Section 1
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Portable x-ray device often uses a stationary anode (doesn't rotate) which limits tube rating
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Section 1
(50 cards)
Portable x-ray device often uses a stationary anode (doesn't rotate) which limits tube rating
Binding force
attractive force between electrons and protons strength of this force is inversely proportional to the square of the distance between the nucleus and the electrons K shell electron has a larger binding force than electron in L or M shells direction of this force is pointed towards the nucleus
HVL depends on beam filtration
if the beam is filtered, higher HVL less filtration, lower HVL *average photon energy determines penetration capacity and increased HVL
10th HVL
thickness of the material that can attenuate an x-ray to 90%
Beam intensity
number of x-rays X energy
Because velocity is fixed, frequency and wavelength have an inverse relationship.
Quality
overall energy of the beam changes in average energy reflect changes in quality
Half value layer
amount of material required to attenuate an x-ray to 1/2 the original output the higher the average photon energy, the more penetrating it will be, the larger HVL is
Filtration diagram
HVL depends on anode material (Tungsten)
Electron stream Actual focal spot Apparent (effective) focal spot
apparent focal spot defines the amount of blur
Single phase generator
produces polyspectrum energy
K, L, M
K shell (inner most)- 2 electrons L shell- 8 electrons M shell- 18 electrons
-69.5 keV is the binding energy of K shell electron
X-ray focal spot
.6 and 1.2mm
Heel effect
bc x-rays on anode side must pass through greater thickness of anode, have reduction in intensity of x-rays heel effect is worse with: 1. smaller anode angles 2. closer object to source distance 3. larger field of view heel effect is better with: 1. large anode angle 2. larger object to source distance 3. smaller field of view mammo- thicker part of the chest wall aligned with the cathode chest x-ray- cathode side down ap thoracic spine xray- cathode side lower thoracic spine due to increasing soft tissues
What is the purpose of a focusing cup?
help the electron beam strike the target in an acceptable size
Line focus principal
small angle = small effective spot = better spatial resolution highly testable tradeoff: - angle is too small, then x-ray beam might not be large enough to expose a standard image receptor at a typical 40 inch source to image distance - steeper (smaller) angle = greater heel effect
Mammo focal spot
.3 and .1mm
The larger the atomic number Z, the more x-rays will be produced due to Bremsstrahlung
mA
ampere- describes the current or movement of electrons number of electrons per second moving from the filament to the target direct relationship quantity of x-rays produced by the tube depends on the number of electrons that flow from the filament kVp affects both the quality (ability to penetrate and energy) and the quantity (number of x-rays). mA and exposure time (mAs) affect only the quantity
At the same kVp, a single phase vs a triple phase generator will
at the same kVp: a single generator has less quantity and quality. a triple phase generator has more quantity and quality.
Anode
only 1% of the electrons that strike the target will result in the production of x-rays. rest of the energy is converted to heat. electrons on the target = heat
Characteristic x-ray
inner electron is forced out and filled by an outer electron exchange results in release of potential energy in form of a characteristic photon making the spiky thing on spectrum diagrams
Velocity =
velocity = frequency X wavelength
* of energy to remove an electron from an atom
15eV of energy to remove an electron from an atom
Mass number
number of protons + neutrons A
kVp vs keV
kVp describes the electron stream as a whole with the number selected being the peak keV describes a single individual electron within that stream
Atomic number
# protons Z
Max energy = max kVp
Wider spot "blooming"
high mA low kVp
The average energy is between 1/3 to 1/2 the max energy
x-ray tube picture
Quantity
total number of x-rays or area under the curve in a spectrum diagram
In nucs, low Z materials (plastic) shield beta emitters in order to minimize bremsstrahlung production
Bound particles have negative energy. The closer to the nucleus, the greater the negative potential energy.
Bremsstrahlung
radiative losses as electrons slow down and approach the nucleus they have 3 responses: 1. strike nucleus giving off max energy to x-ray 2. come close the nucleus and give of medium energy to the x-ray 3. travel distant from the nucleus and give off little energy to the x-ray most 80% of x-rays are produced this way larger the atomic number Z, more brehmsstrahlung
What is the purpose of the tube/glass enclosure/ envelope?
primary purpose is to maintain a vacuum allows the amount and speed of the electrons to be controlled independently without a vacuum the electrons accelerating towards the anode would collide with gas
mAs = quantity
direct relationship
Interactions at the target
bremsstrahlung and ionization cause x-rays and excitation does not
K shell binding energy of tungsten is
-69.5 keV
What is the purpose of a rotating anode?
spread the heat produced over a large surface area to prevent the anode from melting
Focal spot
area of x-ray production on the anode smaller anode better spatial resolution but can't disperse heat. address by: angling the anode and using a rotating anode.
HVL does not depend on mAs and does depend on kVp
Alpha particles
2 protons, 2 neutrons net charge is 2 heavy positive charge allows them to suck electrons trivia: can't travel far can't penetrate used for treatment, not imaging
Cathode
negative part of the tube / filament made of tungsten gets really hot allows enough energy to escape the metal and form a cloud of electrons
Energy =
Energy = frequency X constant "h"
mono-energetic beam has higher HVL than poly-energetic beam at the same kVp
Auger electrons
lighter elements emit auger electrons a secondary ionization occurs when the electron ejected form the aroma has enough kinetic energy to cause addition ionization events ejected electrons called delta rays
increase kVp by 15%, you decrease the mA by half and maintain the same x-ray density on film if you decrease the kVp by 15% you need double the mA to main the same density on film
Section 2
(50 cards)
Quantum mottle
x-ray beam not perfectly uniform and at low doses you notice it more
High energy, linear attenuation and half value layer
high energy, decreased linear attenuation, increased half value layer
a major determinant of spatial resolution of digital images is the pixel size and spacing (pixel pitch)
if you want to increase the x-ray exposure you can
double your mAs or increase the kVp by 15%
K edge
photo electric effect peaks at the binding energy for the inner shell electron select kVp between 65-90 when using barium (k edge 37 keV) and iodine (k edge 33 keV)
Increased filed size
increased field size = decreased collimation = decreased quantum noise = increased dose and scatter
Classical
aka coherent, thomson gets absorbed and re-emitted without a loss of energy, but it's easier to think about it just bouncing off and changing direction x-rays are low in energy (less than 10keV) do not cause ionization do not contribute to the image adds a small amount of dose to the patient
Modulation transfer function
tells you how good the system is so you can compare digital film systems or plain film systems
If given a choice to increase mA or kVp to decrease noise, you should choose?
increase mA if you go too high with the kVp
What if you want to lower the dose but maintain a constant exposure?
raise the kVp by 15% lower the mAs by 50% this works bc the higher kVp x-rays will penetrate more easily and deliver a lower dose
wider window width decreases contrast
DEXA
relies on transmission measurements at two different photon energies 2 methods: 1. using a filter that drops the k-edge in the middle of the spectrum- separating the curve into two peaks- or two different energies (around 40 and 70) 2. switch the tube voltage between low and high Dose for DEXA very low .001 mSv
spatial resolution
how close 2 lines can be to each other and still be resolved as separate quantified with "line pairs per mm"
Grid ratio
describes how dense the grid is ratio of the height of the lead to the distance between them higher the ratio, less scatter, better the contrast higher grid ratio, higher dose
What determines the amount of scatter?
kVp field of view thickness
High density, linear attenuation and half value layer
increased density, increased linear attenuation, decreased half value layer
pixel pitch
measurement from the center of one pixel to the next
Detective quantum efficiency
prediction of dose measure of efficiency of a detector converting x-ray energy into an image the higher the DQE, the lower the dose DQE is directly proportional to MTF DQE is inversely proportional to SNR DQE is better at low spatial resolution DQE of DR is 0.45 better than CR or plain film which is 0.25
center point (or midpoint) of the level is going to determine the brightness
imaging processing software is capable of producing an image with a specified level of contrast even when higher kVp
narrow widow width increases contrast
decreased pixel pitch = better spatial resolution
inverse square law
energy twice as far from the source is spread out over four times the area 1/4th the intensity
Compton scattering
occurs at high energies x-ray strikes an outer shell electron. electron is ejected. original x-ray is a scattered photon. causes 3 things: 1. frees an electron 2. ionizes an atom 3. deflects the incoming x-ray as a scattered photon compton contributes to dose to patient major source of occupational exposure compton is the dominant force contributing to scatter/fog does NOT depend on the Z of the atom dependent on density of the material
What if you want to visualize low contrast objects / tissue?
keep the kVp constant and increase the mAs
sharpness- ability of the x-ray system to define an edge resolution- ability of x-ray system to differentiate two close objects Modulation transfer function- relationship between sharpness and resolution
kVp still influences contrast, but digital systems have a much wider dynamic range
How do you reduce noise?
use a grid use an air gap increase the mA
More x-rays on the film = darker = more radiographic density
What increases scatter?
high kVp technique large field of view thick people
increased pixel density = better spatial resolution
Photoelectric interaction
dominates at lower energy levels relative to compton x-ray strikes an inner shell electron contributes to image contrast probability of photo electric effect is 1/Energy^3 probability of photo electric effect is proportional to the Z^3
Grid cut off
risk of using a grid you block so many photos that you cause a quantum mottle (noisy image) happens with grid is aligned incorrectly
Classical, Compton, PE chart
Linear attenuation
more attenuation occurs with dense object more attenuation occurs with higher Z material lower attenuation occurs with higher kVp higher attenuation occurs at K edge unlike mass attenuation, the linear attenuation of ice, water, and water vapor is different
Attenuation in tissue depends on 3 things:
1. effective atomic number in tissue 2. x-ray beam quality 3. tissue density
Bucky factor
mAs required with grid / mAs without the grid
Collimation reduces noise
to compensate for the decreased photons hitting the image receptor when you collimate (decreased signal to quantum noise ratio) - the mAs is increased
Geometric unsharpness: small focal spot = * blur closer source is to image = * blur closer the object is to the detector = * blur more magnification = * blur
small focal spot = less blur closer source is to image = more blur closer the object is to the detector = less blur more magnification = more blur
magnification =
source to detector distance / source to object distance
What is the primary factor influencing image contrast in digital systems? What is the primary factor influencing image contrast in film systems?
