DI FINAL EXAM Flashcards
Define mA
amount of current flowing across tube during exposure
What does mA influence?
amount of radiation
What does time (s) control?
duration of exposure
Define secondary radiation
radiation coming from another source (ex. wall, floor)
Define primary radiation
radiation between tube and patient - has full E
Define remnant radiation
radiation between patient and IR - “exit” radiation has lower E
Define scatter radiation
primary radiation that has changed direction
What is attenuation
decrease in beam intensity / radiation intensity
Causes of attenuation
absorption & scatter
What does an attenuated beam mean?
a beam that has decreased in intensity
Define SID
source to image receptor distance
Define OID
object to image receptor distance
Define SOD
source to object distance
Factors affecting absorption
thickness, atomic number, density
Increase density =
increase absorption
High contrast vs low contrast
High C has black & white values with more detail, low C has more grey values with less detail
Cathode to anode process
Cathode (negative) expels electrons to anode, anode (positive) produces xrays
Higher kvp = _________ penetration
Increased penetration
Higher kvp = _________ energy
higher energy produced / beam energy
Higher kvp = _______ absorption
decreased absorption (higher penetration occuring)
Explain kvp
Max voltage difference between the cathode and the anode (quality)
What does kvp control?
penetration and quality of x-ray beam / radiation
Explain mAs
Current + time: number of electrons flowing through tube during exposure - total quantity
What does an increase in mAs mean for the xray tube?
an increase in number of xray photons
What does increase in mAs mean for patient?
increased dose
Explain mA
amount of current passing through tube during exposure (higher mA = more electrons) QUANTITY of xrays travelling
What does increasing mA cause
an increase in radiation exposure
Dosimeter purpose
measures intensity
Intensity measured in
mR
What do EI/DI values prove?
amount of radiation that reached the IR
Double mA results in what?
double rad to IR and double PT dose
Define reciprocity
techniques that are different, but will produce similar images (they all have the same mAs - so same rad exposure)
Advantages of reciprocity
control motion, breathing techniques, focal spot size
Smaller focal spot has better what compared to large focal spot?
SR and detail
Focal spot is limited by what technique?
mA
Which has less intensity: 100 cm or 180cm
180 cm (beam divergence / intensity decreases with distance)
Inverse square law formula
I 1 / 1 2 = (d2/d1) ^ 2
explain inverse square law
States that intensity reaching IR is affected by SID (distance: closer to the source = more intensity)
What would you change, technique wise, to get same intensity to IR at 180cm vs 100cm
mAs
The square law formula
mAs / mAs = (SID / SID) ^2
Define subject contrast
determined by absorption of radiation / anatomy (DETERMINES the differences in attenuation of anatomy shown)
Define image contrast
brightness values determined by subject contrast, can be high or low - SHOWS the representation of the differences in attenuation of anatomy
Image contrast depends on
scatter, subject contrast (wont have image without), post processing (windowing), algorithm (puts brightness and contrast values where they should be for proj)
Subject contrast depends on
density, thickness, atomic number, kvp, absorption
Which technique affects contrast?
kvp (penetration - how much or little it penetrates will show on image)
Which kvp produces a better contrast, high or low?
low kvp (less penetration so allows differences to show better)
What happens if you have a low kvp?
increased absorption (so, better contrast and detail), increased subj C, decreased energy
What happens if you have high kvp?
increased penetration (less absorption), decreased subj C, increased energy
Which technique is the potential difference?
kvp
Define potential difference
amount of energy required to move an electric charge from one point to another (kvp - moves electrons cathode to anode)
Define differential absorption
difference in how materials absorb xrays produced
What does differential absorption produce image wise?
contrast/details of image
Absorption: higher Z / thickness / density =
more absorption - less to IR
Absorption: lower Z / thickness / density =
less absorption - more to IR
Does mAs affect absorption?
no, mAs is just the number of xray photons travelling, kvp affects absorption
How do you control motion issues with chest x-rays?
shorter exposure, or use longer exposure if you need to blur out ribs
kvp & intensity formula
I 1 / I 2 = (kvp / kvp) ^2
what does kvp and intensity formula measure
the primary beam/radiation intensity (tube to pt)
kvp & IR exposure formula
I 1 / I 2 = (kvp/kvp) ^5
What does kvp and IR exposure measure
remnant beam/radiation
Explain 15% rule of thumb
increase in kvp by 15% approximately doubles intensity
a high kvp lowers patient dose when you _______ mAs
decrease
Cons of scatter
degrades image, makes grey shades blend together, low E
How to control scatter
grids, collimation, kvp, thickness
How do grids work to control scatter?
