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XR Physics > 3. XR Concepts > Flashcards

3. XR Concepts Flashcards

1
Q

noise and relationship with photons

A

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

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2
Q

quantum mottle? how to decrease

A

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

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3
Q

majority of scatter is?

A

compton interactions

characteristic radiation is generally too low energy

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4
Q

determinants of the amount of scatter

A

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)

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5
Q

role of collimator

A

restrict x-ray beam to limit patient exposure and reduce scatter by reducing field of view

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6
Q

effects of changing collimator

A

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

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7
Q

role of grid

A

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

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8
Q

what is grid ratio

A

describes density of grid; ratio of height of lead to distance between them

higher ratio the less scatter and the better the contrast

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9
Q

how does grid impact dose

A

higher grid ratio increases dose

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10
Q

what is the bucky factor?

A

mAs required with the grid/mAs without the grid

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11
Q

what is a bucky grid

A

moving grid that moves back and forth

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12
Q

grid cut off

A

too many photons are blocked so quantum mottle/noisy image occurs

happens when grid is misaligned

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13
Q

air gap technique

A

separate patient from film/receptor to decrease scatter radiation from radiation missing the target (mammo)

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14
Q

how do decrease quantum mottle?

A

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

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15
Q

inverse square law ?

A

energy twice as far from source is spread out over 4x the area; quadrupling the xray photons will cut mottle in half

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16
Q

mA is the most important factor for sharpness, contrast, or noise/mottle?

A

noise/mottle

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17
Q

if you increase mA from 50–> 200 for an XR, how will this change mottle?

A

decrease 50%

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18
Q

for 4 cm of tissue, what is the increase needed in mA

A

double mA

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19
Q

which exam would use the lowest mA? AP chest, abdomen, foot?

A

foot. thinnest tissue.

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20
Q

which will reduce noise?

use of grid, use of airgap, increase mA

A

all of the above, however increase mA best answer

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21
Q

cause of most grid cut off

A

reposition grid first before increase mA

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22
Q

SNR

A

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)

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23
Q

factors that influence contrast to noise ratio

A

increase in MA (reduces noise, no effect on contrast) improves ratio

reduce kVP (improves contrast, noise is fixed); improves ratio

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24
Q

AEC

A

automatic exposure control

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25
main image characteristics?
noise, spatial resolution, contrast
26
define spatial resolution
how close two lines can be to each other and still be visibly resolved; line pairs per mm
27
types of unsharpness
motion, system, geometric
28
motion unsharpness
loss of spatial resolution due to patient mostion
29
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
30
geometric unsharpness
caused by focal spot, source to object distance, object detector distance, magnification
31
size of focal spot on geometric unsharpness
smaller focal spot, less unsharpness, less blur
32
effect of SOD to geometric unsharpness
closer source to image, more blur
33
object detector distance to geometric unsharpness
closer to object, less blur
34
magnification to geometric unsharpness
more mag, more blur
35
small focal spots are used with what type of exam?
mammo, extremity exams to maximize spatial resolution
36
large focal spots are used when?
minimize motion artifacts (CXR) or when more mAs are used, to allow for reduction in exposure time
37
how is magnification calculated
SID/SOD, source to detector distance/source to object (patient) distance
38
sharpness definition
xray ability to define an edge
39
resolution definition
abiliy to differentiate between 2 closely approximated things
40
MTF
modulation transfer function: relationship between sharpness or contrast resolution/spatial resolution ratio is less than 1: information recorded/information available
41
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
42
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
43
higher DQE translates to?
lower dose
44
DQE relationships
proportional to MTF inversely proportional to SNR better at low spatial resolution; inversely proportional to spatial resolution/frequency
45
general DQE of DR
DR is usually around 0.45; CR or plain films ar around 0.25
46
increase in pixel density increases?
spatial resolution
47
pixel pitch
measurement from center of one pixel to the next aka pixel spacing
48
decreased pixel pitch
increases spatial resolution (pixels are closer together, can make out more detail)
49
what type of detector has the best spatial resolution?
photoconductor
50
bringing patient closer to source will have what effect on geometric unsharpness
make it worse
51
SOD decreased during exam, what can be changed to compensate for blur?
decrease focal spot size
52
increase in focal spot size improves?
blur/motion
53
relationship between kvp and contrast
more kvp , less contrast kvp = quality
54
how od grid/collimation affect scatter? resolution?
reduce scatter, improve contrast resolution
55
film contrast
sensitometric curves?
56
high vs low subject contrast examples
high subject contrast: chest low subjcect contrast: belly, mostly soft tissue
57
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
58
synonym for linear attenuation coefficient
thickness attenuation occurs as compton or PE interaction
59
linear attenuation coefficient equation
compton LAC = density/KV PE LAC = density x atomic number ^3 /kV
60
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)
61
filtering the beam impact on beam energy
filteration = higher beam energy (low energy filtered out) and less contrast
62
increase number of bits? increased?
increase shades of gray
63
how does window width affect contrast
increased window decreases contrast | decreased window increases contrast
64
look up table
histogram of known input intensities and corresponding grayscale ; seen with digital imaging and affects contrast
65
primary factor influencing image contrast in digital vs film systems
look up table vs kVp
66
dynamic range in digital imaging vs film screen
wider dynamic range in digital range
67
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
68
overexposed
high density number, blackening of film, too much mAs, small focal spot to detector/film distance too many photons made it to detector
69
underexposed
increased mottle, too little mAs
70
exposure time
total time electrons hit target to create x-rays
71
relationship between exposure time, dose, mottle
- increase --> increase dose - quadruple --> half mottle/decrease noise - increase --> more motion/blur
72
AEC
automatic exposure control seen in digital imaging to stop exam after correct radiation registered
73
when is AEC used
digital systems, not usually seen with portable or bedside exams
74
increasing filter thickness will ? image contrast
decrease contrast only highest xray energies will make it through
75
? has no change in contrast on a study ?
mAs; quantity will change amount of noise, not contrast kV and grid ratio will change contrast
76
highest kV? abdomen, chest, extremity?
chest - high intrinsic contrast within subject - must penetrate ribs
77
is portable done with a grid?
usually no, typically has lower kV
78
?% change in mAs to see a change in density?
30% increas in mAs to change density
79
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
80
every 4 cm of tissue requires ? change in mA
double
81
SP RATIO
SECONDARY X RAY (scatter) VS PRIMARY X RAY
82
s/P ratio dependent on?
thickness of tissue and area of beam (thicker bodies/bigger areas create more scattor, increasing SP ratio)
83
ways to reduce scatter/improve contrast
collimate, compress(mammo), lower kVP (reduce compton, increase PE with increased dose to skni), use grid/air gap
84
technique for imaging newborns? - grid - kvp - mA
``` don't use grid lower kVP lower mA (or similar) ```
85
moving children technique?
use pigg o stat | -increase kVP slightly to lower mAs
86
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)
87
p 105 chart on kvp in different disease states...
x

XR Physics (6 decks)

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