What is the primary factor influencing image contrast in digital systems? LUT What is the primary factor influencing image contrast in film systems? kVp
Mass attenuation
supposed to reflect the attenuation important point is mass attenuation of ice, water, and water vapor is the same
Noise
data that contributes nothing useful to the image
Grid
multiple thin metallic lead stripes with interspersed radiolucent spaces blocks off angle x-rays and allows the ones coming straight into contact the image receptor
Photoelectric effect is the fundamental method by which contrast is developed
Smaller spot "thinning"
high kVp
Bucky grids
moving grid wiggles back and forth rapidly, too fast to be seen if the grid motion fails and stops moving, you will end up with grid lines
Different names for classical
coherent, thomson
Higher kVp = smaller attenuation coefficients = less contrast
Section 3
(50 cards)
Digital mammo vs analog: -lower spatial resolution - less dose -can post process window and level afterwards, so fewer repeats -noise is fixed
Storage phosphor (CR)
uses a phosphor called barium fluorohalide x-ray causes an electron in the phosphor to change to a metastable state (one that it can hold for several days) to read it out the plate is exposed to a laser that sweeps back and forth red light of the laser liberates the trapped electrons and they return to their shells as they return to their shells, they release a blue-green light which is directed onto a photodetector which then amplifies the light signal and converts to an electronic signal amount of light detected is proportional to the intensity of the incident x-ray photo stimulated phosphorus have a wide detectable range, tolerating x-ray intensities 100 times higher and 100 times lower than the 5 micro Gy needed for an old school screen film plate is reset by forcible exposure to bright white light- which erases it. if you forget to do this, you will get ghosting artifacts.
What is the difference between direct and indirect?
indirect (scintillators) xray -> light -> charge direct (photoconductors) xray-> charge
Automatic exposure control
acts like a timer which decides when to terminate exposure works by using an ionization chamber, placed between the patient and the image receptor duration of the exposure is determined by the density and thickness of the area of the patient placed over the ionization chamber controls only the quantity of radiation reaching the image receptor- no effect on kVp must be re-calibrated if that receptor changes (you upgrade from film to a CR digital system)
K edge filtration in mammo
moly or rhodium filters are used to remove energies lower than 15 and higher than 20keV filters block both high and low photons to remove low energy photos which only give dose and high energy photos that don't help with contrast Rh/Rh used for dense breasts bc it is higher energy compared to Mo/Mo Mo anode with Rhodium filters used to produce an intermediate end spectra between Mo/Mo and Rh/Rh NEVER use an Rh target (21 kev) with a Mo filter (20 kev k edge) Mo anode can also be combined with an aluminum filter, for a harder beam to penetrate dense breasts some digital systems will use Tungsten / Rho and Tungsten / Silver- this creates a higher energy spectrum for increased penetration and lower dose. contrast is lost- but some post processing can bring it back.
kVp and Mas for CXR, abd, foot
PPV2
cases where biopsy was recommended Br4,5 benchmark 25.4%
Direct conversion vs indirect conversion
Grad ratio
height/width between grids
Ghosting
in digital imaging residual image from prior exposure burned into the detector this is why lead is not allowed on flat panel digital systems
a higher kVp can be used (adjusted by 15% rule) and a quality image still produced
Target anodes for mammo?
molybdenum or rhodium anode are used over tungsten bc they have lower energy combination of low kVp and low Z give molybdenum a high peak of characteristic x-ray with low bremsstrahlung around 18 keV (instead of 69.5 like tungsten)
Why lower energy in mammo?
photoelectric effect dominates at lower kVp photoelectric effect is directly proportional to atomic number cubed- Z ^3 and indirectly with energy 1/E^3
Types of digital detectors
storage phosphor (CR)- type of indirect flat panel detector (DR)- direct and indirect
Mag view in mammo
breast is moved away from the detector and closer to the source. typically the distance to the source is cut in half, which doubles the mag. and increases the dose used to evaluate Ca, so you need a smaller focal spot for better spatial resolution less mA about a quarter of conventional contact mamma longer exposure time (about triple that of conventional contact mammo)
Air gap
you don't use a grid for mag views, instead you have an air gap off axis scatter misses the target
Direct systems that avoid lateral dispersion of light have better spatial resolution
Increased pixel density = better spatial resolution Decreased pixel pitch (measurement from center of one pixel to the next) = better spatial resolution
Processor QC
daily
Direct flat panel detector
x-rays go through the amorphous selenium and electrons are released without being converted to light converted to a digital signal stored by TFT
Indirect flat panel detector
x-ray activates thallium doped cesium iodide scintillator emit light in response to absorbed x-ray light gets emitted then converted by a photodiode into an electric charge the electric charge is captured and transmitted by the thin film transistor (TFT) to the workstation
Compression mammo
reduced thickness = less scatter = lower kVp lower kVp = less scatter = improved contrast reduced thickness = less mAs needed = less dose breast doesn't move = less motion artifact breast smashed closer to bucky = less geometric magnification less motion and less geometric mag = improved spatial resolution
Darkroom cleanliness
daily
Beryllium window
diagnostic imaging uses pyrex glass. pyrex glass is not used in mammo bc it causes excess attenuation. mamma uses beryllium
Focal spot, mA, exposure time in mammo vs plain film
mamma uses smaller focal spots, lower mA, and longer exposure times
Film vs Digital
Digital- larger dynamic range + linear response to exposure - bc has larger dynamic range, you can adjust the brightness and contrast -bc you can adjust the contrast and brightness afterwards, you can increase the kVp by 15% and half your mA to decrease the dose Film- narrow dynamic range + curvilinear response to exposure
Bit depth
determines the number of shades of gray displayed on a computer monitor 2 to the x power greater the pixel bit depth, the more shades of gray available for image display and better contrast resolution
Spatial resolution for DR is superior to CR bc the pixel detector is built into the DR flat panel
Grid mammo
mammo uses a smaller grid ratio, 4-5. general x ray 6-16. bucky favor is 2 for mammo, 5 for general x ray. dose is increased with a grid
What is the optimal kVp for mammo?
20keV 25kVp
Lateral dispersion of light
problem with indirect methods that use phosphors light diffuses laterally after it leaves the site of conversion from an x-ray loss of spatial resolution, which get worse with increasing thickness of the crystal making the crystal thin has its own problems, bc your sensitivity for collecting the x-ray is going to drop off Gadolinium oxysulfide has more lateral dispersion than CsI columnar structure makes a thick crystal
Recall rate
5-7%
Plain film vs Digital summary
PPV1
positive screening cases any br0,3,4,5 benchmark is 4.4%
Digital has lower dose than film bc you can post process. Aka digital has higher dose efficiency.
Imaging newborns
do not use a grid lower the kVp- around 65 kVp lower mAs- around 2-4 mAs
Heel effect mammo
cathode goes near chest wall
Plain film has better spatial resolution than digital, bc they don't have the limitation of individual units (pixels)
Contact mammo vs mag view mammo chart
MQSA does not have specific line pair requirements for digital mammo
Mammo vs General Dx chart
Spatial resolution for flat panel detectors is limited to the DEL (detector element); smaller detector elements = better spatial resolution
Pixels gone bad
in digital imaging manifests as a square or a streak
This vs that direct vs indirect digital radiography
Indirect- cesium iodide scintillator (CSI) Direct- amorphous selenium photoconductor (PADS) both direct and indirect use a TFT read out array
Ways to reduce scatter (improve contrast)
collimate compress the part which reduces the thickness lower kVp grid/air gap- reduces the number of scatter photons that strike the film detector
PPV3
results of biopsy aka positive biopsy rate, biopsy yield of malignancy benchmark 31.0%
4cms of patient tissue will require them to double the mA
You need to lower energy to a nearly mono-energetic beam to enhance attenuation differences and increase spatial resolution to see microcalcs
Spatial resolution lp/mm screen film mammo digital mammo digital radiograph CT MRI
screen film mammo 15 lp/mm digital mammo 7 lp/mm digital radiograph 3 lp/mm CT 0.7 lp/mm MRI 0.3 lp/mm
Cancers / 1000 screened
3-8
Section 4
(50 cards)
Saturation artifact
dose increases to penetrate dense obj like metal and regions around it are bright
What improves spatial resolution? What is the main limiter?
Electronic Mag Quality of the tv display
Image intensifier basic set up
Image intensifier
Minification gain
reducing image size from the input phosphor to the output phosphor increase brightness
The ability of the image intensifier to increase brightness deteriorates with the age of the tube which requires more dose to produce the same output brightness. older I.I.= more dose Image intensifier is replaced when the conversion gain falls to 50%
Where is the ideal place to stand?
on the same side of the patient as the I.I. preferabel to position the C arm x-ray tube under the table
Darkroom fog
semi-annually
Flare or glare artifact
transition form heavy attenuation to minimal attenuation you can see bright white "glare" at the periphery near the decreased attenuation this is from an overproduction of x-rays in this thin area, to compensate for the nearby thick area image becomes brighter with transition to less attenuation
Phantom eval
weekly
Lag artifact
move the I.I. and ghosted image remains superimposed from the prior field
* is behind MQSA
FDA
Flux gain
high voltage between the photocathode and the output phosphor causes electrons to accelerate and yields a gain in energy accelerating electrons increases brightness
Vignetting artifact
edges are darker than the center
Places that do mamma have to be accredited and certified i.e. pay money every * years
3
Last image hold vs fluoro spot
S distortion
interference with earth's magnetic field on flow of electrons towards the I.I. seen large field of view adding mu metal improves the artifact bc it deflects the magnetic field
Best position of the I.I. and the x-ray tube?
x-ray tube source far away from patient I.I. close to the patient 1. decreases patient dose 2. decreases scatter to the operator 3. increase image sharpness by decreasing focal spot blur and magnification
MQSA requires a resolution of 12 line pairs per mm for screen film and manufacture specs for digital (7lp/mm)
Spatial resolution vs electronic mag
if the input field is cut in half, the size of the object being viewed is cut in half, but the output is still fixed. relative to the original setting, the object has undergone a 2X magnification, and a double in spatial resolution. the dose goes up bc when you halve the input field, the brightness decrease by 1/3, so to compensate the radiation at the input quadruples. more radiation = more dose
Automatic brightness control
device that optimizes image exposure in regular x-ray automatic exposure control systems primarily mess with mA (quantity) and less so the kVp (quality) for fluoro, ABC used during electronic mag
Viewbox conditions
weekly
Conversion gain
describes efficiency of an I.I. in changing incident x-rays into light
What is the general radiation change with each magnification?
