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AJT Physics > PHYSICS - CT > Flashcards

PHYSICS - CT Flashcards

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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
Q

Effect of increasing tissue density on μ (LAC)

A

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

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26
Q

μ (LAC) for photon energies at the k-edge is increased or decreased?

A

increased μ (more beam attenuation per unit length of tissue at the k-edge)

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27
Q

Relationship between μ (LAC) and HVL

A

inversely related; materials with a higher μ have a smaller HVL

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28
Q

Mass attenuation coefficient

A

describes attenuation per unit mass of tissue; expressed in g^-1; same for ice, water, and water vapor

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29
Q

Water is always ___ HU

A

0 HU

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30
Q

Effect of reconstructing thicker slices from thinner slices

A

increased SNR, increased partial volume artifact, decreased spatial resolution in the Z-direction

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31
Q

Effect of a sharpening filter

A

increased spatial resolution, increased noise (decreased SNR)

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32
Q

Effect of a smoothing filter

A

decreased spatial resolution, decreased noise (increased SNR)

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33
Q

T/F - filters/kernals (in FBR) do not change voxel data

A

false; filters change voxel data (unlike window/level)

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34
Q

Advantage of iterative reconstruction

A

increased SNR at a given dose (or dose can be reduced while maintaining SNR), reduced streak artifact

35
Q

180 degree axial interpolation (vs. 360 degree)

A

better spatial resolution, more accurate reconstructions, less partial volume artifact

36
Q

Effect of decreasing pixel size on resolution and SNR

A

increased spatial resolution, decreased SNR (fewer photons per pixel)

37
Q

Major factors affecting spatial resolution

A

pitch, slice thickness, DEL/pixel size, # of projections per slice, focal spot size, patient motion, reconstruction algorithm

38
Q

Increased slice sensitivity profile (SSP) equates to…

A

decreased spatial resolution in the Z-direction; increased pitch => increased SSP => decreased resolution

39
Q

SSP is increased by…

A

increasing pitch, using 360 interpolation (vs. 180), and detector binning => wider SSP => lower spatial resolution

40
Q

How to: increase number of photons per voxel

A

increase slice thickness, increase DFOV, increase mAs, or decrease pitch; more photons per voxel => less noise

41
Q

Effect of decreasing kVp

A

increased contrast, increased quantum mottle, decreased dose; may need to increase mA to maintain signal

42
Q

How to: reduce dose

A

decrease kVp, decrease mAs, increase pitch, tube current modulation; also better reconstruction techniques

43
Q

Cardiac imaging best performed during which part of the cardiac cycle

A

diastole

44
Q

Prospective cardiac imaging

A

lower dose, always axial, no functional imaging, susceptible to motion (beta-blocker is essential)

45
Q

Retrospective cardiac imaging

A

higher dose, helical (low pitch), functional imaging is possible, less susceptible to motion; use if CI to beta-blocker

46
Q

Contraindication to beta-blocker - use prospective or retrospective gating?

A

retrospective

47
Q

CT fluoro

A

lower mAs, same kVp; continuous and intermittent modes

48
Q

Continuous mode CT fluoro

A

near real time; “first in, first out” image reconstruction

49
Q

Advantages of dual source CT

A

faster acquistion, tissue characterization (with dual energy)

50
Q

Contrast timing techniques

A

empirical, test bolus, bolus tracking

51
Q

CT phantom sizes

A

16 cm for head, 32 cm for abdomen; contains central and peripheral pencil ionization chambers

52
Q

Central and peripheral dose for head and abdomen

A

same for CT head; central dose is less than peripheral in CT abdomen

53
Q

Dose (CTDIvol) is over or under-estimated for obese patients?

A

over-estimated

54
Q

Dose (CTDIvol) is over or under-estimated for pediatric patients?

A

under-estimated

55
Q

Adding doses together across multiple acquistions/phases

A

DLPs can be added together, while CTDIvol cannot

56
Q

Z-axis variation

A

“tails” of radiation along the edge of the area being scanned; radiation profile is not limited to the primary area being scanned

57
Q

Z-axis variation greater for SDCT or MDCT?

A

MDCT has greater Z-axis variation

58
Q

How to: fix beam hardening artifact

A

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
Q

How to: fix partial volume artifact

A

acquire thinner slices

60
Q

How to: fix partial volume averaging

A

decreasing pitch, use thinner detector rows, use axial acquisition (instead of helical), reconstruct with thinner slices

61
Q

How to: fix photon starvation

A

tube current modulation, adaptive filtration to correct attenuation profile

62
Q

How to: fix undersampling

A

acquire as many projections as possible per rotation (for view aliasing) or utilize high resolution techniques (for ray aliasing)

63
Q

How to: fix streak artifact from metal

A

thinner slices, higher kVp, remove metal (if possible), interpolation software

64
Q

How to: fix patient motion

A

align scanner in the primary direction of motion, overscanning, gating, faster scanner, restrain patient

65
Q

How to: fix incomplete projection artifact

A

position patient properly (e.g. move arms above head)

66
Q

Ring artifact is specific to…

A

3rd generation CT scanners

67
Q

How to: fix stair step artifact

A

thinner slices, reconstruct with overlapping slices

68
Q

CTDIvol and DLP for typical head CT

A

60 mGy and 1000 mGy-cm

69
Q

CTDIvol and DLP for typical abdomen/pelvis CT

A

15 mGy and 500 mGy-cm

70
Q

Standard matrix size and bit depth

A

512 x 512, bit depth is 12

71
Q

Overscanning

A

pitch <1

72
Q

Effect of lower kVp on HU

A

lower kVp => higher HU, and vice-versa

73
Q

Risk of fetal thyroid dysfunction with…

A

maternal IV contrast administration; considered a negligible risk

74
Q

kVp for standard adult

A

120 kVp (range is 80-140); increased to 140 kVp for obese patients

75
Q

Increasing number of photons 2x increases SNR by what factor?

A

1.4x

76
Q

Changing kVp generally requires a change in…

A

mAs to maintain balance between dose and quantum mottle

77
Q

mAs and kVp adjustments for pediatric imaging

A

decrease kVp (because thinner object) => need to increase mAs to avoid quantum mottle

78
Q

Breast dose reduction strategies

A

reduce mA, tube current modulation, bismuth shield

79
Q

Noise is proportional to…

A

1 / sqrt(number of photons per voxel); relates to mA, exposure time, and slice thickness

80
Q

FBP vs. BP

A

FBP sharpens sinogram data prior to back projection; reduces streaking

81
Q

Better SSP: 180 vs. 360 interpolation?

A

180 degree interpolation

82
Q

Better SSP: axial vs. helical acquisition?

A

axial acquisition (no interpolation required)

83
Q

Size specific dose index (SSDI)

A

SSDI = f * CTDIvol, where f >1 for obese and f <1 for peds

84
Q

Sinogram axes

A

x-axis is DEL, y-axis is projection number

AJT Physics (7 decks)

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