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Imaging Review CCPM > CT and radiography > Flashcards

CT and radiography Flashcards

1
Q

CT matrix size

A

512x512, 2 bytes/pixel, 0.5 MB

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2
Q

CT spatial resolution

A

100 um FWHM
-depends on FOV, detector size, slice thickness

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3
Q

daignostic CT effective dose

A

15 mSv

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4
Q

dose for CT taken for CT/PET for anatomy and attenuation correction purposes

A

2 mSv

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5
Q

what must be done to mAs if kV is increased by 15% to maintain Kair at image receptor

A

halve mAs

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6
Q

k edge of iodine

A

33 keV

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7
Q

what are filters made from

A

mostly Al, some from Cu (unless mammo)

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8
Q

lung Z and bone Z

A

Z = 7.5 lung
Z= 12 bone

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9
Q

what is beam intensity proportional to?

A

kV^2
mAs

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10
Q

CT beam quality

A

HVL is 6-10 mm Al

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11
Q

radiography beam quality

A

HVL is 5 mm Al

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12
Q

Kair at image receptor

A

a few uGy

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13
Q

Mo kedge

A

20 keV

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14
Q

CsI kedge

A

36 and 33 keV

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15
Q

Rh kedge

A

23 keV

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16
Q

Ag k edge

A

25 keV

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17
Q

W k edge

A

70 keV

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18
Q

Pb k edge

A

88 keV

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19
Q

Ba k edge

A

36 keV

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20
Q

O2 k edge

A

0.5 keV

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21
Q

Ca k edge

A

4 keV

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22
Q

film with steeper gradient means what contrast?

A

better contrast

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23
Q

what % of xrays does film absorb vs screen film?

A

-film absorbs 1 %
-screen film absorbs 50%

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24
Q

what has BEST resolution?

A

photoconductor
-because charge is not dispersed by detection

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25
Q

why is there less scatter for lower tube kV and high Z?

A

PE will dominate over Compton

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26
Q

when to use high vs low latitude

A

latitude= dynamic range
high latitude- chest radiography (don’t need high contrast)

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27
Q

dynamic range of digital detectors vs screen film

A

digital handles 100X higher than screen-film

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28
Q

max film OD

A

2 (1% transmittance)
Useful range is 0.3 (50% transmittance) to 2

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29
Q

how many bits in a byte

A

8

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30
Q

radiography and film matrix size

A

2560x2048, 2 bytes/pixel, 10 MB

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31
Q

grid ratio

A

strip height H/gap width D

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32
Q

bucky factor

A

dose with grid/dose without grid

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33
Q

OD

A

log(Io/I)

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34
Q

target film OD

A

1.5

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35
Q

base+ fog film OD

A

0.2

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36
Q

what can human eye resolve?

A

5 lp/mm

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37
Q

Nyquist frequency

A

limiting resolution for sampling of 2 pixels/mm is 1 lp/mm
i.e. half the sampling is limiting

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38
Q

CT resolution in lp/mm

A

0.7 lp/mm

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39
Q

radiography resolution

A

5 lp/mm

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40
Q

what is MTF

A

resolution as a function of frequency
-output to input modulation

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41
Q

does motion blur depend on magnification?

A

no but magnification increases time because focal spot is smaller, so motion can increase

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42
Q

relate sampling frequency to pixel size

A

sampling frequency = 1/pixel size

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43
Q

number of photons /mm2 for radiography

A

10^5/mm2

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44
Q

Rose Model

A

SNR>5 = lesion detected. CNR has no absolute meaning, it is only relative
SNR= contrast*rootN
N= number of photons/area
contrast=(phi-phiROI)/phi

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45
Q

specificity

A

TN/(TN+FP)

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46
Q

sensitivity

A

TP/(TP+FN)

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47
Q

negative predictive value

A

TN/(TN+FN)

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48
Q

positive predictive value

A

TP/(TP+FP)

PP= 3 ps

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49
Q

effect on contrast with kV

A

increasing kV reduces contrast but decreases mottle due to more penetration
total effect on CNR depends on Z

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50
Q

relate FWHM to lp/mm

A

lp/mm = 1/2FWHM

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51
Q

what do you plot on ROC curve

A

sensitivity vs 1- specificity

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52
Q

accuracy of ROC curve

A

(TP+TN)/(TP+TN+FP+FN)