increases dose by 1.4- 2.0 times you get less light out, the ABC turns up the juice to maintain the picture
KERMA
kinetic energy release per unit mass
Double the distance from the tube does what to dose?
decreases it by a factor of 4 (inverse square law)
Screen film contrast
semi-annually
2 diff types of image acquisition in fluoro:
1. image intesifier 2. flat panel detector
Phantom limit with and without a grid
3 mGy, measured with a GRID dose limit wihtout a grid is 1 mGy no actual regulation of wha ta human breast can endure
Collimators
pro: reduce dose pro: reduce scatter con: alter exposure
You have to have * documented hours of mammo education
60
Brightness gain is the combined effects of flux gain and magnification gain
Compression test
semi-annually
Kerma area product (KAP)
amount of kerma X cross sectional area of the beam total radiation incident on a patient measurement of total radiation dose in the exam
Air KERMA
estimation of how many photons are in a unit of air prior to the energy striking the skin
GU radiology set up
GU radiology set up with bladder closer to the receptor with source above the table instead of below the operator lens dose is higher with the source above the table
Geometric mag vs Electronic mag
Geometric mag: - mammo - move the obj closer to the source Electronic mag: - decrease the input field by half, the automatic exposure control will increase the juice to compensate for brightness
Regular Dx vs Fluoro
Dx higher mA and short exposure time Fluoro lower mA due to very long exposure time
Electronic mag increases the air KERMA and therefore the skin dose, bc you increased the juice. Electronic mag does not increase the KAP bc even though you increased the juice, you decreased the cross sectional area.
You have to do * months of formal training
3
* LP/mm in the anode-cathode direction
13
Pincushion distortion
bent lines at the periphery seen with large field of view
Max automatic exposure control Normal air kerma limit: High level control:
normal size: 87 mGy/min 10 R/min high level control/obese: 176 mGy/min 20 R/min in high level mode, you must have audible or visual alarms in addition the normal time alarm used in fluoro
The relationship between intrinsic receptor resolution and the focal spot size (R/F) will determine if geometric mag will increase or decrease spatial resolution R/F > 0.5 mag will reduce unsharpness R/F < 0.5 mag will increase unsharpness
Less magnification (small FOV) = magnified = less bright More minification (larger FOV) = less magnified = more bright
During the last 2 years you have to read * mammograms during a 6 month period under direct supervision
240
Both electronic and geometric mag increase the dose, but geometric increases it more. Focal spot blur also occurs with Geometric.
* LP/mm in the left right right direction
11
Repeat analysis
quarterly
breast phantom average breast
4.2cm 50% adipose 5-% glandular
Section 5
(50 cards)
Frame averaging
increases SNR (less mottle), but more susceptible to blur
DSA takes a single frame image and subtracts it from a post contrast image. it removes everything that is still and leaves stuff that's moving, like blood.
Additional high dose situations: later and oblique views
People always use a drop of 30 to 15 frames per second as an example bc that equals 1 dose reduction of 30% It is not a direct 1:1 thing 50% reduction in pulse rate = 30% reduction in dose
Dose area product / KAP
measures radiation dose to air in mGy X collimator area unit = mGy/cm *measure is independent of beam location as you move the beam away from the patient, the intensity decreases, but the beam spreads out more KAP is NOT dependent on location
Motion creates ghosting
Be careful how the question is worded. A lower frame rate will have more mA per individual pulse- but the overall mAs will be decreased relative to regular fluoro below 30 frames per second.
Flat panel detector system
new fluoro machine
increased kVp = less skin dose, slightly more organ dose. if you compensate for the increased kVp with a drop in mA then dose decreases. bc if you increase kVp by 15%, then you can decrease mA by 50%. for CT, if you increase kV = increased dose.
Pixel binning
binning increases pixel size reduced spatial resolution (but improves SNR)
Pitch and fill factor
Pitch- linear dimension of a detector element Fill factor- the portion of the area that's sensitive to light fill factor = sensitive area / pitch ^2
How is spatial resolution tested for?
lead bar pattern
When does pulse fluoro reduce dose?
if you drop the rate below 30 pulse/second
II systems are limited by?
TV systems (1.0-2.0 lp/mm for GI and 2.0-4.0 lp/mm for angio)
If you measure spatial resolution at a diagonal (45 degrees to the raster lines) does it get better or worse?
better. spatial resolution improves by a factor of square root of 2 over the vertical or horizontal resolution
Pulsed fluoro is good for moving patients (wiggling babies)- gives you sharper images with less motion blur
Barium study kVp
because the amount of barium you give is higher than the amount of iodine, you have to penetrate it. and pick a higher kV. pick a kV of 100+ k-edge of barium is 37 keV
FPD systems are limited by?
detector element size (2.5-3.0 lp/mm)
total dose for IR = dose per frame rate X frame rate X duration X number of runs
A fatter patient gets more skin dose, because the automatic brightness control sees less penetration- then cranks up the dose.
Grids in IR are not used for extremities or peds
Regular vs pulsed fluoro
Regular- continuous low mA with higher S = mAs Pulsed- pulsed high mA with lower S= mAs regular fluoro mAs = pulsed fluoro mAs
What limits the spatial resolution of an I.I.?
television display system resolution of TV depends on the raster scan lines, the bandwidth, and the FOV
Interventional reference point
measures radiation emitted from the source likely underestimates patient skin dose
kVp for a low volume iodinated study = 70kV. chosen to max out K-Edge. kVp for a high volume barium study = >100kV. chosen to max out penetration.
Patient and operator dose doubles with a lateral view compared to a PA
Are artifacts of I.I. the same as flat panel detectors?
No, flat panel detectors do NOT have pincushion, S distortion, vignette, glare, or saturation
Bad pixel
appear as white or black spots on flat panel detectors interpolating to fill data corrects for this artifact
Dose spreading
change angle of gantry to spread skin dose over a broad area
Regulatory dose with no high level control present
10R/min 87 mGy/min
How is distortion checked for?
mesh screen or plate look for straight lines not pincushion or S distortion
Pulsed fluoro
dose is administered in pulses, instead of continuously. you get less motion artifact (better spatial resolution in moving objects) and overall less dose (if less than 30 frames per second)
Pulsed fluoro can reduce dose when the frame rate is below 30 frame/second
50% of the dose is delivered in the superficial 3-5cm of skin/fat. the depth of the 50% depends on the kVp and filtration (higher kVP + copper filtration = more penetration)
Dynamic range
only an issue for I.I. with variability n very dense or transparent stuff
Does the display limit the spatial resolution of a flat panel detector system?
no, FPD usually have displays with the same matrix as the image receptor
Vertical resolution is limited by?
number of raster lines that the display monitor uses must alternate white and black to reproduce a line pair image on the I.I., so the max number of line pairs in the vertical direction is half the number of lines used. you use the Kell factor to correct for the lines not matching up. vertical resolution = raster lines X kell factor / 2 x FOV in mm decreasing the FOV improves the spatial resolution
Smaller FOV, better spatial resolution
Better pure spatial resolution FPD vs II?
II systems are better and change with FOV
Iodine study kVp
pick a kV of 70 places the average energy around 35 k-edge of iodine is 33 keV
If the matrix size is 1100 X 1100 and the FOV is 25cm, wha tis the pixel size of the detector element?
pixel = FOV/matrix needs to be in mm: 25 cm = 250mm 250mm/1100 = 0.23mm pixel size
Dose outside of lead, standing 1 meter away form the patient is 1/1000 of the dose received by the patient you get 0.1% of what the patient gets at 1 meter
Best place to stand
stand on the image receptor side of the patient you are trying to avoid the large amount of compton scatter radiation produced where the beam enters the patient
Binning
taking several detector elements and making a large detector element *binning improves SNR
typical dose is 0.3-0.5 mGy per gram at the retrace skin position (10x-20x more per image than fluoro)
Digital subtraction angiography (DSA) is performed at a kVp of 70 to exploit the k-edge of iodine
*Magnification will increase air kerma, but not KAP
Source to skin distance
if you are using under the table positioning the SSD depends on the table height small SSD = high dose short angiographers stand on a platform so the source to image receptor distance is kept at 100cm or more.
Most IR systems use a pulsed fluoro to help reduce dose
Lag artifact (ghosting)
occurs with both I.I. and flat panel detector occurs if the exposure uses high radiation
Section 6
(50 cards)
Increase beam width: reduced scan time reduced motion artifact increases partial volume mAs is unchanged, bc a larger area of tissue is scanned
What is the relationship between HU and x-ray attenuation?
when HU increases by 10 HU, x-ray attenuation increases by 1%
Regular Dx vs CT chart
Filtered back projection
sharpening of the projection data prior to back projection
Does changing the keV mess with the HU? Why?
Yes since PE dominates at a lower energy, low keV will create a higher HU
conceptus dose limit is * mSv per month
0.5 mSv per month for conceptus dose limit
Helical scanning
tables moves at a constant speed, tube is on the entire time. advantages: primary advantage = faster secondary advantage= post acquisition flexibility
Nitroglycerin
0.8-1.2 mg glycerole trinitrate 5mg isosorbide dinitrate dilates coronary arteries contraindications: aortic stenosis hypertrophic cardiomyopathy phosphodiesterase inhibitor like sildenafil for 48 hours prior to exam
is mAs the same on CT as plain film?
no CT uses effective mAs which is tube current mA x exposure time
How do you calculate pixel size?
field of view / matrix
How does dual energy CT work?
