CT and radiography Flashcards
1
Q
CT matrix size
A
512x512, 2 bytes/pixel, 0.5 MB
2
Q
CT spatial resolution
A
100 um FWHM
-depends on FOV, detector size, slice thickness
3
Q
daignostic CT effective dose
A
15 mSv
4
Q
dose for CT taken for CT/PET for anatomy and attenuation correction purposes
A
2 mSv
5
Q
what must be done to mAs if kV is increased by 15% to maintain Kair at image receptor
A
halve mAs
6
Q
k edge of iodine
A
33 keV
7
Q
what are filters made from
A
mostly Al, some from Cu (unless mammo)
8
Q
lung Z and bone Z
A
Z = 7.5 lung
Z= 12 bone
9
Q
what is beam intensity proportional to?
A
kV^2
mAs
10
Q
CT beam quality
A
HVL is 6-10 mm Al
11
Q
radiography beam quality
A
HVL is 5 mm Al
12
Q
Kair at image receptor
A
a few uGy
13
Q
Mo kedge
A
20 keV
14
Q
CsI kedge
A
36 and 33 keV
15
Q
Rh kedge
A
23 keV
16
Q
Ag k edge
A
25 keV
17
Q
W k edge
A
70 keV
18
Q
Pb k edge
A
88 keV
19
Q
Ba k edge
A
36 keV
20
Q
O2 k edge
A
0.5 keV
21
Q
Ca k edge
A
4 keV
22
Q
film with steeper gradient means what contrast?
A
better contrast
23
Q
what % of xrays does film absorb vs screen film?
A
-film absorbs 1 %
-screen film absorbs 50%
24
Q
what has BEST resolution?
A
photoconductor
-because charge is not dispersed by detection
25
why is there less scatter for lower tube kV and high Z?
PE will dominate over Compton
26
when to use high vs low latitude
latitude= dynamic range
high latitude- chest radiography (don't need high contrast)
27
dynamic range of digital detectors vs screen film
digital handles 100X higher than screen-film
28
max film OD
2 (1% transmittance)
Useful range is 0.3 (50% transmittance) to 2
29
how many bits in a byte
8
30
radiography and film matrix size
2560x2048, 2 bytes/pixel, 10 MB
31
grid ratio
strip height H/gap width D
32
bucky factor
dose with grid/dose without grid
33
OD
log(Io/I)
34
target film OD
1.5
35
base+ fog film OD
0.2
36
what can human eye resolve?
5 lp/mm
37
Nyquist frequency
limiting resolution for sampling of 2 pixels/mm is 1 lp/mm
i.e. half the sampling is limiting
38
CT resolution in lp/mm
0.7 lp/mm
39
radiography resolution
5 lp/mm
40
what is MTF
resolution as a function of frequency
-output to input modulation
41
does motion blur depend on magnification?
no but magnification increases time because focal spot is smaller, so motion can increase
42
relate sampling frequency to pixel size
sampling frequency = 1/pixel size
43
number of photons /mm2 for radiography
10^5/mm2
44
Rose Model
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
45
specificity
TN/(TN+FP)
46
sensitivity
TP/(TP+FN)
47
negative predictive value
TN/(TN+FN)
48
positive predictive value
TP/(TP+FP)
PP= 3 ps
49
effect on contrast with kV
increasing kV reduces contrast but decreases mottle due to more penetration
total effect on CNR depends on Z
50
relate FWHM to lp/mm
lp/mm = 1/2FWHM
51
what do you plot on ROC curve
sensitivity vs 1- specificity
52
accuracy of ROC curve
(TP+TN)/(TP+TN+FP+FN)
53
Median kerma area product in radiography
1 Gy cm2
54
usual radiography skin dose
<10 mGy
55
entrance Kair
0.1-10 mGy
56
integral dose for 70 kg patient who gets 1 Gy
1 J/kg * 70 kg = 70 J
57
LET of xray gamma ray, beta
1 keV/um
58
LET of alpha particles
100 keV/um
59
effective dose of most radiography exams
0.1-1 mSv
60
effective dose of CT exams
1 mSv to over 10 mSv
61
scatter dose at 1 m from patient
0.1% of entrance skin dose
62
how to reduce heel effect
-increase anode angle
-increase SID
-decrease FS
63
exposure index
measure of Kair at image receptor
1 uGy is EI of 100
64
entrance Kair order
a few mGy
65
resolution of digital detectors vs screen film
3 lp/mm vs 6 lp/mm
66
equation for magnification
SID/SOD
67
HVL for kV os 60,80,100,120
2,3,4,5 mm Al
68
deviation index
quantifies how closely Kair at receptor matches target value
+/- 3 means exposure is double or half of target Kair
69
SID
usually 100 cm
180 cm for chest xray
70
does contrast increase with mAs?
no, but CNR increases because noise is proportional to root(mAs)
71
dose index
peak skin dose/IRP Kair, 0.5-0.8
72
what is IRP
interventional reference point
15 cm closer to focal spot than system iso
IRP Kair excludes backscatter, is conservative estimate of patient skin dose
73
KRP in interventional radiography
200 Gycm2
74
effective dose interventional radiography
30 mSv
75
typical peak skin dose interventional radiography
2 Gy
76
entrance Kair interventional radiography
3 mSv/image
77
receptor Kair interventional radiography
3 uGy/image
78
where would worker get most scatter from patient?
