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XR Physics > 3. XR Concepts > Flashcards

3. XR Concepts Flashcards

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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
Q

main image characteristics?

A

noise, spatial resolution, contrast

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26
Q

define spatial resolution

A

how close two lines can be to each other and still be visibly resolved; line pairs per mm

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27
Q

types of unsharpness

A

motion, system, geometric

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28
Q

motion unsharpness

A

loss of spatial resolution due to patient mostion

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29
Q

system unsharpness

A

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
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30
Q

geometric unsharpness

A

caused by focal spot, source to object distance, object detector distance, magnification

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31
Q

size of focal spot on geometric unsharpness

A

smaller focal spot, less unsharpness, less blur

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32
Q

effect of SOD to geometric unsharpness

A

closer source to image, more blur

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33
Q

object detector distance to geometric unsharpness

A

closer to object, less blur

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34
Q

magnification to geometric unsharpness

A

more mag, more blur

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35
Q

small focal spots are used with what type of exam?

A

mammo, extremity exams to maximize spatial resolution

36
Q

large focal spots are used when?

A

minimize motion artifacts (CXR) or when more mAs are used, to allow for reduction in exposure time

37
Q

how is magnification calculated

A

SID/SOD, source to detector distance/source to object (patient) distance

38
Q

sharpness definition

A

xray ability to define an edge

39
Q

resolution definition

A

abiliy to differentiate between 2 closely approximated things

40
Q

MTF

A

modulation transfer function: relationship between sharpness or contrast resolution/spatial resolution

ratio is less than 1: information recorded/information available

41
Q

importance of MTF

A

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
Q

DQE

A

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
Q

higher DQE translates to?

A

lower dose

44
Q

DQE relationships

A

proportional to MTF
inversely proportional to SNR
better at low spatial resolution; inversely proportional to spatial resolution/frequency

45
Q

general DQE of DR

A

DR is usually around 0.45; CR or plain films ar around 0.25

46
Q

increase in pixel density increases?

A

spatial resolution

47
Q

pixel pitch

A

measurement from center of one pixel to the next aka pixel spacing

48
Q

decreased pixel pitch

A

increases spatial resolution (pixels are closer together, can make out more detail)

49
Q

what type of detector has the best spatial resolution?

A

photoconductor

50
Q

bringing patient closer to source will have what effect on geometric unsharpness

A

make it worse

51
Q

SOD decreased during exam, what can be changed to compensate for blur?

A

decrease focal spot size

52
Q

increase in focal spot size improves?

A

blur/motion

53
Q

relationship between kvp and contrast

A

more kvp , less contrast

kvp = quality

54
Q

how od grid/collimation affect scatter? resolution?

A

reduce scatter, improve contrast resolution

55
Q

film contrast

A

sensitometric curves?

56
Q

high vs low subject contrast examples

A

high subject contrast: chest

low subjcect contrast: belly, mostly soft tissue

57
Q

penetrance of kvp through thick vs thin tissue? effect of increasing kvp?

A

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
Q

synonym for linear attenuation coefficient

A

thickness

attenuation occurs as compton or PE interaction

59
Q

linear attenuation coefficient equation

A

compton LAC = density/KV

PE LAC = density x atomic number ^3 /kV

60
Q

name some ways contrast can be improved?

A

decrease kV, increase atomic number (contrast, oral barium), increasing density (barium vs water vs gas)

reduce scatter (grid, air gap)

61
Q

filtering the beam impact on beam energy

A

filteration = higher beam energy (low energy filtered out) and less contrast

62
Q

increase number of bits? increased?

A

increase shades of gray

63
Q

how does window width affect contrast

A

increased window decreases contrast

decreased window increases contrast

64
Q

look up table

A

histogram of known input intensities and corresponding grayscale ; seen with digital imaging and affects contrast

65
Q

primary factor influencing image contrast in digital vs film systems

A

look up table vs kVp

66
Q

dynamic range in digital imaging vs film screen

A

wider dynamic range in digital range

67
Q

window lvel vs width

A

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
Q

overexposed

A

high density number, blackening of film, too much mAs, small focal spot to detector/film distance
too many photons made it to detector

69
Q

underexposed

A

increased mottle, too little mAs

70
Q

exposure time

A

total time electrons hit target to create x-rays

71
Q

relationship between exposure time, dose, mottle

A
  • increase –> increase dose
  • quadruple –> half mottle/decrease noise
  • increase –> more motion/blur
72
Q

AEC

A

automatic exposure control seen in digital imaging to stop exam after correct radiation registered

73
Q

when is AEC used

A

digital systems, not usually seen with portable or bedside exams

74
Q

increasing filter thickness will ? image contrast

A

decrease contrast

only highest xray energies will make it through

75
Q

? has no change in contrast on a study ?

A

mAs; quantity will change amount of noise, not contrast

kV and grid ratio will change contrast

76
Q

highest kV? abdomen, chest, extremity?

A

chest

  • high intrinsic contrast within subject
  • must penetrate ribs
77
Q

is portable done with a grid?

A

usually no, typically has lower kV

78
Q

?% change in mAs to see a change in density?

A

30% increas in mAs to change density

79
Q

effect of changing kVp on density?

A

greater impact on density at low kVp
q
decrease kVP by 15%, double mA to maintain same degree of density

80
Q

every 4 cm of tissue requires ? change in mA

A

double

81
Q

SP RATIO

A

SECONDARY X RAY (scatter) VS PRIMARY X RAY

82
Q

s/P ratio dependent on?

A

thickness of tissue and area of beam (thicker bodies/bigger areas create more scattor, increasing SP ratio)

83
Q

ways to reduce scatter/improve contrast

A

collimate, compress(mammo), lower kVP (reduce compton, increase PE with increased dose to skni), use grid/air gap

84
Q

technique for imaging newborns?

  • grid
  • kvp
  • mA
A 
don't use grid
lower kVP
lower mA (or similar)
85
Q

moving children technique?

A

use pigg o stat

-increase kVP slightly to lower mAs

86
Q

image optimization tactics for pediatrics? casts?

A

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
Q

p 105 chart on kvp in different disease states…

A

x

XR Physics (6 decks)

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