HU of each pixel obtained at 140 and 80 kVp scan you can do a virtual non-con find out what a stone is made of
Hounsfield unit equation
HU = 1000 X (attenuation of material - attenuation of water) / attenuation of water
Increased kVp = image noise will decrease
Window, Level example
Regulatory dose limit is * mSv
50 mSv regulatory dose
Regulatory dose with high level control on
20R/min 176 mGy/min an audible alarm must be used
Window, Level HU Brian Lung Abd Bone
*Minimal slice thickness is determined by detector element aperture width in a modern CT
if you turn down the mAs what happens to your images?
less mAs = more noise
Secondary ulceration
24 Gy
Axial scanning
tube comes on and takes a picture, tube shuts off, and the table moves advantages: better spatial resolution in the z-dimension since full image sets are taken less partial volume artifact
Temporary epilation (hair loss)
3 Gy
sinogram
represents the data from all the projections of all tube angles
Bow tie filters
compensate for uneven filtration reduce scatter and dose made of teflon, low Z material, to reduce hardening
B blocker
metoprolol (lopressor) 2.5-5.0mg IV lower heart rate to less than 65 bmp for prospective ECG contraindication: <60 HR SBP <100 cardiac failure asthma, COPD active bronchospasm 2nd or 3rd degree AV block antidotes: fluids atropine 0.5mg IV glucagon 50 micrograms/kg IV loading dose followed by continuous infusion 1-15 mg/h
Pitch
table movement / beam width 1 = no overlap >1 = table moved fast creates gap less dose worse spatial resolution <1= table moves slow creates overlap more dose better spatial resolution
Smooth kernel
decreased noise decreased spatial resolution soft tissue kernel
in 1 year, you get * mSv
5 mSv typical dose in 1 year
most CT scanners are 3rd generation
meaning the x-ray tube and the detectors spin around the patient in synchrony
How do you improve spatial resolution?
pixels smaller matrix larger
What is the relationship between pixel width and height to voxels?
voxel has a 3rd dimension (depth) which represents the slice thickness. voxel is a cute, pixel is a square.
Moist desquamation /ulceration
18 Gy
What kind of x-xrays are used with CT?
highly filtered, high kV (average energy 75 keV)
Advise patient for burns, esp 10 days post procedure
2-5 Gy
Telangiectasia
10 Gy
Raw data -> projection back projection filtered back projection iterative reconstruction
back projection- old way filter back projection- modern way, math filter applied iterative reconstruction- corrects for noise so you can use lower dose
Chronic erythema / main erythema
6 Gy
Unlike conventional radiography with automatic exposure control, increasing kVp in CT will increase radiation dose to the patient
Dry desquamation
13 Gy
No action is needed below
2Gy
CT has very high tube currents up to 1000mA
Sharp kernel
increased noise increased spatial resolution bone kernel
Cardiac CT prospective retrospective
performed during diastole prospective: -step and shoot, R-R interval -pro: reduced radiation -con: non-functional imaging, susceptible motion artifact -trivia: always axial, not helical. retrospective: -scans the whole time and then back calculates -pro: functional imaging with contraction and wall motion -con: higher radiation, low pitch, increased dose
Will filtration change the HU?
Yes filtration will remove low energy photons, that increase the average energy and lower HUs seen with cupping artifact
Early transient erythema
2 Gy
Permanent epilation
7 Gy
Collimator is used at the x-ray tube and detector to shape the x-ray beam which defines the thickness of a slice and reduces scatter
Procedure and dose should be reviewed by physics
Above 5 Gy
What is the matrix size for CT? Each pixel is?
Matrix is 512 x 512 each pixel represents 4096 possible shades of gray 12 bits 2^12 = 4096
Anti scatter septa
grids to prevent scatter in CT
Section 7
(50 cards)
Patient motion
to fix: tie the patient down modern fast scanner *align the scanner in the primary direction of motion *over scanning an extra 10% on the 360 rotation, with the repeated portion average.
Beam hardening
lower energy photons removed leaving a harder beam with an increased average energy ex. cupping and dark bands/streak to fix: filtration to remove the lower energy components or adding a bow tie filter calibration correction using a phantom to allow the detector to compensate for hardening effects correction software tilting the gantry to avoid areas causing hardening
Photon starvation
high attenuation areas like the shoulders result in streaking fixing it: 1. automatic tube current modulation- increase the dose in the area of greater attenuation 2. adaptive filtration performed to correct the attention profile and smooth the data
DLP
CTDI vol X length of the scan in cm
The thinner the detector element aperture, the better the spatial resolution in the Z direction
Phantom size
body phantom is 32cm if the patient is larger than the phantom, then dose is over estimated if the patient is smaller than the phantom, the dose is underestimated
Effective dose
k X DLP k is a constant effective dose will be in Sv
Dark bands/ streak
dark bands / streaks between two dense objects x-rays that pass through one are less attenuated than those that pass through both
Embryo dose in a CT abd and pelvis is
30 mGy
CTDI vol head
75 mGy
Incomplete projection
if parts of the patient are hanging outside the field but attenuating x-rays messes with computer's math ex. arms hanging down to fix: position pt correctly
Detector aperture size
as the detector size is reduced the cranial-caudal resolution (z-axis increases) the in-plane x-y axis is NOT affected by aperture size
Number of projects, more data, better spatial resolution
Pediatric considerations
reducing mAs works to reduce dose kVp can also be reduced. 70 kVp is good for peds. reduced techniques are possible bc x-ray penetration is greater in child dose reduction in head CT are more modest than peds belly if the patient is thin, reducing the kV will improve the contrast resolution bc of less compton scatter
Fat person
increase mA increasing rotation time will result in more photons, but dose and movement will also increase. lower the pitch increasing kVp
Holding matrix size constant, decreasing FOV, decreases pixel size. This increases spatial resolution but decreases contrast resolution (less photons per box)
CT angio study = kVp of 100 (120 in a fat person)
Increase signal to noise
higher mA longer rotation time higher kVp larger slice thickness large pixel decreased pitch
Over ranging
with helical scanning you need 1/2 rotation beyond the planned scan length so you can create the first and last image only happens with helical/spiral scans, not axial. higher the pitch, more over ranging over ranging contributes more to the DLP for short scans like pedi and cardiac ct increases with increasing number of slices acquired simultaenously
modulating mA based on the density of the scout
Holding matrix size constant and increasing FOV will increase pixel size. This decreases spatial resolution but increases contrast resolution (more photons per box).
Strategies to reduce breast dose
decrease mA use mA modulation based on density shield breasts with bismuth can degrade image and cause him hardening falsely elevating the HU
Over beaming
radiation extends beyond the active detector area and not used for imaging decreases with increasing number of slices acquired simultaneously
Dose of 1 chest CT is equal to * PA + Lateral CXR
100
Reference dose set by ACR at 75 percentile, doses above that should be investigated and reduced if possible
CTDI vol adult abd
25 mGy for adult abd
Ring artifact
calibration error or defective detector on 3rd generation scanner fixing: recalibrate detector or replace
Partial volume
2 types: 1. partial volume effect- dense object protrudes into the width of an x-ray beam resulting in divergence of the beam manifesting as shading artifacts 2. CT voxels are cubes. if one dense thing takes up half the cube and a low attenuating thing takes up the other half, the machine will average them together and make something of intermediate density. fixing it: thinner slices
Individual dose monitoring is mandated if the occupational dose is greater than 10% of the annual dose limit 500 mrem
Volume CTDI
weighted CTDI / pitch
Body scan vs head scan variation with scan plane
body scan- surface is about twice the central dose head scan- central and surface are very similar
Undersampling
insufficient number of projections used to reconstruct the CT results in misregistration artifacts and poor quality 2 types: 1. view aliasing- under sampling between 2 projections. fine stripes radiating from the edge of a dense object. fixed by slowing the rotation and acquiring the largest number of projections per rotation. 2. ray aliasing- under sampling within a projection you see strips close to the structure. fixed by using specialized high resolution techniques.
increased mA = improved signal to noise increased mA = improved signal to noise but not 1:1. you need 4x the mA to double the SNR. increased mA= decreased quantum mottle
Smooth filter
poor spatial resolution, less noise
Risk of radiation induce cancer per dose
5% per Sv adult 15% per Sv for child 1/10th for someone older than 50
Sharp filter
better spatial resolution, more noise
Signal changes directly with increased x-ray flux. Twice the x-rays, twice the signal. Noise changes by a factor of the square root of N. In other words, twice the x-rays square root of 2 or 1.4 the noise.
Weighted CTDI
1/3 central CTDI + 2/3 peripheral CTDI mGy
Focal spot (smaller = better): determines spatial resolution in the x-y plane, side to side. Detector size (smaller = better): determines spatial resolution in the z plane, cranio-caudal direction, long axis.
Metal artifact
metals with high Z (iron, platinum) have more artifact than those with lower Z (titanium) fix: *increase kVp, *thinner slices, software
in conventional radiography and CT, HVL of soft tissue is 3cm
Decrease kv, * HU
decrease kv, increase HU
Helical artifact in axial plane: single section
artifact from the helical interpolation *main place seen is at the top of the skull (anatomy changing rapidly in the Z direction) higher the pitch, the worse this is to fix: reduce variation in the z direction use a low pitch use 180 degree instead of 360 when possible use thin sections *this is why head CTs are done with axial over helical imaging
CTDI vol peds abd
20 mGy for peds abd (5 year old)
CT of extremities has a low effective dose <1mSv bc they don't contain radiosensitive organs
CT vs plain film with spatial resolution and contrast resolution
Contrast resolution- CT superior Spatial resolution- plain film is superior
Factors that affect spatial resolution and contrast resolution chart
CT contrast resolution (ability to discriminate between structures) is excellent but spatial resolution is poor.
Cupping
center of the image appears darker type of beam hardening x-rays pasting through the middle of a uniform shape like a head are hardened more than those traveling through the shorter path periphery
If you increase the kVp you will increase your CTDIw on both a head and body phantom
Section 8
(50 cards)
Spatial pulse length
number of cycles emitted X wavelength *objects closer than 1/2 spatial pulse length will NOT be resolved
Refraction
bending of sound wave caused by a change in speed what influences refraction? 1. speed change based on tissue compression 2. angle of incidence as sound passes from tissue to fluid there is a change in speed bc of the different compressive abilities of the tissues results in bending of the beam. se this along the edges of fluid filled structures like the GB.