where beam enters patient
79
dose from lateral projections vs AP
double
80
what are bowtie filters made of
Teflon
81
% of incident fluence absorbed by CT detectors
90%
82
sinogram
projection vs xray tube angle
83
CT rotation speed
1 rps
84
effective mAs
true mAs/pitch
85
window/level for head scan
80/40
86
tissue HVL for typical CT
4 cm
87
angular modulation
-change current as xray tube rotates around patient
-current for lat > current for AP/PA
-current in chest < current in abdomen
88
how many projections does CT acquire for a single projection>
1000
89
CT detector width
0.625 mm
typically 64 slices in each beam width
90
how to improve resolution in CT
rays acquired at 0 and 180 degrees are offset by 1/2 detector width
91
window/level for chest
1500/-500 for lung
400/50 for mediastinum
92
how many shades of grey with 12 bits
2^12= 4096
93
window/level for liver
150/60
94
pitch
L/NT
<1 is oversampled
95
axial vs helical CT
axial: table still as tube rotates
helical: table moves as tube rotates. Reduces scan time
96
head CTDI
60 mGy
97
chest CTDI
10 mGy
98
abdo CTDI
15 mGy
99
pelvis CTDI
5 mGy
100
what happens to mottle if you multiply mA by 4?
mottle reduced to 1/2
101
embryo dose from CT
-primarily from internal patient scatter
-Pb apron wouldn't work
-0.1 mGy for chest CT
102
what is kq factor
converts DLP to effective dose in CT
-depends on age, size, phantom used for CTDI
-independent of kV
103
cause of ring artifact
bad detector
104
cause of multiple ring artifacts
CT not calibrated
105
beam hardening artifact image
106
motion artifact image
107
windmill artifact image
108
film speed vs light needed for development
film speed ~ 1/light needed for development
film speed ~ 1/thickness^2
film speed~ # of light photons generated
109
how thick is intensifying screen
200 um
110
conversion efficiency of scintillator
% of absorbed energy converted into light
2-20%
111
CR plate -what does each color do
red light- simulate and empty electron traps
-blue light- emitted and measured
white light- used to erase
112
what is big gamma in film?
max slope of characteristic curve
3.32 max contrast factor
113
change in HD curve for overprocessed film
curve shifts to left (more sensitive)
rise in toe
less contrast (slope is less steep)
114
change in HD curve for underprocessed film
curve shifts right (less sensitive)
less contrast
115
what is detector geometric efficiency proportional to?
area/(source to detector distance)^2
116
amplifier gain
log(Vout/Vin)
117
% contrast required in CT vs radiography for detections
0.3% in CT
3% in radiography
CT improves soft tissue contrast
118
formula to estimate CTDI for head vs body
head- 0.2 mGy/mAs
body- 0.1 mGy/mAs
119
what happens to contrast as latitude increases?
contrast decreases
120
%mAs to use for infant vs large adult
-infant: -45% mAs
-large adult: +60% mAs
121
how many cervical vertebrae do humans have?
7
122
most appropriate exam to check for kidney stone
CT
123
equation for DQE
DQE = (SNRout/SNRin)^2
SNRin= N/root(N)
SNRout=N(1-exp(-ux))/root(N(1-exp(ux)))
124
quantum sink
-limiting stage with worse SNR as it has the fewest quanta
-cannot improve SNR without improving this stage
125
probability of A if A occurs m(A) times in M repetitions
integral(m(A)/M
126
probability of A or B
P(A)+P(B)
127
probability of A and B
P(A)*P(B)
128
probability of A given B
P(A)*P(B)/P(B)
129
probability density function
P(x=X)=integral(p(x)dx)
130
expectation value
131
what is characteristic function
FT of PDF
132
what is wiener spectrum
FT of auto-correlation function
133
approximation of exp(-ux)
=1-ux for u<<<1
134
noise equivalent quanta NEQ
ideal NEQ = q
more noise will make quanta appear to be reduced
DQE=NEQ/true input
135
does amplification improve DQE?
no because it also amplifies noise
136
the three film efficiencies
-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
characteristics of high speed film system
-lower patient dose
-more mottle because less quanta are required
-reduced latitude
-higher contrast
138
equation showing how scatter degrades contrast
w/o scatter, contrast = (P2-P1)/P
with scatter, contrast = (P2-P1)/(P+S)
139
dark field imaging
makes use of the scatter
140
how to calculate slice thickness in helical scanning?
-measure FWHM of bead sensitivity profile as function of z position
141
CT uniformity test
subtract middle from periphery
-should be within 2 HU of baseline and baseline should be within 5 HU
142
IGRT FBCT kV and MV dose
-0.5 to 10 mSv for kV
-10-30 mSv for MV
143
IGRT CBCT kV and MV dose
1-35 mSv kV
30-100 mSv MV
144
chest radiograph imaging parameters
85-140 kVp, 5 mA, 5 ms
145
abdo/pelvis radiograph imaging parameters
80 kVp, 1000 mA, 50 ms
146
skull radiograph imaging parameters
70 kVp, 40 mAs
147
CT imaging parameters
120 kVp, 750 mA, 170-240 mAs
148
typical radiography filter
3 mm Al
149
CT typical filter
2 mm Al + 0.1 mm Cu
150
central slice theorem
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
power rating of CT
100 kW
152
power rating of xray
25 kW
153
film screen resolution vs film resolution
5 lp/mm vs 100 lp/mm
154
CT resolution
0.8 lp/mm
155
kV CBCT resolution
6-9 lp/mm
pretty sure this is wrong
156
MV CBCT resolution
< 0.4 lp/mm
157
for what voltage are tubes space charge limited
<40 kV
158
output proportionality to kV for CT
output proportional to kV^2.6
159
what are pixels in fourier space
pixels are squares which are sincs in fourier space
160
most common detector for a direct radiography system
a-Se
161
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.
162
SI unit of air kerma
1 Gy
