3. XR Concepts Flashcards
noise and relationship with photons
quantum noise (digital imaging); background randomness or background data that does not contribute to the image; function of photons hitting imaging receptor
more photons: more signal relative to noise
less photons: low signal relative to noise
quantum mottle? how to decrease
quantum mottle is an important source of random noise
decrease quantum mottle with more x-rays (mAs) or more efficient detection….basically more xrays on detector
majority of scatter is?
compton interactions
characteristic radiation is generally too low energy
determinants of the amount of scatter
high kVP, thickness, field of view
kvP (PE effects dominate at lower energies, compton scatter at higher KvP)
thicker body pat will have more irradiated tissue/more compton scatter interactions
smaller field of view (narrow beam) will have a larger angle of escape (less area for scatter to land on)
role of collimator
restrict x-ray beam to limit patient exposure and reduce scatter by reducing field of view
effects of changing collimator
decrease collimator: increase field size, more quantum noise, dose and scatter increases
increase collimator, field size shrinks and there is less scatter
collimation decreases noise
role of grid
reduce scatter, increase contrast
multiple thin metallic/lead strips with holes that blocks off angle xrays (presumed scatter) and allows “straight” xrays to contact image receptor
what is grid ratio
describes density of grid; ratio of height of lead to distance between them
higher ratio the less scatter and the better the contrast
how does grid impact dose
higher grid ratio increases dose
what is the bucky factor?
mAs required with the grid/mAs without the grid
what is a bucky grid
moving grid that moves back and forth
grid cut off
too many photons are blocked so quantum mottle/noisy image occurs
happens when grid is misaligned
air gap technique
separate patient from film/receptor to decrease scatter radiation from radiation missing the target (mammo)
how do decrease quantum mottle?
decrease quantum mottle/noise by increase mA/kVP
however increase mA over kVP
increase kVP could shift into Compton scattery territory and create more noiese via scatter
increased kVP would decrase mottle but incrase overall mottle
inverse square law ?
energy twice as far from source is spread out over 4x the area; quadrupling the xray photons will cut mottle in half
mA is the most important factor for sharpness, contrast, or noise/mottle?
noise/mottle
if you increase mA from 50–> 200 for an XR, how will this change mottle?
decrease 50%
for 4 cm of tissue, what is the increase needed in mA
double mA
which exam would use the lowest mA? AP chest, abdomen, foot?
foot. thinnest tissue.
which will reduce noise?
use of grid, use of airgap, increase mA
all of the above, however increase mA best answer
cause of most grid cut off
reposition grid first before increase mA
SNR
signal to noise ratio or contrast to noise ratio
a lesion with high contrast can tolerate noise and still be visible (scoliosis films with reduced mA because of the high contrast in bones)
factors that influence contrast to noise ratio
increase in MA (reduces noise, no effect on contrast) improves ratio
reduce kVP (improves contrast, noise is fixed); improves ratio
AEC
automatic exposure control
main image characteristics?
noise, spatial resolution, contrast
define spatial resolution
how close two lines can be to each other and still be visibly resolved; line pairs per mm
types of unsharpness
motion, system, geometric
motion unsharpness
loss of spatial resolution due to patient mostion
system unsharpness
loss of spatial resolution due to fault of detector
- film (size of grain of photographic chemical)
- CR: computed radiography, size of laser used to read phosphor plate in the cassette reader
- DR: size of individual thermoluminescent transistor
geometric unsharpness
caused by focal spot, source to object distance, object detector distance, magnification
size of focal spot on geometric unsharpness
smaller focal spot, less unsharpness, less blur
effect of SOD to geometric unsharpness
closer source to image, more blur
object detector distance to geometric unsharpness
closer to object, less blur
magnification to geometric unsharpness
more mag, more blur
small focal spots are used with what type of exam?
mammo, extremity exams to maximize spatial resolution
large focal spots are used when?
minimize motion artifacts (CXR) or when more mAs are used, to allow for reduction in exposure time
how is magnification calculated
SID/SOD, source to detector distance/source to object (patient) distance
sharpness definition
xray ability to define an edge
resolution definition
abiliy to differentiate between 2 closely approximated things
MTF
modulation transfer function: relationship between sharpness or contrast resolution/spatial resolution
ratio is less than 1: information recorded/information available
importance of MTF
can be used to compare different digital film systems or plain film etc
MTF rate tells you how efficient information recorded/available is translated
DQE
detective quantum efficiency; estimate of required exposure level to create optimal image; prediction of dose
comparison of image on detector to that of ideal detector via SNR at output to SNR at input as a function of spatial frequency
usually less than 1
higher DQE translates to?