Axial resolution
tell 2 closely spaced objects in the direction of the beam apart minimum required separation between the 2 electors is 1/2 the spatial pulse length *objects closer than 1/2 spatial pulse length will NOT be resolved
Curvilinear probes
type of linear probe that operates with individual elements firing on their own face is curved so that scan lines diverge deeper into the image giving a wider field of view for deeper structures used for abdominal imaging
What determines the strength of echoes?
angle and impedance
Tissue thickness that reduces the US intensity by 3dB is considered the half value thickness
Focusing
try and converge / narrow the ultrasound beam, again with goal of improving lateral resolution
Higher frequency = more scatter
Speed = wavelength X frequency
Is the axial resolution constant at different depths?
yes has nothing to do with depth, just spatial pulse length
Low damping (high Q)
narrow bandwidth *for doppler to preserve velocity info
Relative intensity
dB change of 10 in dB scale corresponds to 2 orders of magnet 100 times dB is based on a log 10 scale reducing the sound intensity to 10% is 10 dB reducing to 1% is 20 dB reducing to .1% is 30 dB
Will increasing the transmit gain (power) help with lateral resolution?
no transmit gain widens the beam you want a nice narrow beam use minimal transmit gain
Heavy damping (low Q)
broad bandwidth *high spatial (axial) resolution fewer interference effects and more uniformity
Linear probes (sequenced)
each element fire and receives on their own width of transducer = width of the sum of ht individual elements linear arrays are good for peds, superficial carotids, leg veins, testicles, thyroids
Operating frequency is determined from the speed of sound and the thickness of the piezoelectric material. Only way to change frequency is to change the probe. Wavelength changes to accommodate changing velocity in different material.
Phased probes
hive mind firing in multiples using interference patterns to steer the beam probes are made smaller good for limited acoustic windows in between ribs, transvaginal
Dampening block
sits behind the piezoelectric crystal and absorbs backward directed energy
Piezoelectric material
functional component of the transducer made of lead-zinc-titanate (PZT) when compressed the dipoles get disturbed which is measured and recorded
Speed is constant in a particular media, so that an increase in frequency decreases the wavelength and vice versa
Total/complete reflection can occur if the speed difference and the angle of incidence exceeds the critical angle
Impedance (Z)
density X speed of sound
Speed effect on frequency
none, frequency is the same in various media *wavelength changes in media
More or less scatter with high frequency probes?
More- smaller wavelengths make surface look rougher (non-specular) causes scatter
Frequency
rate of change between compression and rarefaction Hertz number of times a wave oscillates through a cycle each second
Near field
Fresnel beam converges higher transducer frequency = longer near field larger diameter element = longer near field
Stair step artifact
stair step on edges of multi planar reformatted image occurs when you have a wide collimation of non-overlpaiing intervals less severe with a helical scanner where you get some overlap to fix: thin slices
Focal depth zone
narrowest beam maximum intensity spot between converging and diverging beams get you best echoes and lateral resolution here
loss of 3dB represents a *% loss of signal intensity
loss of 3dB represents a 50% loss of signal intensity
Helical artifact in multi-section
classic windmill appearance where several rows of detectors intersect worsens with increased helical pitch to fix: z filter reduces severity of windmill artifacts
Zebra artifact
reformatted artifact secondary to helical interpolation with increased noise along the z axis manifests as zebra stripes and pronounced on 3D imaging most significant away form axis of rotation- nose is worst off axis
How does the machine know what speed is?
it doesn't justa assumes everything is always 1540 m/s causes artifacts
What is impedance?
degree of stiffness in a tissue determines the strength of surface reflection
Optimal matching layer thickness is equal to 1/4 the wavelength
Stand off pad
block of ultrasound gel or low impedance material that you can scan through works by moving superficial things into the focal zone
What ist he unit used for impedance?
rayl
Is the lateral resolution constant at different depths?
no the lateral resolution worsens in the deeper field
Reflection
differences in acoustic impedance of 2 neighboring tissues results in reflection large difference in impedance results in a large reflection -why gel must be used between transducer and skin to eliminate air pockets
The thickness of the transducer is equal to 1/2 the wavelength. Lower frequency is seen with thicker crystals and higher frequency is seen with thinner crystals.
More or less attenuation with high frequency probes?
More
You will get a big reflection if?
there is a large difference in impedance ex. thats why you need gel between skin and air
As the frequency increases, the HVT decreases you use high frequency probe for superficial stuff
Wavelength
distance between areas of compression
0.5dB per cm per MHz or 0.5 (dB/cm)/MHz
Is the speed of sound constant in tissue?
No changes via wavelength depending on compressibility of the tissue slow in air fast in bones
Matching layer
minimizes the acoustic impedance differences between the transducer and the patient
Speed in different materials
based on compressibility air- very compressible- low speed bone- not compressible- fast speed *ultrasound machine is stupid and assumes everything travels at 1540 m/s in tissue the fact that the machine thinks speed is constant causes artifacts
Far field
Fraunhofer beam diverges
What makes the sound have bend?
changes in the speed of sound occur as it travels through different media creating bending aka refraction as described by snell's law
Lateral resolution
resolving objects perpendicular to the beam the thinner the beam, the more likely each will fall into a separate beam best at the focal zone
Section 9
(50 cards)
Refraction artifact
speed difference in tissues cause refraction resulting in 1. object appears wider than it actually is 2. object can be misplaced to the side of the returning echo 3. object can appear duplicated classic look: "duplicated SMA" occurs secondary to symmetry of geometry
Power doppler
very sensitive look for presence of flow without info on direction
Mirror image artifact
US beam passes through a highly reflective surface and gets repeatedly reflected liver lung interface
Time gain compensation
makes uniform brightness from top to bottom works by compensating for loss of echo strength caused by depth of the reflector treats echoes differently depending on which depth they are returning from you could manually manipulate it by wiggling buttons around (now automated with a button push)
Focal zone
center your focal zone tightly over the object
B mode
B = brightness conversion of A line info to brightness modulated dots
Flash artifact
burst of color filling the screen secondary to transducer or patient motion classic ex. is fetal kick
A mode
A = amplitude only used by ophtho for eye measurements
Summary of lateral resolution: -best at the focal zone -changes with depth -best with a narrow pulse beam width -best with higher frequency -best with higher scan line density -large diabetes transducers have longer focal zones and better lateral resolution
Ring down artifact
fluid trapped between tetrahedron of air bubbles parallel bands extending *posterior to a collection of gas
*Higher frequency = more narrow the beam = better lateral resolution
Shadowing
material attenuates US beam more than the surrounding tissue the beam distal to this is weaker (darker) than the surrounding field
Output power vs. Receiver gain
Output power: -increases brightness by adjusting the sound pulse SENT TO THE BODY -degrades lateral resolution Receiver gain: -increases brightness by adjusting the sound pulse AFTER IT RETURNS to the transducer ALARA favors receiver gain
Increased through transmission
material attenuates sound less than the surround tissue and the strength of the beam distal is stronger (brighter) than the surrounding field
Improving axial, lateral, elevation resolution
Higher scan line density = better lateral resolution
Color bleed
looks like color extending beyond the vessel wall decreases sensitivity to thrombus or stenosis to fix: decrease color gain
Stable cavitation
micro bubbles present in the media expand and contract
Aliasing
high velocities are displayed as negative occurs when the doppler shift is greater than Nyquist limit Nyquist limit (kHz) = 1/2 X pulse repetition frequency to fix: -decrease doppler shift by using a low frequency transducer or using a doppler angle closer to 90 (increasing the angle) -increasing pulse repetition frequency (which will increase your Nyquist) or selecting a sample volume at a lesser depth or increasing the scale
Power doppler is extremely sensitive to flow, even slow flow
Twinkle artifact
occurs behind strongly reflecting surfaces like calcifications manifests as noisy spectrum with rapid fluctuation of red and blue colors this artifact has greater sensitivity for detection of small stones than acoustic shadowing this is highly dependent on machine settings and how round the reflecting surface is (more rough = more twinkle)
Power doppler does NOT exhibit aliasing (both color and spectral can)
US Assumptions: -All echoes originate within the main beam -All echoes return to the transducer after a single 180 degree reflection -Amt of time an echo takes to return to the probe depends directly on the object depth -Speed of sound in human tissue is constant 1540 m/s -Sound beam and its echo travel in a straight path -Acoustic energy is uniformly attenuated
Speed displacement artifact
speed of sound slows down in fat, relative to liver the beam takes longer to return in fat and is perceived by the machine as being further away creating an appearance of discontinuous and focally displaced liver border
You can't place the probe perpendicular to the vessel, bc gives you false impression of no flow
Harmonics
involves transmitting at one frequency and receiving at another this is possible bc body tissues distort the wave wave has to travel some distance to become distorted enough to generate harmonics harmonics are NOT produced in the near field (they haven't traveled far enough) process IMPROVES lateral resolution reduction in artifacts- specifically reverberation because you are dealing with higher frequency, you do lose some depth penetration (rmr high frequency attenuated faster) makes a solid look like a cyst by turning it anechoic
Doppler angle is not as important with color doppler since the information is semi quantitative
Artifacts from multiple echoes chart: reverberation comet tail ring down mirror image
Elevation resolution (slice thickness)
similar to lateral resolution, but in the orthogonal plane worst measure of resolution depends on height of transducer element source of volume average
Compound imaging
image an object in multiple different directions with electronic strong of the ultrasound beam will sharpen edges loss of posterior shadowing makes a cyst look solid by removing posterior features
Thermal index
max temp. rise in tissue secondary to energy absorption
Axial, lateral, elevation resolution dependent on
Most intensity = spectral pulsed doppler mode
Reverberation artifact
sound wave hits two parallel highly reflective surfaces multiple equidistant spaced linear reflections
Mirror image
occurs secondary to a vessel adjacent to a highly reflective surface, such as lung. results in duplication of the structure being evaluated classic locations are sub diaphragmatic region of the liver and supraclavicular region
Spatial resolution of color doppler is less compared to gray scale imaging (although smaller vessels are better seen)
Power doppler has NO dependence on doppler angle- you can measure doppler totally perpendicular to the vessel
Tissue vibration
secondary to turbulent flow doppler shows mixture of red and blue classic ex. AV fistula in a kidney post biopsy
Why must the doppler angle be more than 30?
angles less than 20 cause refraction and loss of signal aliasing becomes an issue
M mode
M = motion B mode information used to display echoes from a moving organ like heart valves
Doppler angle
should be between 30-60 cos 90 = 0, then the numerator would be 0
Pseudoflow artifact
things that move like blood but are not classic example: ureteral jets
Beam width artifact
US beam exists the transducer with the same width as the transducer. it then narrows to the focal zone and diverges in the far field. if the divergent beam encounters a strong reflector, it sends signal back that is assumed to be from the main beam. classic location: bladder with peripheral echoes improved with: 1. adjusting the focal zone to the level of interest 2. placing the transducer at the center of the image
Side lobe artifact
off axis / side beam hitting a strong reflector incorrectly assumed as coming from the main beam *happens more in linear arrays seen when incorrectly placed echoes overall an anechoic structure bladder, "pseudo sludge" GB
Comet tail artifact
form of reverberation 2 parallel highly reflective surfaces closer together with the sequential echoes closely spaced space between them may be less than 1/2 the spatial pulse length and as a result the displayed echoes will look like a triangle
Pulsed wave (spectral) doppler
spectrum of doppler shifts instead of a single frequency obtains the direction of blood flow
Mechanical index
how likely cavitation will occur matters the most with contrast enhanced US
Color doppler
direction of flow with colors red and blue color intensity varies depending on the flow intensity obtains samples of each pixel multiple times and displays average shift
Harmonics vs compound imaging
compound- makes a cyst look like a solid by removing posterior features harmonics- makes a solid look like a cyst by making it anechoic
What is the relationship between transducer width and lateral spatial resolution?