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53
Q

Median kerma area product in radiography

A

1 Gy cm2

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54
Q

usual radiography skin dose

A

<10 mGy

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55
Q

entrance Kair

A

0.1-10 mGy

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56
Q

integral dose for 70 kg patient who gets 1 Gy

A

1 J/kg * 70 kg = 70 J

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57
Q

LET of xray gamma ray, beta

A

1 keV/um

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58
Q

LET of alpha particles

A

100 keV/um

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59
Q

effective dose of most radiography exams

A

0.1-1 mSv

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60
Q

effective dose of CT exams

A

1 mSv to over 10 mSv

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61
Q

scatter dose at 1 m from patient

A

0.1% of entrance skin dose

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62
Q

how to reduce heel effect

A

-increase anode angle
-increase SID
-decrease FS

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63
Q

exposure index

A

measure of Kair at image receptor
1 uGy is EI of 100

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64
Q

entrance Kair order

A

a few mGy

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65
Q

resolution of digital detectors vs screen film

A

3 lp/mm vs 6 lp/mm

66
Q

equation for magnification

A

SID/SOD

67
Q

HVL for kV os 60,80,100,120

A

2,3,4,5 mm Al

68
Q

deviation index

A

quantifies how closely Kair at receptor matches target value
+/- 3 means exposure is double or half of target Kair

69
Q

SID

A

usually 100 cm
180 cm for chest xray

70
Q

does contrast increase with mAs?

A

no, but CNR increases because noise is proportional to root(mAs)

71
Q

dose index

A

peak skin dose/IRP Kair, 0.5-0.8

72
Q

what is IRP

A

interventional reference point
15 cm closer to focal spot than system iso
IRP Kair excludes backscatter, is conservative estimate of patient skin dose

73
Q

KRP in interventional radiography

A

200 Gycm2

74
Q

effective dose interventional radiography

A

30 mSv

75
Q

typical peak skin dose interventional radiography

A

2 Gy

76
Q

entrance Kair interventional radiography

A

3 mSv/image

77
Q

receptor Kair interventional radiography

A

3 uGy/image

78
Q

where would worker get most scatter from patient?

A

where beam enters patient

79
Q

dose from lateral projections vs AP

A

double

80
Q

what are bowtie filters made of

A

Teflon

81
Q

% of incident fluence absorbed by CT detectors

A

90%

82
Q

sinogram

A

projection vs xray tube angle

83
Q

CT rotation speed

A

1 rps

84
Q

effective mAs

A

true mAs/pitch

85
Q

window/level for head scan

A

80/40

86
Q

tissue HVL for typical CT

A

4 cm

87
Q

angular modulation

A

-change current as xray tube rotates around patient
-current for lat > current for AP/PA
-current in chest < current in abdomen

88
Q

how many projections does CT acquire for a single projection>

A

1000

89
Q

CT detector width

A

0.625 mm
typically 64 slices in each beam width

90
Q

how to improve resolution in CT

A

rays acquired at 0 and 180 degrees are offset by 1/2 detector width

91
Q

window/level for chest

A

1500/-500 for lung
400/50 for mediastinum

92
Q

how many shades of grey with 12 bits

A

2^12= 4096

93
Q

window/level for liver

A

150/60

94
Q

pitch

A

L/NT
<1 is oversampled

95
Q

axial vs helical CT

A

axial: table still as tube rotates
helical: table moves as tube rotates. Reduces scan time

96
Q

head CTDI

A

60 mGy

97
Q

chest CTDI

A

10 mGy

98
Q

abdo CTDI

A

15 mGy

99
Q

pelvis CTDI

A

5 mGy

100
Q

what happens to mottle if you multiply mA by 4?

A

mottle reduced to 1/2

101
Q

embryo dose from CT

A

-primarily from internal patient scatter
-Pb apron wouldn’t work
-0.1 mGy for chest CT

102
Q

what is kq factor

A

converts DLP to effective dose in CT
-depends on age, size, phantom used for CTDI
-independent of kV

103
Q

cause of ring artifact

A

bad detector

104
Q

cause of multiple ring artifacts

A

CT not calibrated

105
Q

beam hardening artifact image

A
106
Q

motion artifact image

A
107
Q

windmill artifact image

A
108
Q

film speed vs light needed for development

A

film speed ~ 1/light needed for development

film speed ~ 1/thickness^2
film speed~ # of light photons generated

109
Q

how thick is intensifying screen

A

200 um

110
Q

conversion efficiency of scintillator

A

% of absorbed energy converted into light
2-20%

111
Q

CR plate -what does each color do

A

red light- simulate and empty electron traps
-blue light- emitted and measured
white light- used to erase

112
Q

what is big gamma in film?

A

max slope of characteristic curve
3.32 max contrast factor

113
Q

change in HD curve for overprocessed film

A

curve shifts to left (more sensitive)
rise in toe
less contrast (slope is less steep)

114
Q

change in HD curve for underprocessed film

A

curve shifts right (less sensitive)
less contrast

115
Q

what is detector geometric efficiency proportional to?