lower dose
DQE relationships
proportional to MTF
inversely proportional to SNR
better at low spatial resolution; inversely proportional to spatial resolution/frequency
general DQE of DR
DR is usually around 0.45; CR or plain films ar around 0.25
increase in pixel density increases?
spatial resolution
pixel pitch
measurement from center of one pixel to the next aka pixel spacing
decreased pixel pitch
increases spatial resolution (pixels are closer together, can make out more detail)
what type of detector has the best spatial resolution?
photoconductor
bringing patient closer to source will have what effect on geometric unsharpness
make it worse
SOD decreased during exam, what can be changed to compensate for blur?
decrease focal spot size
increase in focal spot size improves?
blur/motion
relationship between kvp and contrast
more kvp , less contrast
kvp = quality
how od grid/collimation affect scatter? resolution?
reduce scatter, improve contrast resolution
film contrast
sensitometric curves?
high vs low subject contrast examples
high subject contrast: chest
low subjcect contrast: belly, mostly soft tissue
penetrance of kvp through thick vs thin tissue? effect of increasing kvp?
more xrays will strick the thinner tissue, however by increasing the kvp through different tissues the contrast will get closer to zero and no significant contrast
at low kvp, the difference between tissues is higher
higher kvp, smaller attenuation coefficients, less contrast
synonym for linear attenuation coefficient
thickness
attenuation occurs as compton or PE interaction
linear attenuation coefficient equation
compton LAC = density/KV
PE LAC = density x atomic number ^3 /kV
name some ways contrast can be improved?
decrease kV, increase atomic number (contrast, oral barium), increasing density (barium vs water vs gas)
reduce scatter (grid, air gap)
filtering the beam impact on beam energy
filteration = higher beam energy (low energy filtered out) and less contrast
increase number of bits? increased?
increase shades of gray
how does window width affect contrast
increased window decreases contrast
decreased window increases contrast
look up table
histogram of known input intensities and corresponding grayscale ; seen with digital imaging and affects contrast
primary factor influencing image contrast in digital vs film systems
look up table vs kVp
dynamic range in digital imaging vs film screen
wider dynamic range in digital range
window lvel vs width
level/center = change for brightness
wideth: change for contrast
level up/higher pixel value for looking at dark stuff/lungs
level down/lower pixel value: looking at bones
overexposed
high density number, blackening of film, too much mAs, small focal spot to detector/film distance
too many photons made it to detector
underexposed
increased mottle, too little mAs
exposure time
total time electrons hit target to create x-rays
relationship between exposure time, dose, mottle
- increase –> increase dose
- quadruple –> half mottle/decrease noise
- increase –> more motion/blur
AEC
automatic exposure control seen in digital imaging to stop exam after correct radiation registered
when is AEC used
digital systems, not usually seen with portable or bedside exams
increasing filter thickness will ? image contrast
decrease contrast
only highest xray energies will make it through
? has no change in contrast on a study ?
mAs; quantity will change amount of noise, not contrast
kV and grid ratio will change contrast
highest kV? abdomen, chest, extremity?
chest
- high intrinsic contrast within subject
- must penetrate ribs
is portable done with a grid?
usually no, typically has lower kV
?% change in mAs to see a change in density?
30% increas in mAs to change density
effect of changing kVp on density?
greater impact on density at low kVp
q
decrease kVP by 15%, double mA to maintain same degree of density
every 4 cm of tissue requires ? change in mA
double
SP RATIO
SECONDARY X RAY (scatter) VS PRIMARY X RAY
s/P ratio dependent on?
thickness of tissue and area of beam (thicker bodies/bigger areas create more scattor, increasing SP ratio)
ways to reduce scatter/improve contrast
collimate, compress(mammo), lower kVP (reduce compton, increase PE with increased dose to skni), use grid/air gap
technique for imaging newborns?
- grid
- kvp
- mA
don't use grid lower kVP lower mA (or similar)
moving children technique?
use pigg o stat
-increase kVP slightly to lower mAs
image optimization tactics for pediatrics? casts?
pediatrics:
- 0-5yo use 25% of mA as adult
- 6-12 use 50% mA as adult
casts:
fiberglass: n/a
plaster: increase mAs (2x when dry, 3x when wet)
p 105 chart on kvp in different disease states…
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