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physics factoids > CT > Flashcards

CT Flashcards

1
Q

CT mA, kVp and focal spot vs Xray

Main diff

A

HIGHER mA in CT

CT kVp 80-120 Xray 50-120

focal spot CT 0.6-1.2 Xray 1.0-1.2

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2
Q

CT filters

material

A

copper or aluminum

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3
Q

Bow tie filters

purpose? material?

A

compensate for uneven attenuation of beam by patient (head)

low Z materials

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4
Q

heel effect in CT

A

minimized - anode cathode axis positioned perpendicular to imaging plane

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5
Q

CT scatted reduction

collimator?

septa?

A

collimator at both the xray tube and at detector

“defines section thickness on a single slice”

anti-scatter septa (like grid)

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6
Q

iterative reconstruction

effect on dose

A

allows more noise

(less dose same quality)

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7
Q

Modern vs dino CT detectorys

A

scintillator vs Gas-filled

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8
Q

what determines number of slices obtained simultaneously?

A

number of detectors in Z direction

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9
Q

“isotropic resolution”

A

MDCT can do non-axial recons without stretching pixels

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10
Q

minimum slice thickness determined by?

A

detector element aperture width

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11
Q

kVp and mA adjustments in modern CT

A

using scout before acquisition

on the fly adjustment with continuous modulation

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12
Q

“ray”

A

total xray attenuation along a particular line from focal point to single detector

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13
Q

“projection”

A

all rays at a given tube position/angle of xray tube

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14
Q

‘kind’ of xrays used in CT?

A

highly filtered, high kV

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15
Q

Matrix size in CT?

bits?

shades of gray?

A

512 x 512

12 bits

2 ^12 = 4096 shades of gray

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16
Q

pixel size = ?

better res = ?

A

FOV/MATRIX

better res = larger matrix

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17
Q

Pitch equation

A

Table movement / beam width

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18
Q

relationship bt HU and Xray attenuation

A

increase in 10HU = 1% increase in x-ray attenuation

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19
Q

effect of keV on HU

A

low keV –> higher HU

more attenuation thanks to k edge

contrast HU at 140 kV = 100HU

at 80 kV = 400HU

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20
Q

filtration and HU’s

A

increases average energy (beam hardened)

higher energy beam –> lower HU’s

phenomenon of ‘cupping’ with lower HU in center as beam is filtered by body

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21
Q

Window ‘level’ =

‘width’

A

level = middle of grayscale, want it at attenuation of thing youre lookin at

width = width… want it narrow if things look same (gray and white matter)

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22
Q

what HU is black if level set at 100 and window 300

A

less than -50

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23
Q

Typical presets (W,L)

Brain

A

W 80, L+40

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24
Q

Typical presets (W,L)

Lung

A

W 1500, L-400

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25
Q

Typical presets (W,L)

Abdomen

A

W 400, L +50

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26
Q

Typical presets (W,L)

A

Bone W 1600, L +500

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27
Q

Process of making an image in CT

starts with detector…

A

Raw data

I

processing (filter back or iterative)

I

Mathematic filter ‘Kernel’ applied

I

Ready to read/store

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28
Q

Acquisition

Axial

A

Stationary table, tube on, picture, tube off

table moves, tube on, another picture

Better Z resolution with no partial volume effect along long axis

29
Q

Acquisition

helical

A

table moves, tube on

way faster

30
Q

effects of increased beam width

A

faster

less motion

INCREASES PARTIAL VOLUME

DOESNT affect dose (faster but larger area)

31
Q

more kVp –> ?

A

Increased dose (always in CT)

Decreased noise

32
Q

Kernel

A

trade off decision between spatial resolution and noise

sharp kernel = better res, more noise (bone)

soft kernel = low noise, reduced res (brain)

33
Q

Cardiac

Prospective pro/con

A

step and shoot - R-R interval

Less radiation

Can’t do functional imaging

ALWAYS axial

34
Q

Cardiac

Retrospective

A

CAN do functional

scanning whole time

higher radiation

35
Q

Dual energy kVp’s?

A

80 and 140 kVp

36
Q

SNR and xray flux (mA)

A

Signal is directly related twice the xrays, twice the signal

Noise (also concerned with scatter, not just mottle), changes by square root of change. Twice the xrays square root 2 x the noise

EXAMPLE quadruple the mA, what is change in SNR

4 times the signal, square root of 4 times the noise

4/2 = SNR doubles

37
Q

to improve SNR

mA?

rotation time?

kVp?

slice thickness?

pixel size?

pitch?

