better tissue contrast, and is faster. Artifacts linked to EPI: -Magnetic susceptibility- can be improved with segmented sequences instead of single shots -Ghosting: gradient imperfections mess with spatial encoding -Chemical shift (due to use of narrow readout bandwidth) DWI: base sequence uses either fast GRE or EPI

Back

What are the dose thresholds for infertility?

Front

Females: threshold is age-depenent -Close to puberty: ~10 Gy -Closer to menopause: ~2 Gy Males: -Temporary sterility: ~0.15-2.5 Gy -Permanent sterility: 5 Gy

Back

At what dose threshold (with what finding) must you hospitalize a patient?

Front

2-5 Gy, vomiting 1-2 hours after exposure, skin redness 8-15 hours after exposure Vomiting < 1 hour after exposure, skin redness 1-6 hours after exposure: from dose > 4 Gy Hospitalize and send to specialized radiation center.

Back

5 G line

Front

5 gauss line gauss exclusion line risk to implanted devices

Back

Cell survival curve: what is the shoulder, and what is the slope?

Front

Shoulder = repair shoulder, or quasithreshold -Repair mechanisms are keeping the cells alive. -Measure of sub-lethal damage -Only exists with LOW-LET radiation curves 1 / slope = D0 - describes the linear portion of the curve and the radiosensitivity of the cell population -The higher the Do the more radio-resistant the cell is. -Slope is the mean lethal dose Higher dose rate- creates small shoulder and steeper drop in curve Oxygen will also cause a steeper drop in curve (more pronounced with low LET).

Back

What artifact is generated when gradients are rapidly turned on and off?

Front

Eddy currents: can be located in the magnet, the cables, the wires, or even the patient -Looks like distortion (contraction or dilation of image) or shift/shear -Most severe with DWI pulses -Improved by optimizing the sequence of gradient pulses.

Back

What are the weekly QC tests for accredited MRI scanners?

Front

Performed by MR technologist Center frequency Table positioning Setup and scanning Geometric accuracy High contrast resolution (phantom) Low contrast resolution Artifact analysis Film QC Visual checklist

Back

Section 8

(6 cards)

Shewhart chart

Front

Shewhart chart = control chart "Process-behavior" type of chart to see if a process is under control (with stable variation) or not. Upper and lower controls chosen based on the system's definition of success and failure. Staying between the controls demonstrates a stable process. Numerator = samples of success Denominator = total opportunities

Back

Commonly used billing codes come from what entities?

Front

CPT (current procedural terminology: from AMA editorial panel. Gives "uniform" description of procedure. ICD-9: from WHO

Back

Type 1 vs type 2 error

Front

Type 1: false positive Type 2 : false negative Decreased by strong power (larger sample size)

Back

ACR appropriateness ratings

Front

1-3 = usually not appropriate 4-6 = might be appropriate 7-9 = usually appropriate Also a 1-6 relative radiation score for each test

Back

Environmental safety tours: when and by whom?

Front

JC requires 2 of these in a patient care area and 1 of these in a non-patient care area per year Safety champion often gives these Safety champion title should rotate among staff Safety officer trains the safety champion

Back

Effects of disease prevalence on sensitivity, specificity, PPV, and NPV

Front

No effect on sensitivity or specificity Higher Prevalence: higher PPV, lower NPV Lower prevalence: lower PPV, higher NPV

Back