A

area/(source to detector distance)^2

116
Q

amplifier gain

A

log(Vout/Vin)

117
Q

% contrast required in CT vs radiography for detections

A

0.3% in CT
3% in radiography
CT improves soft tissue contrast

118
Q

formula to estimate CTDI for head vs body

A

head- 0.2 mGy/mAs
body- 0.1 mGy/mAs

119
Q

what happens to contrast as latitude increases?

A

contrast decreases

120
Q

%mAs to use for infant vs large adult

A

-infant: -45% mAs
-large adult: +60% mAs

121
Q

how many cervical vertebrae do humans have?

A

7

122
Q

most appropriate exam to check for kidney stone

A

CT

123
Q

equation for DQE

A

DQE = (SNRout/SNRin)^2
SNRin= N/root(N)
SNRout=N(1-exp(-ux))/root(N(1-exp(ux)))

124
Q

quantum sink

A

-limiting stage with worse SNR as it has the fewest quanta
-cannot improve SNR without improving this stage

125
Q

probability of A if A occurs m(A) times in M repetitions

A

integral(m(A)/M

126
Q

probability of A or B

A

P(A)+P(B)

127
Q

probability of A and B

A

P(A)*P(B)

128
Q

probability of A given B

A

P(A)*P(B)/P(B)

129
Q

probability density function

A

P(x=X)=integral(p(x)dx)

130
Q

expectation value <x^n>

A

<x^n>=integral(x^np(x)dx
<(x-xbar>^n=integral(x-xbar)^np(x)dx

131
Q

what is characteristic function

A

FT of PDF

132
Q

what is wiener spectrum

A

FT of auto-correlation function

133
Q

approximation of exp(-ux)

A

=1-ux for u«<1

134
Q

noise equivalent quanta NEQ

A

ideal NEQ = q
more noise will make quanta appear to be reduced
DQE=NEQ/true input

135
Q

does amplification improve DQE?

A

no because it also amplifies noise

136
Q

the three film efficiencies

A

-xrays absorbed by screen/xrays incident = 30-80%
-number of optical photons x their energy/number of photons absorbed times their energy = 5%
-optical energy that escapes screen= 50%

137
Q

characteristics of high speed film system

A

-lower patient dose
-more mottle because less quanta are required
-reduced latitude
-higher contrast

138
Q

equation showing how scatter degrades contrast

A

w/o scatter, contrast = (P2-P1)/P
with scatter, contrast = (P2-P1)/(P+S)

139
Q

dark field imaging

A

makes use of the scatter

140
Q

how to calculate slice thickness in helical scanning?

A

-measure FWHM of bead sensitivity profile as function of z position

141
Q

CT uniformity test

A

subtract middle from periphery
-should be within 2 HU of baseline and baseline should be within 5 HU

142
Q

IGRT FBCT kV and MV dose

A

-0.5 to 10 mSv for kV
-10-30 mSv for MV

143
Q

IGRT CBCT kV and MV dose

A

1-35 mSv kV
30-100 mSv MV

144
Q

chest radiograph imaging parameters

A

85-140 kVp, 5 mA, 5 ms

145
Q

abdo/pelvis radiograph imaging parameters

A

80 kVp, 1000 mA, 50 ms

146
Q

skull radiograph imaging parameters

A

70 kVp, 40 mAs

147
Q

CT imaging parameters

A

120 kVp, 750 mA, 170-240 mAs

148
Q

typical radiography filter

A

3 mm Al

149
Q

CT typical filter

A

2 mm Al + 0.1 mm Cu

150
Q

central slice theorem

A

taking a one-dimensional Fourier transform of a projection is equivalent to taking the two-dimensional Fourier transform and evaluating it along one direction in frequency space.

151
Q

power rating of CT

A

100 kW

152
Q

power rating of xray

A

25 kW

153
Q

film screen resolution vs film resolution

A

5 lp/mm vs 100 lp/mm

154
Q

CT resolution

A

0.8 lp/mm

155
Q

kV CBCT resolution

A

6-9 lp/mm

pretty sure this is wrong

156
Q

MV CBCT resolution

A

< 0.4 lp/mm

157
Q

for what voltage are tubes space charge limited

A

<40 kV

158
Q

output proportionality to kV for CT

A

output proportional to kV^2.6

159
Q

what are pixels in fourier space

A

pixels are squares which are sincs in fourier space

160
Q

most common detector for a direct radiography system

A

a-Se

161
Q

For the adult, the values of k (mSv/mGy-cm) are 0.0021, 0.0059, 0.014, 0.015, and 0.015 for adult head, neck, chest, abdomen, and pelvis, respectively. Note that for children, the corresponding coefficients are higher, indicating higher effective doses per unit of DLP for children than for adults.

A
162
Q

SI unit of air kerma

A

1 Gy

Imaging Review CCPM (11 decks)

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