made of radiopaque strips that absorb the radiation, radiolucent material lets through radiation needed for the image
Higher the kvp, ________ the grid ratio used
higher
Increase kvp = _______ E for scatter
increased energy for scatter
Factors affecting scatter
kvp, field size (want small), thickness, material
Thicker = _______ scatter
more (more matter = more scatter)
Higher the grid ratio = _______ scatter
decrease in scatter (16:1 - less scatter than 8:1)
Define grid frequency
number of strips per cm
Higher grid frequency = _________ strips
more
Con of grids / high grid ratio
can get rid of useful radiation (absorb)
List grid types:
linear parallel, linear focused, crossed
Define grid cut off
unnecessary absorption of primary radiation by grid - “cut off” from reaching IR - partial or complete
Grid cut off is caused by what?
incorrect centring / position of grid, wrong SID used, tube angled against the grid lines
How does grid cut off affect the image?
poor penetration - resolution loss / grainy / grey spots
Linear focused grid vs parallel
Linear focused - grid lines are angled, Linear parallel - grid lines run straight side by side
Increased grid cut off with what grid ratio?
higher grid ratio
Con of focused grid?
need to be accurate with centring, SID must be accurate, can cause improper absorption if upside down
As grid ratio increases = ________ pt dose & why
patient dose (because you have to use a higher technique / mAs with a higher grid ratio)
What grid type gets rid of the most scatter?
crossed grids
List and explain grid errors (4) that lead to grid cut off
off center (CR not in center of grid), off level (tube and grid not perpendicular), off focus (incorrect SID), upside down (tube side not facing towards the tube)
Explain air gap technique
alternative to using grid to reduce scatter reaching IR by increasing distance between patient and IR (OID), helps reduce patient dose
Magnification factor formula
Mag factor = SID / SOD (convert: SID - OID = SOD)
What causes magnification (distortion)
increased OID, decreased SOD / SID
foreshortening vs elongation
Foreshortening - from angled object with a CR thats perpendicular to IR and elongation is from object being parallel to IR with a CR that’s angled (or other way around)
What controls spatial resolution?
pixel size / acquisition pixel size (the more pixels = the better the SR)
What affects spatial resolution?
focal spot size, movement, OID and SID
Define spatial resolution
ability to see difference between two objects close together
What is analog used for?
analog xray images are processed - converts analog signal to digital image
More line pairs =
better detail (lp/mm)
Small focal spot vs large focal spot
small has increase sharpness and spatial resolution and large has decreased sharpness and decreased spatial resolution
Explain anode heel effect
angle on tube, photons on anode side have to go through thick target material - decreased intensity on anode side. Place thinner anatomy on anode side
Do the size and shape of the physical AEC detectors change sizes?
No, only the outlines do
With AEC, what happens if spine isn’t over detector?
AEC detector will see more rad – shorter exposure time – lower mAs (over ST instead of bone) – lower DI value
Explain AEC
terminates exposure once ionization chambers receive proper amount of radiation
Explain falling load generators
start at HIGH mA and gradually decreases during exposure (kvp remains constant) - purpose is to get the lowest time possible
What do you pick with falling load generator?
kvp
What do you pick with AEC?
kvp and mA (NOT time)
Backup mAs/time rule?
1.5-2x the anticipated mAs (50%-100%)
Explain backup mAs with AEC and why have it
max mAs exposure can have incase of system failure or error by technologist - avoids overexposure to patient
Explain min response time
shortest possible exposure time (only with AEC)
2 types of windowing
brightness and contrast
Explain window level
brightness values (higher WL = brighter image)
Explain window width
contrast values (Increase WW = decrease constrat)
List post process examples
adding text, windowing, magnification, flipping/rotation, inversion
Advantages of digital radiography
lower patient dose, fewer repeats (have automatic rescaling), post processing, higher contrast resolution
Explain detective quantum efficiency (DQE)
How well the detector is at converting x-ray energy into an image
The higher the DQE =
less mAs / rad required = less patient dose
Disadvantages of digital radiography
lower spatial resolution, dose creep (because of post processing / fixing over exposures - may end up using higher mAs value than required)
Subject contrast affected by
scatter rad
How much subject contrast is required for an exposure in film vs digital radiography
film: 10% and digital %1 (so, less needed now with DR to see differences in structures)
DDR acquisition types
indirect and direct
Explain indirect acquisition + example
2 step process - offers high DQE - converts xray photons to light - CR cassettes
Explain direct acquisition + example
1 step - better SR - flat panel detectors
When comparing DQE between different detectors / systems, must be compared with same what?