A

Higher SNR with

higher mA

Longer rotation time

higher kVp

thicker slices

bigger pixels

LOWER pitch

38
Q

CT vs xray

contrast and spatial resolution

A

CT has waaay better contrast res

CT spatial res not so much

39
Q

Why is contrast res so good in CT?

limiting factor for contrast res

A

minimal scatter hitting detector

tight collimation (twice)

windowing

NOISE limits contrast res

40
Q

ENEMY OF CONTRAST RES =

41
Q

Spatial res

in CT = ?

mammo = ?

A

1 lp/mm

11lp/mm

42
Q

Effect on spatial res

focal spot

A

Smaller better

43
Q

Effect on spatial res

Mag

A

mag blurs, worsens spatial res

44
Q

Effect on spatial res

Detector aperture size

A

Larger aperture improves Z axis spatial res

NO EFFECT ON X, Y

45
Q

Effect on spatial res

Pixel size, DFOV, matrix size

A

pixel size = DFOV/matrix

bigger matrix, smaller DFOV = smaller pixel size

smaller pixel size = BETTER SPATIAL RES

46
Q

CT X-Y spatial res

Z spatial res

A

X-Y determined by focal spot

Z determined by detector size

47
Q

pixels, Matrix, FOV and contrast vs spatial res

A

constant matrix, decreased FOV will make pixels smaller

better spatial res

WORSE CONTRAST RES (fewer photons per pixel)

BIGGER FOV, bigger pixels, worse spatial res better contrast res (more photons per box)

48
Q

CT dose distribution

body vs head

A

Body

middle gets 50% of skin

Head

All the same

49
Q

CTDI

“weighted”

“volume”

A

radiation dose, normalized to beam width

“weighted” = 1/3 central CTDI + 2/3 Peripheral CTDI

“volume” = Weighted CTDI / PITCH

CTDI vol

50
Q

Pitch and dose

A

proportional

2.0 halves the dose

51
Q

DLP = ?

A

CTDI Vol x length of scan

52
Q

CT effective dose

unit?

A

k x DLP

k = body part constant

effective dose unit = Sv

53
Q

Body phantom size

what if patient bigger or smaller

A

32 cm

size 32

bigger patient = OVER estimated dose

smalle patient = UNDER estimated dose

54
Q

‘average’ dose and effective dose (CTDI) for

CT head

CT abd

Peds belly

A

CT head = 58 mGy (1-2 mSv)

CT abd = 18 mGy (8-11mSv)

Peds belly = 15 mGy

55
Q

ACR Established reference values (75th percentile)

Head

adult belly

peds belly

A

Head = 75 mGy

Adult belly = 25 mGy

Peds belly = 20 mGy

above these doses, “should be investigated and reduced if possible”

56
Q

Risk of radiation induced cancer per dose?

adult per Sv

child per Sv

A

adult = 5% per Sv

Child = 15 % per Sv

0.5% per Sv over 50 yo

57
Q

2 view chests per CT radiation wise

58
Q

embryo dose in CT abd/pel ?

59
Q

CT artifacts

Cupping

A

Dark center of image 2/2 beam hardening

(center sees higher average energy)

60
Q

CT artifacts

beam hardening dark bands/streaks

A

in the setting of two dense objects

objects that pass through one are less attenuated than those that pass through both

results in dark streaks between them

61
Q

CT artifacts

How to fix beam hardening

A

Filtration - pre-hardening to remove lower energy components or bow tie filter

Calibration correction- using a phantom to allow the detector to compensate

iterative software

62
Q

Partial volume

2 patterns

how to fix?

A

dense object protrudes partially into the widthof an xray beam. Beam diverges, this looks like shading adjacent to the object

Dense thing and low attenuation thing each occupying half of 3d voxel. machine averages together and it look intermediate

ex = skull base averaging with csf or brain to look like blood

Fix with thinner slices

63
Q

Photon starvation

A

typically shoulders or other high attenuating areas result in streaking when beam travels horizontally through shoulders

Fix = automatic modulation, more mA there

64
Q

Under sampling

A

Insufficient number of projections

leads to mis-registration

View aliasing - under sampling between projections, see fine stripes radiating from the edge of a dense object

fix = slow rotation speed

Ray aliasing - undersampling within a projection. See strips appearing close to structre

65
Q

Metal artifact

fixes

A

Beam hardening, partial volume, aliasing

worse with higher Z (fe, platinum)

FIX = increase kVp, THINNER SLICES

66
Q

Incomplete projection

A

body parts or IV contrast outside field but still attenuating xrays

67
Q

Ring artifact

A

calibration error of defective detector

recalibrate or replace

68
Q

helical artifact in axial plane

A

top of skull, anatomy changing rapidly in Z direction

worse with higher pitch

lower pitch, use thinner sections

physics factoids (8 decks)

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