Longer transducer = longer focal zone length = longer near field
Section 10
(50 cards)
Internal conversion
energy emitted from gamma emission is instead transferred and expels an inner shell electron with a downward cascade of electrons form higher levels this results in emission of an x-ray or auger electron *bad bc you are emitting particles which case harm and are not imageable gamma photons
Spectral doppler deposits more heat compared to gray scale ultrasound
Scintillation crystal
sodium iodine doped with thallium when struck with a photon will produce a pulse of light thick crystal = better sensitivity, worse resolution thin crystal = better resolution, worse sensitivity thin crystals allow the the photomultiplier tube to sit closer and improve resolution but don't catch as many photons as a thick crystal
Band of stability
Metastable state
in most cases the transition from an isobaric to isometric transition is fast, but sometimes you enter an intermediate "metastable" state
Pinhole collimator
magnifies and inverts used for thyroid and small parts magnification: f = b, no mag f > b, mag f < b, gets smaller if you move the pinhole far enough way, the image will get smaller
Converging hole collimator
magnifies without inverting
Bombardment
you can create tracers with bombardment in a nuclear reactor or a cyclotron a cyclotron is helpful bc of transmutation where you don't need to clean up the parent element
Alpha decay
occurs in heavier unstable atoms helium nuclei (2 protons, 2 neutrons) slow, fat, and worthless for imaging can be used for treatment with bone pain in cancer mets via Radium 223
QA on dose calibrator (ionizing chamber)
Consistency -should be within 5% of computed activity -performed daily Linearity -performed quarterly Accuracy -performed at installation and annually Geometry -performed at installation and moving the device
Pulse height analyzer
removes background scatter and only evaluates tracer you're looking at
How long do you keep a radioactive material?
10 half lives
Ionizing chamber
used when higher doses are expected no dead time problem detects exposure from 0.1 to 100R/h
Thyroid probe
modified version of the NaI well counter primary use to calculate thyroid uptake dose is compared to a calibrated capsule of the same radionuclide
Thermal induced damage is a threshold phenomenon (you get no tissue damage until a certain temperature is reached)
Sodium iodide well counter
same thing as a gamma camera hole in a block of NaI crystal in which the sample is placed can be overwhelmed if the sample exceeds 5000 counts per second, it will be under reported good for in-vitro blood or urine samples good for wipe tests samples
Diverging collimator
minimizes object
Ring badge
worn on dominant hand, index finger, label inwards towards the source/palm, under a glove to avoid contamination
Star artifact
intense energy results in septal penetration of the hexagonal collimator holes seen in the thyroid bed after high therapeutic dose (using a medium ene. collimator instead of high)
You don't wear lead in nucs bc it won't stop the gamma rays
Half life
amount of time needed for a radionuclide to be reduced into half of it's existing activity physical half life biologic half life- how long it takes to pee or poo half life effective half life- takes both physiologic and biologic half life into account 1/effective = 1/physical + 1/biologic
Center of rotation
performed monthly ensures gamma cameras used for SPECT are centered correctly 5 small 99mTc point sources along the axis of rotation axis should be straight
whatever gives more counts = more sensitivity.
Photomultiplier tube
convert light to electric signal record location on an x-y axis that goes to a computer records Z signal intensity which goes to the pulse height analyzer
Electron capture
captures an electron from the k shell isobaric- decreased atomic number, but mass does not change excellent for imaging bc it is coupled to isomeric transition results in emission of gamma photons
Sensitive tissues on fetus: embryo less than 8 weeks after conception head, brain, spine any fetus eyeball any age
Parallel hole collimator chart -septa length -hole diameter -septa thickness
sensitivity and resolution have an inverse relationship more counts, better sensitivity but degrades resolution distance affects resolution, NOT sensitivity short septa, better sensitivity long septa, better resolution wide holes, better sensitivity narrow holes, better resolution high energy = long thick septa + wide hole low energy = short thin septa + narrow hole
Scanning maternal uterine arteries with doppler is ok as long s the fetus is outside the beam
Cavitation is most likely to occur with low frequency and high pressure
Isomeric transition
after undergoing an isobaric transition, there is often left over energy that then undergoes isometric transition
perform SPECT when improved spatial resolution is needed
Geiger muller counter
detects small amount of radioactive contamination gas filled chamber that beeps dead time- device is very sensitive but can be overloaded - max dose 100mR/h does not provide info on radiation type
Flood test
tests if the camera can produce a uniform image along the entire crystal surface 2-5% of non-uniformity is allowed, 1% if SPECT 2 types: 1. Daily- Extrinsic with a collimator 2. Weekly- Intrinsic with no collimator -Na99mTcO4 or Co57 source Recommended counts for both extrinsic and intrinsic range is between 5-10 million
Energy window
performed daily you can use a string, vial, or patient for Tc you would use a 20% window centered at 140 keV
Thermal index: Below 0.7 for OB imaging between 1.0-1.5, US not exceed 30 minutes between 2.5-3.0- US should not exceed 1 minute greater 3.0 US should not be used
Per NCRP a risk benefit decision when the TI exceeds a value of 1.0 and the MI exceeds a value of 0.5
Activity
amount of disintegrations per second historically measured in Curie (Ci) which is 3.7 X 10^10 disintegrations per second new SI units Becquerel (Bq) which is one disintegrations per second
Pulsed doppler (spectral, power, color) should not be used on embryo
Keep TI under 1.0 (some sources say 0.7)
Collimator type chart and pics -parallel -pinhole -converging -diverging
Transient cavitation
bubble oscillation become so large the bubbles collapse resulting in shock waves causing damage
Image linearity and spatial resolution
performed weekly lead bar phantom w parallel lines placed between collimator and Co57 sheet tests image resolution and linearity resolution- ability differentiate 2 distinct points linearity- are all bars straight
Film badge
should be worn on the collar at the chest/neck level
Rate of energy absorption increases with frequency and the rise in temperature slows secondary to conduction and perfusion
Soft tissue index should be used in early gestation. Bone index should be used after 10 weeks when bone ossification s evident.
M mode US is recommended instead of spectral doppler US to document heart rate of fetus
Beta Minus
neutron turns into proton emits a beta particle and a neutrino isobaric transition- atomic number changed, but atomic mass is unchanged not good for imaging you want a plastic shield bc it has a low Z preventing bremmstahlung x-rays
Downscatter
high energy photons spill into window of a low energy emitter image with Xe before Tc image lower photon energy tracers first
Beta positive
proton turns into neutron emits positron and neutrino takes 1.02MeV of energy positron travels, meets an electron, and annihilates resulting in TWO 511 keV photons emitted 180 degrees apart from each other 511 keV + 511 keV = 1022 (1.02 MeV)
Fission
another way to create tracers where you fire neutrons into large atoms which split them into multiple tracers
Section 11
(50 cards)
PET limitation: angulation
the two 511 keV photons are not totally 180 degrees apart
PET is depth independent PET attenuation is depth dependent
What if it gets on my clothes?
take them off and they'll be held by the RSO
TR
time to repetition time between two successive RF pulses
Agreement states
states that reach an agreement with the federal government more strict but not less strict then the national agency
T2 vs T2*
T2* = tissue spin + field inhomogeneity T2 = tissue spin * T2 always decay faster than T2
Steps for major/minor spill
Major: 1. clear area 2. cover spill but do NOT clean it up 3. clearly indicate boundaries of spill area 4. shield source 5. notify radiation safety officer 6. decontaminate person Minor: 1. protect the patient 2. confine the spill 3. clean up the spill 4. survey clean up items 5. survey clean up people
With PET, attenuation correction is applied before reconstruction which is an advantage over SPECT.
Major spill and doses of radio tracers
call radiation officer to clean up 100 mCi of Tc, Ti 10 mCi of In, I-123, Ga 1 mCi of I-131
2D system
pro: lead or tungsten septa that block unwanted scatter con: also blocks correctly oriented photons
SUV equation
tissue radioactivity concentration X weight / injected dose activity
Minor spill
you clean it up
Code of federal regulations (CFR) 10 CFR part 19 10 CFR part 20 10 CFR part 35
10 CFR part 19- notices, instructions, reports to workers 10 CFR part 20- protection against radiation 10 CFR part 35- medical use of byproducts
68 Ge/ 68 Ga
Germanium 68 will decay into Gallium 68, similar to Tc and Moly Gallium 68 emits 511 keV positrons, but has a half life of 68 minutes so it's easier to user Germanium that has a half life of 271 days
Glucose and SUV
High glucose = lower SUV
Normalization scan
done monthly expose detectors to a uniform flux of positrons
Wipe test does not work on xenon
Noise equivalent counts (NEC)
signal to noise ratio of PET scan ratio of true coincidences over total coincidences (true + false)
Free induction decay
after you give an RF pulse, the protons sync up and the signal decreases bc of T2*
RF pulse
decreases longitudinal magnetization and establishes a transverse magnetization
Uncorrected PET
hot skin, hot lungs
PET
proton converts to a neutron with positron emission positron and electron annihilate creating TWO 511 kEv photons that are emitted 180 degrees from each other which is detected by the PET scanner Computer assumes two 511 kEv photons travel in a straight line 2 criteria for photons to be considered a true coincidence pair and calculated along the Line of Response: 1. stay within 25-30% of the full width half maximum centered at the 511 photo peak 2. photons struck detector at nearly the same time If the photons meet with the above criteria and a Line of Response has been generated, that data will make a sinogram which is converted into a picture by iterative reconstruction
PET limitation: scatter
detectors try hard not to get fooled by having coincidence detection
Pre FDG Prep
Fast 4 hours if they eat or take insulin, drives FDG into muscles oral hydration frequent voiding decreases bladder dose no exercise for 24-48 hours metformin is ok to take. warm room, propranolol, diazepam given to prevent brown fat
PET limitation: crystal thickness
PET requires thick crystals to handle high energy 511 photons thick crystals limit resolution this is the primary limiting factor of resolution for PET
T2
transverse magnetization decay aka spin spin relaxation signal has decayed to 37% of its original value
Smaller than 1cm and SUV
smaller than 1cm = lower SUV
Fat vs Skinny SUV
SUV in fat people are overestimated (fat people- eat more sugar- hotter)
PET limitation: positron range
positron travels prior to annihilation event so the detectors cannot track the original location of emission
PET has a lot more 10X more counts that SPECT so it has less mottle
You need an odd number of protons for MRI
Is T1 different in a stronger magnet?