kvp (so beam has same penetration)
Explain quantum mottle (noise)
image has a grainy look - determined by number of photons hitting IR to create signal (pronounced QM - means you can SEE it) - can distract you from the image
low kvp and mAs regarding QM
few photons hit IR, low signal, more noise (patchy)
high kvp and mAs regarding QM
many photons hit IR, high signal, less noise = better image
Define signal
determined by number of photons / information reaching IR (known as image forming xrays - or remnant radiation)
What does high signal mean for the image?
more radiation hitting the IR = less noise = better image (want a high SNR)
What can cause QM
larger patient, SID, grid, too low of technique, scatter, lack of information
Signal is caused by
attenuation, technique(increase mAs-more photons to IR), x-ray photons (more = better chance of converting into something)
How to get higher signal to noise ratio
increase mAs
What percent of the original latent image is retained by PSP and for how long after exposure?
75% and up to 8 hours
Exposure indictors predict what?
If IR received enough radiation and if you have a good SNR
More lp/mm =
better spatial frequency
What can affect the EI value?
collimation, technique
Explain target index?
programmed into system - changed to match procedure. TI is what the IR SHOULD see (How DI is detected)
Explain DI value
compres measured value (EI) and ideal value (TI) - how much it “deviated” from TI
DI formula
DI = log (EI/TI) x 10
List DI value terms
optimal range, acceptable range, out of desired range
Explain dynamic range
identified by bit depth (# of brightness values each pixel can have)
Explain pixel pitch
PP - distance from centre of one pixel to the centre of the one next to it (smaller PP = better SR)
Explain pixel density
The higher the # of pixels, the greater pixel density within a space / FOV (better SR)
Explain digital
matrix of pixels represented by numerical values
Explain CR - computed radiography
Taken to a reader to be processed, considered indirect digital (more than one step) - excite electrons to a higher E level
Explain photostimulable storage phosphor (PSP)
Stores energy inside a CR cassette, laser beam interacts, is exposed and converted into a digital image - stores and releases E
Indirect acquisition process CDC
Scintillator converts x-ray photons to light, photodetector (CDC) converts light into electronic signal - ADC converts image and produces
Direct acquisition process
Directly converts x-ray photons to an electronic signal (amorphous selenium)
How is digital image formed?
matrix of pixels - displayed on a monitor
Define pixel
represented by numerical value, contains series of bits / bit depth that determine the brightness/details
Define image matrix
electronic image that is laid out in rows and columns
How many different values does 2^4 have
16 different values
The more bit depth =
the more brightness values within an image - the more CR
Explain contrast resolution
ability to distinguish many shades of grey from black and white (higher the CR = the more shades) - determined by the bits
Does a larger matrix increase CR or SR?
SR
CR plates produce what?
the latent image
What captures the latent image?
PSP - stores the latent image until processing
What does noise (QM) limit regarding resolution?
contrast
Increase pixels = increase _______
SR
Increase bit depth = increase _______
CR
What resolution does indirect acquisition have?
CR
What colour is laser hitting photons?
red
Laser colour that clears cassette signals
white
What acquisition has higher DQE
indirect (due to scintillation layer)
What does a higher signal to noise ratio mean?
IR saw more photons (want this) has clearer image
What does signal to noise ratio mean?
compares level of signal (wanted) to noise (unwanted). Decreased signal = more noise
What affects EI value?
technique, collimation, algorithm, scatter, SID
Increase EI = ______ patient dose
increase
Fuji Optimal number
S# - 200-400
Carestream optimal number
EI - 800-2200
Explain fuji mAs process
cut mAs in half = double s#, double mAs = half s#
Under 200 with fuji means? and over 400 means?
under 200 overexposure, over 400 underexposure
What does doubling/halving exposure do with carestream EI?
changes EI by 300
What is DI optimal number? colour?
