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AJT Physics > PHYSICS - CT > Flashcards

PHYSICS - CT Flashcards

1
Q

Most modern CT scanners are what generation?

A

3rd generation (tube and detector spin in synchrony)

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2
Q

4th generation CT scanner

A

360 degree ring of DELs with rotating x-ray tube (no ring artifact)

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3
Q

Axial acquisiton

A

a.k.a. step-and-shoot; better spatial resolution in Z-direction (more projections per slice), no partial volume artifact in Z-axis

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4
Q

Helical acquisition

A

faster, flexible slice selection, reduced stair-step artifact, susceptible to partial volume artifact (due to interpolation), less discontinuity of moving anatomy between different slices (e.g. bowel)

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5
Q

Effective mAs

A

(mA * exposure time) / beam pitch, or mAs / beam pitch

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6
Q

Tube current modulation methods

A

based on topogram and/or on-the-fly

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7
Q

Anode-cathode axis relative to imaging plane

A

perpendicular (to decrease heel effect)

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8
Q

Slice width in SDCT

A

determined by beam collimation and equal to beam width

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9
Q

Slice width in MDCT

A

determined by DEL width and whether signal from adjacent rows are summed

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10
Q

Cone-shaped beam

A

3-dimensional, used in MDCT

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11
Q

Fan-shaped beam

A

2-dimensional, used in SDCT

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12
Q

Detector aperture size

A

a.k.a. DEL width (in Z-direction)

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13
Q

Determinants of in-plane spatial resolution

A

focal spot size, DEL size

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14
Q

Slice pitch

A

a.k.a. detector pitch; table movement per tube revolution / single DEL width

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15
Q

Collimator pitch

A

a.k.a. beam bitch; table movement per tube revolution / beam width

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16
Q

Relationship between pitch and spatial resolution

A

higher pitch => fewer projections => more interpolation required => lower Z-axis spatial resolution

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17
Q

Effect of a larger pitch

A

faster scan, lower dose, decreased spatial resolution, decreased SNR

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18
Q

Effect of a smaller pitch

A

longer scan, higher dose, increased spatial resolution, increased SNR

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19
Q

Pre-patient collimation

A

occurs in x/y and z directions; defines beam width and reduces overscan

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20
Q

Post-patient collimation

A

occurs only in z direction; scatter reduction

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21
Q

Geometric efficiency

A

% of beam that hits the detector; SDCT > MDCT with more rows > MDCT with fewer rows

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22
Q

Linear attenuation coefficient

A

describes attenuation per unit length of tissue; varies with kVp and tissue properties (Z and density); expressed in cm^-1; different for ice, water, and water vapor

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23
Q

Effect of increasing kVp on μ (LAC)

A

smaller μ (less beam attenuation per unit length of tissue)

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24
Q

Effect of increasing tissue Z on μ (LAC)

A

larger μ (more beam attenuation per unit length of tissue)