Stronger magnet makes T1 longer stronger magnet gives protons more energy so it takes longer for them to relax
PET is a set of rings and detectors SPECT few cameras rotating around the patient
T1
longitudinal relaxation aka spin-lattice relaxation involves exchange of thermal energy T1 defined as the time at which longitudinal magnetization is 63% of it's final value short T1 bright, long T1 dark
Signal cannot be measured in the longitudinal direction
Truncation artifact PET
The field of view is bigger in a PET scanner compared to a CT. Therefore if the patient is fat, a peripheral lesion may not be see on the CT but can be seen on the PET. Fat people can have artificially lower or higher SUVs in peripheral lesions secondary to FOV differences between CT and PET.
What if it gets on my skin?
wash with soap and water
SPECT
single photon system (unlike PET that uses 2 photons) slow, total scan time is 15 minutes uses iterative reconstruction to make the picture SPECT has the advantage of improved contrast from overlapping structures over planar imaging SPECT is depth-dependent (PET is not)
True, Scatter, Random coincidence
The CT part of a PET scan is performed so you can see where things are and for attenuation correction
What if xenon leak?
Leave room asap close the door
Iterative reconstruction and SUV
more iterations, higher SUV
Time of flight
uses quick detection to estimate the point of annihilation used for large objects iwht low contrast *improved spatial resolution and image contrast trivia: SUV measurements with TOF images are higher than those on standard PET
Coordinate system MRI
Timing and SUV
the longer you wait to scan, the higher the SUV
Blank scan
PET QA done daily performed with "nothing in the field of view" 2 methods: 1. By using positron 68Ge or 137Cs alone in the scanner 2. phantom filled with 511 keV positron emitting 68Ge/Ga
What cases protons to lose their T2 relaxation?
1. inhomogeneities in the external field 2. inhomogeneities in the actual tissues
Larmor equation
precession frequency = magnetic field strength in tesla X gyromagnetic constant
3D system
do NOT use septa pro: large increase in sensitivity and allows you to decrease amount of tracer you are giving cons: 1. dead time- you can overwhelm the detectors with high count rates 2. too many random events cause noise 3. lot of scatter
PET deals with high energy 511 keV so requires thicker crystal Bismuth Germinate, Lutetium oxyorthosilicate (LSO), Lutetium Ytrium Oxyorthosilicate (LYSO) SPECT medium energy photons
Section 12
(50 cards)
Type 2 chemical shift artifact
black line in all directions a fat water interface to fix: adjust TE spin echo sequence
Inversion recovery
*start with a 180 degree preparation pulse wait for the thing you want to saturate to hit its null point, and then slam it with a 90 degree pulse
Spin echo
90 RF pulse with 180 rephasing pulse 180 pulse administered at 1/2 TE 180 pulse given to improve heterogeneous field and gives T2, not T2 has a long TR, and long duration time
Truncation/Gibbs
high contrast interfaces both frequency and phase encoding, but more commonly in the phase encoding CSF-cord mimics syrinx limited sampling of free induction decay to fix: increase matrix decrease bandwidth decrease pixel size
Zipper artifact
stray RF signals defective shielding pulse ox monitor in the room to fix: close the door remove electronic devies repair RF shielding
When do you deliver the 180 refocusing pulse?
1/2 the TE or time to echo
GRE
gradient echo use flip angle less than 90 no 180 degree pulse faster acquisition bc the flip angle is less than 90 you get T2* bc there is no 180 degree pulse so you get more susceptibility artifact less heating and Specific absorption rate
Better spatial resolution
small voxel small field of view larger matrix thinner slices (large slice selection gradient, thin transmit bandwidth)
Gadolinium and T2 effects
At high concentration, T2 dominates pseudo layer from T2 shortening at high Gd concentration
Better signal to noise
noise is random variation in the signal that causes degradation larger voxel = better SNR stronger magnet long TR big FOV large slice (shallow slice selection gradient, thick transmit bandwidth) more NEX short TE small matrix small receiver bandwidth correct coil size
Phase encoding is longer than frequency encoding so it is done on the thinner portion
Spin echo sequences will get rid of india ink but not chemical shift
K space is filled at each TR, TR contributes to the duration of the sequence.
Duration of a 2D study
repetition time X number of phase encoding steps X number of excitations exception- fast spin echo and 3D imaging
Fast spin echo duration equation
1/ echo train length
Type 1 chemical shift artifact
bright on one side, dark on the other *frequency encoding direction (diff from most) fix: flip PE/FE use STIR increase receiver bandwidth
Partial volume MR artifact
different signal intensities overlap in a single volume results in averaging to fix: make pixels smaller
Echo planar imaging
EPI- noisy one fastest acquisition turn phase and frequency encoding on and off rapidly, fast filling of k space. DWI very vulnerable to magnetic susceptibility
Motion artifact
to fix: stop moving breath hold respiratory gating respiratory ordered phase encoding- phase encoding steps are ordered with respiration breathing navigator apply fat sat band across abdomen switch phase encoding direction blade/propeller to oversample center k-space resulting in redundant information for error correction
Steps of a basic spin echo sequence
Flow on spin echo and gradient echo
Spin echo: flow looks dark, moving blood gets hit with 90 degree pulse, moves out of the way before 180 pulse, so no signal Gradient echo: flowing blood looks bright
Echo train length
number of echoes in the same TR in fast spin echo sequence
Contrast agent chart: Magnevist Multihance Eovist Gadavist
Modification relationship to SNR, SR, duration chart
Aliasing
undersampling phase encoding direction fix: FOB bigger flip the PE/FE surface coils sat bands
Phase encoding direction abd head breast
shorter distance is used for the phase encoding direction abd- front to back head- side to side breast- side to side spine- sagittal direction
Good and bad sequence for susceptibility artifact
no susceptibly artifact- STIR more artifact- echo planar imaging and GRE (no 180 pulse)
Gadolinium
must chelate gado with DTPA to prevent toxicity causes T1 shortening bc it's paramagnetic with 7 unpaired electrons the T1 shortening gets stronger with stronger field strength
Receiver bandwidth vs transmit bandwidth
STIR -less susceptible to metallic artifact and field inhomogeneity -cant be used with gadolinium bc it has a similar time to inversion as fat and will be nulled out
K space
center- contrast periphery- spatial resolution
Functional MRI
increased blood flow to areas that are active resulting in a decrease of deoxyhemoglobin fMRI depends on the T2* effects of deoxyhemoglobin
Magic angle artifact
MSK tendon artifact angle 55 degrees seen short TE sequences like T1, PD, GRE NOT seen in T2 sequences with long TE to fix: high field strength results in greater T2
Spin Echo vs Gradient Echo TR and TE
Fat saturation technique vs STIR
fat sat technique is used with gado STIR cannot be used with gado Directions for fat sat: 1. select the fat peak 2. send an RF pulse at fat resonance, so they cannot give signal 3. everything else will give signal except for fat STIR: 180 pulse, followed by 90, good for metal artifact
Thin slices
Large slice selection gradient Thin transmit bandwidth
Thick slices
Small slice selection gradient Thick transmit bandwidth
Fast spin echo
apply multiple 180 RF pulses to reduce TR and duration of the study Echo train length- number of echoes in the same TR *T2 fat signal is longer with fast spin echo bc of J coupling interference Duration- 1/ echo train length
Duration of a 3D study
TR x Phase encoding steps x Number excitations x #slices 3D studies have better SNR but take longer so performed less
B factor
B factor controls how much ADC contributes to the study the higher the B factor, the greater the diffusion weighting B Zero = long TR, long TE, poor mans T2
Proton density has excellent SNR
Long TR - big FOV --> big voxels --> better SNR Short TE- small matrix --> big voxel --> better SNR
1. Select the desired slice -place the gradient perpendicular to the desired plane 2. Encoding spatial information in the phase encoding direction 3. Encoding spatial information in the frequency encoding direction
ADC
made by taking the negative logarithm from the difference between the B0 and B500-1000
The phase matrix is another way of saying number of phase encoding steps or lines you need to fill
You must get the out of phase images at 2.2 msec. hepatic steatosis (microscopic fat)- dark on all out of phase hemochromatosis- gets darker on in phase which progresses over time
How to fix T2*
Hit it with a 180 refocusing pulse that 1. turns T2* into T2 2. creates an echo
T1, T2, PD TR TE
Cross talk artifact
to fix: increase gap betweens actions interleave slices (all odds, all evens) 3D not susceptible to this artifact
Receiver bandwidth
fat bandwidth gives you rapid sampling of data, which picks up more noise, worse SNR narrow bandwidth gives you slow sampling of data, less noise, better SNR
What makes a flow void?
no flow: gets 90 and 180= makes an echo slow flow: gets some 90 and 180= makes an echo fast flow: gets 90, misses 180 = no echo
Section 13
(50 cards)
MR Artifact chart
If there is a code in the scanner. MRI techs get pt out of the scanner, start CPR, stabilize pt, and get them out into zone 2 for the code team. you can code them in zone 3, but never zone 4
How to prevent bore from touching the skin?
pad the skin
Inhomogeneous fat suppression
local field inhomogeneities cause fat protons to precess at different frequencies which prevents fat suppression to fix: STIR esp with metal
Zone 4
restricted room where the patient gets scanned
Contrast agent safety
nephrogenic systemic fibrosis GFR>30 to get contrast dialysis does not make it ok bc you can't dialyze the gado out
MR scanner keeps the wires cold by submerging them in liquid helium
Kerma is the same as the absorbed dose with low energy photons, bc the photoelectric effect dominates. Kerma is more than the absorbed dose with high energy photons, bc photons shoot through and don't contribute to dose.