-3 to 2, green
Indirect acquisition CCD process
x-ray photons converted to light with the scintillation layer (cesium iodide), CCD converts light to electronic signal, ADC produces image (scintillation, CDC, ADC)
Indirect acquisition TFT process
x-ray photons converted to light through scintillation layer (cesium iodide), photodiode layer converts light to electronic signal, TFT holds charge briefly, sends signal to ADC to produce image (scintillation - photodiode - TFT - ADC)
Direct acquisition process
One step (directly converts photons) X-ray photons interact with photoconductor (amorphous selenium) which creates electronic charge, storage capacitors in the DELs collect the charge briefly, released to ADC, digital image produced (Photoconductor - DEL - ADC)
Explain PSL (Photostimulable Luminescence) process
Phosphor stimulated, Emits light when exposed to a light source (infared), light given off is collected and converted into a digital image
Algorithm process
selected prior to processing - puts brightness/contrast values in proper range
What happens if you under/over expose?
Automatic rescaling
What is below -5 or above 4 with DI? colour?
out of desired range, orange
Factors that affect S# or EI value?
size of plate, technique, collimation, centring, scatter, beam (should match plate size), algorithm
Explain the read process
PMT converts light to electrical signal, electrical signal is amplified, A/D converter produces binary number
What acquisition has better SR?
direct, because no light divergence
What is ADC? what does it produce?
analog-to-digital-converter (raw data) produces image and binary number
What acquisition is general radiography?
indirect CCD and TFT
Purpose of cesium iodide indirect acquisition
Helps focus the light and improve SR
difference between an underexposed vs overexposed image appearance
dark, bright
What is bit depth dependent on?
amount of radiation reaching each pixel (will determine shade of pixel)
Write 155 binary number
done on paper
Increase thickness = ________ absorption
increase (so, less rad to IR)
Order of absorption rates for the body (more - least)
bone, soft tissue, air
Other name for focal spot blur
pneumbra, geometric unsharpness
What is focal spot blur
whole image is affected and image has a blur to it - loss of sharpness - more noticeable on cathode side
Causes of focal spot blur
increased OID, larger focal spot
How to avoid/fix focal spot blur
increase SID, when smaller focal spot when possible
What causes QM?
not enough photons hitting IR (less signal)
For same body part, every 4cm thickness change, do what with the mAs?
double it
kvp should never be ______ than optimum for that body part
less than (should be HIGHER than usual)
need to have at least _____ subject contrast
1%
Why does a nongrid chest have a lower kvp than with a grid?
because grids absorb good radiation - so have to compensate for this with grid chest
List group 2 that have the same techniques
AP/PA knee (if no grid), lateral skull, C-spine, AP shoulder
List group 1 that have the same techniques
AP abdomen, AP pelvis, AP lumbar spine, AP hip, townes skull
Some generators produce more ________
output
What is Anatomically Programmed Radiography (APR)
Used to select procedure / body part (selects correct mAs, kvp, SID, grid, time, FSS) - “fixed technique”
AEC vs APR
AEC you don’t select time, you select kvp and mA, have to select detectors, back up mAs.
What do you select with falling load generator?
kvp
kvp for: PA wrist
65-70
kvp for: AP/PA knee
70-85
kvp for: AP abdomen
70-80
kvp for: PA chest with grid
110-125
kvp for: chest non gird
80-90
kvp for: AP chest
120
Why is it important to know anticipated mAs with AEC?
want them to be the same - tells us if we overexposed pt
anticipated 10 mAs, actual was 20, what could have caused this?
Improper centring (under more or less density), grid ratio, too much filtration
why is it important to Know the appropriate EI/DI ranges for each exposure with AEC
image quality
What are Single Variable Technique Charts, what do they provide?
standardized techniques for procedures that are programmed into system (provides consistency and fewer repeats/errors)
What ISN’T standardized in Single Variable Technique Charts
thickness (changed based on patient)
x-ray image should have:
proper penetration (more rad to IR), contrast, controlled scatter, high SNR, and translucency between structures
What does translucency mean for image?
means you can see overlying structures (ribs, heart shadow, can see spine detail THROUGH the heart tissue)
Signs of improper penetration
can’t make out detail (blurry)
Whats included in Single Variable Technique Charts
exposure factors (mAs,kvp), projections, IR (bucky or table), processing (brightness/contrast values), FS size, AEC, density selectors, grid
What do density selectors allow control of?
density within the image/ radiation reaching IR - can reduce scatter hitting IR in high scatter situations
Optimum kvp penetrates a range of ________
thicknesses
For optimum mA, need to know the following:
Focal spot size (to use small FS, has to be within range) and exposure time (short exp time - higher mA)
higher or lower mA for PA chest
Higher, higher mA allows for a shorter exposure time (so, pt doesnt have to hold breath long)
As patient size changes:
time and mAs changes (adjust mAs based on thickness)
Can use similar technique for:
AP lumbar spine, AP abdomen, AP pelvis, townes skull
15% rule explain
cut mAs in half = increase kvp by 15%, double mAs = decrease kvp by 15%
AP knee = _____ AP ankle
x 2 the technique (times knee technique by 2 to get the technique for ankle)
Purpose of bucky factors
Also called grid factors, ratio that determines the extra rad exposure a patient will receive when using a grid (need to compensate for changes)
What can cause lateral decentering?