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25
Effect of increasing tissue density on μ (LAC)
larger μ (more beam attenuation per unit length of tissue)
26
μ (LAC) for photon energies at the k-edge is increased or decreased?
increased μ (more beam attenuation per unit length of tissue at the k-edge)
27
Relationship between μ (LAC) and HVL
inversely related; materials with a higher μ have a smaller HVL
28
Mass attenuation coefficient
describes attenuation per unit mass of tissue; expressed in g^-1; same for ice, water, and water vapor
29
Water is always ___ HU
0 HU
30
Effect of reconstructing thicker slices from thinner slices
increased SNR, increased partial volume artifact, decreased spatial resolution in the Z-direction
31
Effect of a sharpening filter
increased spatial resolution, increased noise (decreased SNR)
32
Effect of a smoothing filter
decreased spatial resolution, decreased noise (increased SNR)
33
T/F - filters/kernals (in FBR) do not change voxel data
false; filters change voxel data (unlike window/level)
34
Advantage of iterative reconstruction
increased SNR at a given dose (or dose can be reduced while maintaining SNR), reduced streak artifact
35
180 degree axial interpolation (vs. 360 degree)
better spatial resolution, more accurate reconstructions, less partial volume artifact
36
Effect of decreasing pixel size on resolution and SNR
increased spatial resolution, decreased SNR (fewer photons per pixel)
37
Major factors affecting spatial resolution
pitch, slice thickness, DEL/pixel size, # of projections per slice, focal spot size, patient motion, reconstruction algorithm
38
Increased slice sensitivity profile (SSP) equates to...
decreased spatial resolution in the Z-direction; increased pitch => increased SSP => decreased resolution
39
SSP is increased by...
increasing pitch, using 360 interpolation (vs. 180), and detector binning => wider SSP => lower spatial resolution
40
How to: increase number of photons per voxel
increase slice thickness, increase DFOV, increase mAs, or decrease pitch; more photons per voxel => less noise
41
Effect of decreasing kVp
increased contrast, increased quantum mottle, decreased dose; may need to increase mA to maintain signal
42
How to: reduce dose
decrease kVp, decrease mAs, increase pitch, tube current modulation; also better reconstruction techniques
43
Cardiac imaging best performed during which part of the cardiac cycle
diastole
44
Prospective cardiac imaging
lower dose, always axial, no functional imaging, susceptible to motion (beta-blocker is essential)
45
Retrospective cardiac imaging
higher dose, helical (low pitch), functional imaging is possible, less susceptible to motion; use if CI to beta-blocker
46
Contraindication to beta-blocker - use prospective or retrospective gating?
retrospective
47
CT fluoro
lower mAs, same kVp; continuous and intermittent modes
48
Continuous mode CT fluoro
near real time; "first in, first out" image reconstruction
49
Advantages of dual source CT
faster acquistion, tissue characterization (with dual energy)
50
Contrast timing techniques
empirical, test bolus, bolus tracking
51
CT phantom sizes
16 cm for head, 32 cm for abdomen; contains central and peripheral pencil ionization chambers
52
Central and peripheral dose for head and abdomen
same for CT head; central dose is less than peripheral in CT abdomen
53
Dose (CTDIvol) is over or under-estimated for obese patients?
over-estimated
54
Dose (CTDIvol) is over or under-estimated for pediatric patients?
under-estimated
55
Adding doses together across multiple acquistions/phases
DLPs can be added together, while CTDIvol cannot
56
Z-axis variation
“tails” of radiation along the edge of the area being scanned; radiation profile is not limited to the primary area being scanned
57
Z-axis variation greater for SDCT or MDCT?
MDCT has greater Z-axis variation
58
How to: fix beam hardening artifact
tilt gantry or patient positioning, pre-harden beam (bowtie filter), calibration using a phantom, reconstruction algorithms; increasing kVp would also decrease beam hardening (per Ram)
59
How to: fix partial volume artifact
acquire thinner slices
60
How to: fix partial volume averaging
decreasing pitch, use thinner detector rows, use axial acquisition (instead of helical), reconstruct with thinner slices
61
How to: fix photon starvation
tube current modulation, adaptive filtration to correct attenuation profile
62
How to: fix undersampling
acquire as many projections as possible per rotation (for view aliasing) or utilize high resolution techniques (for ray aliasing)
63
How to: fix streak artifact from metal
thinner slices, higher kVp, remove metal (if possible), interpolation software
64
How to: fix patient motion
align scanner in the primary direction of motion, overscanning, gating, faster scanner, restrain patient
65
How to: fix incomplete projection artifact
position patient properly (e.g. move arms above head)
66
Ring artifact is specific to...
3rd generation CT scanners
67
How to: fix stair step artifact
thinner slices, reconstruct with overlapping slices
68
CTDIvol and DLP for typical head CT
60 mGy and 1000 mGy-cm
69
CTDIvol and DLP for typical abdomen/pelvis CT
15 mGy and 500 mGy-cm
70
Standard matrix size and bit depth
512 x 512, bit depth is 12
71
Overscanning
pitch <1
72
Effect of lower kVp on HU
lower kVp => higher HU, and vice-versa
73
Risk of fetal thyroid dysfunction with...
maternal IV contrast administration; considered a negligible risk
74
kVp for standard adult
120 kVp (range is 80-140); increased to 140 kVp for obese patients
75
Increasing number of photons 2x increases SNR by what factor?
1.4x
76
Changing kVp generally requires a change in...
mAs to maintain balance between dose and quantum mottle
77
mAs and kVp adjustments for pediatric imaging
decrease kVp (because thinner object) => need to increase mAs to avoid quantum mottle
78
Breast dose reduction strategies
reduce mA, tube current modulation, bismuth shield
79
Noise is proportional to...
1 / sqrt(number of photons per voxel); relates to mA, exposure time, and slice thickness
80
FBP vs. BP
FBP sharpens sinogram data prior to back projection; reduces streaking
81
Better SSP: 180 vs. 360 interpolation?
180 degree interpolation
82
Better SSP: axial vs. helical acquisition?
axial acquisition (no interpolation required)
83
Size specific dose index (SSDI)
SSDI = f * CTDIvol, where f >1 for obese and f <1 for peds
84
Sinogram axes
x-axis is DEL, y-axis is projection number

AJT Physics (7 decks)

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