Noisy MRI sequence
noise comes from the gradient coils gradient intensive sequences like echo-planar imaging are the loudest
Eddy current
occurs when gradients are turned on and off rapidly at bone brain interface most severe with DWI to fix: optimize the sequence of gradient pulses
Exposure
ability x-rays ionize air measured in Roentgens
Dark blood sequence
spin echo double inversion recovery- 2 consecutive 180 pulses good for anatomy
Kerma
stands for kinetic energy release per unit mass estimates how much primary energy gets transferred when you expose a person to x-rays
Crisscross artifact / herringbone
obliquely oriented stripes throughout the image data processing/ reconstruction error to fix: reconstruct the image again
Absorbed radiation dose
amount of energy absorbed per unit mass measured in Gy or Rads (1 Gy=100 rads)
If you double the duty cycle (1/2 the TR), what do you do to the SAR?
doubles
Equivalent dose
biologic effect corrected for by type of radiation Sievert
Annual QC for MRI scanners
performed by medical physicist magnetic field homogeneity slice position accuracy slice thickness accuracy radio frequency coil check display monitor check
SAR equation
specific absorption rate SAR = B02 x alpha2 X duty cycle B02- strength magnet squared alpha-flip angle squared duty cycle= cool down. longer TR- more cool down increasing your TR, decreases your duty cycle
Inversion recovery cardiac
nulls myocardium to look for delayed enhancement
Effective dose
biologic effect on tissue type Sievert closest measure of cancer risk bc it considers both radiation and tissue type
Damage to fetal ossicles is a theoretical risk for fetal MRI
What sequence has the highest SAR?
spin echo, bc has higher flip angles
Pad areas between extremities and prevent them from touching each other which can cause burns
Implant rupture screening will NOT have contrast, but instead have fat and water saturated sequence (only silicone is bright)
Tissue doses are higher than air kerma bc x-ray interaction with tissue creates scatter and secondary electrons which contribute to dose
How to reduce neurostimulation during MRI?
electrical currents result in painful neurostimulation typically in arms and legs were the gradient magnetic field is changing most rapidly occurs in high bandwidth readouts and rapid gradient switching (echo-planar imaging) to fix: 1. reduce readout bandwidth 2. increase TR
Weekly QC for MRI
performed by MRI techs center frequency table positioning set up and scanning geometric accuracy high contrast resolution verified by phantom low contrast resolution artifact analysis film quality control visual checklist
5G Line
Gauss exclusion zone magnetism outside of this line won't mess up implanted devices like pacemaker, insulin pump, vagus nerve stimulator this does not mean pulling won't occur
Zone 1
no restriction outside the building
Collimation effect on air kerma and KAP
Collimation increases air kerma bc of automatic bright control increasing the juice but decreases KAP
Susceptibility artifact
metal, ca, gado, bone air interface like sinus most severe with gradient echo lease severe with spin echo bc of the 180 degree refocusing pulse decreasing T2* to fix: use spin echo and fast spin echo instead of gradient echo reduce field strength high receiver bandwidth thin slices align the longitudinal axis of the metal implants with axis of the main field STIR does way better than selective fat suppression blooming artifact from metal clips worse on in phase bc it is done later (more timing = more blooming). the T2* effect worsens over time.
Kerma area product (KAP)
dose area product KAP = kerma dose X cross sectional area total radiation incident on the patient *independent of source distance geometric magnification increases entrance skin KERMA but does not change the KAP
If you double the flip angle, what do you do to the SAR?
quadruples
Antenna effect
abandoned intracardiac pacemaker lead results in the electric field coupling with the wire
Dielectric effects / standing waves
worse with stronger magnet large ascites bellies dark signal int he central abdomen to fix: apply dielectric pads parallel RF transmission
Zone 2
no restriction waiting/ dressing room screen patients
If you double B0, what do you do to the SAR?
quadruples
Quench
shut scanner down fast venting all liquid helium out of the room to the outside if there is a crack in the pipe, the helium will enter the room. it displaces the oxygen and pressure pins the door closed. a code in the scanner is not a reason to quench
Signal flair in breast
if the breast is too close to the cold element it will not fat sat and looks bright to fix: reposition the patient
What are the SAR limits?
FDA limits 4W/kg over 15 minutes 3W/hg over 10 minutes
Zone 3
restricted room control room for MR techs lock on door between zone 2 and 3
Shimming
improves field homogeneity combination of passive and active shimming is done passive shimming- phantom is scanned and shim plate positioned active shimming- uses the coil before or after each patient
Chemical shift artifact in breast
seen in the breast at the fat water interface corrected by increasing the bandwidth to capture both fat and water in the same phase
Bright blood sequence
gradient sequence SSFP- primary gradient sequence for wall motion and volume analysis. CINE.
Medicated patches can burn the patient during MR
FDA max for noise in MRI
140 dB for MR 99dB for patients with hearing protection
Motion breathing cardiac in breast
breathing and heart beating is corrected for by running the phase encoding direction side to side
Translational force
getting pulled into the magnet don't bring o2 tank into zone 4 transitional force has nothing to do with the 5G line
High yield trivia: KAP represents the total energy incident on the patient KAP does not indicate risk of skin burns KAP is a good indicator of stochastic risk Air kerma is an indicator of deterministic risk
Section 14
(50 cards)
Radon (alpha emitter) is the single largest contributor to effective dose
Radiation induced sterility in males has a latent period between irradiation and sterility
8-15 weeks is the most vulnerable time
Final number of double stranded DNA breaks is more important than the initial number of breaks bc some will be repaired.
Double stranded DNA breaks are the most important lesions caused by x-rays
Stochastic
shit happens follows a linear quadratic no threshold model committed dose- stochastic vocab word- means probability of cancer induction and genetic damage
Survival curve
repair shoulder- when repair mechanisms can work the repair shoulder only exists with low LET radiation curves higher dose rate makes a smaller repair shoulder and a steeper drop in the curve oxygen will make a steeper drop in the curve
Radiation induced sterility in females causes menopausal symptoms
Deterministic
x amount of radiation causes x effect *once you cause the threshold, you get the result *after you cross the threshold, the more dose you get, the more severe that thing gets
First 2 weeks of implantation 50-100mGy causes fetal loss. If the fetus survives, she will have no lasting effects. All or nothing.
Cell phase order of sensitivity
most sensitive during mitosis M>G2>G1>S G1 phase is the most variable in length
Syndrome with most sensitivity to x-rays
ataxia telangiectasia
What is the most sensitive blood cell in the body?
lymphocytes dose of 0.25 Gy is enough to depress the amount circulating in the body
After 15 weeks, the brain is less sensitive to radiation
Risk of radiation induced cancer 1 Sv = 5.5% chance of developing cancer
What is the most radiosensitive part of the body?
GI tract
Transient skin erythema can be seen in hours with the main wave occurring after 10 days
Teratogenicity of radiation is dose and time dependent
Fetal thyroid dose NOT take up iodine prior to week 8. so if mom gets I-131 prior to week 8, the fetus will not be hypothyroid.
Kill effect
As LET increases, RBE will increase to a certain point Above 100 keV/micrometer of tissue, RBE decreases with increasing LET
Damage to tissues is result of electron ionization
Acute radiation syndrome
clinical response when the body is hit with a large amount of radiation 4 phases: 1. flu symptoms 2. then you feel better during the latent phase 3. then your syndrome subtype manifests according to the chart 4. then you either recover or die
Lethal dose 50/30
Dose kills 50% of people in 30 days *LD50/30 for a person is around 3-4Gy without treatment
Latency
time between radiation exposure and cancer leukemia has the shortest latency 5-7 years
Malmo rooms drywall should be
2 layers of 5/8 in dry wall
Relative biologic effectiveness
capability of radiation with different LETs to produce a particular biologic reaction RBE = dose of 250 kV x-rays / dose in Gy of test radiation
Cataracts caused by radiation are in the posterior lens
Deterministic vs stochastic effect chart
risk of radiation induced cancer in adult, child, elderly
adult: 4-5% risk per Sv child: 15% per Sv 1/10th that for someone older than 50
Low LET radiation cases single strand breaks which can be repaired High LET radiation cases double stranded breaks which are harder to repair mutation results when the radiation change the nitrogenous base which is transferred when the cell divides
Male gonad shield
placed just below the pubic symphysis
Law of bergonie and tribondeau
cell sensitivity directly related to reproductive activity and inversely related to their differentiation more a cell turns over like skin, blood, gi tract, reproductive cells, more sensitive they are the less turn over and more differentiated they are brain, nerves, muscles, the less sensitive they are
60% of x-ray damage to biologic material is mediated by free radicals
Triaging patients with possible acute radiation syndrome
the earlier the symptoms appear, the worse pt will do *early vomiting marker of severity / poor prognosis
Carcinogenic by radiation is stochastic (all or nothing) based on Beir 5 or UNSCEAR committees
total body dose of 0.75 - 1.25 Gy will cause nausea 30% of the time
Mammo doors should be equivalent to 1 mm of steel
Female gonad shield
placed 1 inch medial to the anterior superior iliac spine and covers from the pubic symphysis to the sacrum
Doses over 100-200 mGy associated with reduced head diameter and mental retardation
Working population has a risk of 4-5% / Sv for cancer
IQ drops 30 points per 1 Sv with the reins of retardation being 40% at 1 Sv
Radon workers get more lung cancer
Oxygen enhancement ratio (OER)
biologic tissue is more sensitive to radiation in an oxygenated state OER only matters for low LET radiation
Takes a low dose to increase risk of childhood leukemia (just a few radiographs)
Direct vs indirect ionization radiation, including chart.
direct- act directly on DNA indirect- act on water majority of irradiation is the result of indirect action on water creating free radicals
Majority of energy received by biologic material from x-rays is transferred by electrons
Greatest source of exposure to ionization radiation for the general population of the US DUE TO HUMAN ACTIVITY is medical imaging
Radiation induced sterility in males does NOT affect hormone levels or libido
X-ray or gamma ray dose of 1000 Gy in a period of second/minutes will cause instant death of a large number of cells
Linear energy transfer
average amount of energy deposited per unit path 2 types: 1. High LET- neutrons, protons, alpha particles, heavy ions- much more damaging 2. Low LET- photons, gamma rays, electrons, positrons
Section 15
(2 cards)