Being out of D-tent lock
Will DI change with AEC with change in SID for PA chest? what about mAs?
no, Di will remain the same, but mAs will be lower
What will happen with off center AEC detector with spine, regarding technique?
lower mAs than anticipated, lower DI = underexposed image
AEC detectors: single knee vs bilateral
centre detector with single knee, two laterals with bilateral
If you need to repeat an image taken with AEC, what do you do?
switch to fixed technique unless you know reason
If tube is off centre with AEC, but anatomy is overtop of AEC, what happens?
grid cut off (not centred to grid), DI remains the same (IR always sees same rad with AEC), longer exposure time, higher mAs, more rad reaching patient = more dose, amount reaching IR stays the same
From the lab, what can you use to prove intensity changes with SID change?
use dosimeter (dosimeters measure intensity) for two images and compare
From lab, how would you prove that going from a table top exposure for an AP shoulder to a table bucky exposure for the same shoulder would require an increase in technique
take 2 exposures (1 table, 1 bucky) using same technique for AP shoulder and compare results
What could cause new unit set up with the same techniques, to start showing DI values as underexposures?
System could have different DQE, different filtration, generator performance (direct, indirect, etc)
What can lateral decentring cause?
grid cut off, lower DI / higher mAs if using fixed technique
Does AEC affect lateral decentring?
no
With AEC, which will have a higher mAs 100 cm or 180cm
180 cm (takes longer for IR to receive proper rad)
With AEC, which will have a higher mAs 6:1 or 12:1
12:1 (takes longer for IR to receive proper rad)
with AEC, will thinner or thicker patient see more rad exposure?
thicker (more pt dose)
Whats the exception when pt would see less dose with a higher mAs?
adding additional filtration (filter out low E photons)
How can you tell if its a fixed technique or AEC?
mAs is given if its a fixed technique, back up is given if its AEC
How to know if you can use small FS?
mA, if its within the correct range
Why do you lose radiation going from 100 cm to 180 cm? if thats all you’re doing?
inverse square law - loss of intensity reaching IR - beam divergence (will miss IR)
the grid 100-180 range has a what grid ratio? high or low? why?
Low, allow for SID range
Increase kvp = _______ scatter
increase
will direct or indirect show better SR
direct acquisition
will direct or indirect show better CR
indirect
Higher contrast resolution shows higher or lower DQE?
higher (can use lower mAs values)
Does changing the collimator turn the AEC detectors?
No, they remain the same direction
With AEC, what happens if you’re supposed to be over ST but you’re centred over bone?
longer exposure time, higher mAs, DI increases higher than anticipated
With AEC, what happens if you’re supposed to be over bone but you’re centred over ST?
shorter exposure (premature), and lower mAs
AEC won’t compensate for what?
improper centring, wrong detectors, +/- density selectors left on
What happens if you have 100 cm selected for PA chest that should be at 180cm? with mAs and DI value
mAs lower than anticipated (closer to IR - 3x less), DI value stays the same
Why doesn’t DI change if you use the wrong SID?
because you’re still properly centred, using correct detectors, and penetrating proper tissue - so same rad will hit AEC
can you use AEC if someone has hardware?
no, use fixed (would lead to high mAs and DI value - because very dense)
Grid errors can cause what?
grid cut off
underexposure or overexposure if youre laterally decentered with fixed technique?
underexposure
What to remember about MRT with AEC?
you can’t go lower than the MRT
Risk when imaging metal / radiopaque material
overexposure
With falling load, when having issues with motion, change what?
80% to 100% setting
what does the 100% setting on falling load gen affect?
just the length of the exposure (shorter exposure time - doesnt affect mA)
Does AEC compensate for lateral decentring of grid? will this affect DI value?
yes , DI value not affected
What will change DI value with AEC?
density selector left on
Does changing mA affect patient dose?
no, doesnt affect penetration
More matter (bigger field size) = ______ mAs and exposure
lower and shorter
If anatomy is off centre with AEC detectors, what happens?
