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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
Typical presets (W,L) Abdomen
W 400, L +50
26
Typical presets (W,L)
Bone W 1600, L +500
27
Process of making an image in CT starts with detector...
Raw data I processing (filter back or iterative) I Mathematic filter 'Kernel' applied I Ready to read/store
28
Acquisition Axial
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
Acquisition helical
table moves, tube on way faster
30
effects of increased beam width
faster less motion INCREASES PARTIAL VOLUME DOESNT affect dose (faster but larger area)
31
more kVp --\> ?
Increased dose (always in CT) Decreased noise
32
Kernel
trade off decision between spatial resolution and noise sharp kernel = better res, more noise (bone) soft kernel = low noise, reduced res (brain)
33
Cardiac Prospective pro/con
step and shoot - R-R interval Less radiation **Can't do functional imaging** **ALWAYS axial**
34
Cardiac Retrospective
CAN do functional scanning whole time higher radiation
35
Dual energy kVp's?
80 and 140 kVp
36
SNR and xray flux (mA)
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
to improve SNR mA? rotation time? kVp? slice thickness? pixel size? pitch?
Higher SNR with higher mA Longer rotation time higher kVp thicker slices bigger pixels LOWER pitch
38
CT vs xray contrast and spatial resolution
**CT has waaay better contrast res** **CT spatial res not so much**
39
Why is contrast res so good in CT? limiting factor for contrast res
minimal scatter hitting detector tight collimation (twice) windowing NOISE limits contrast res
40
ENEMY OF CONTRAST RES =
NOISE
41
Spatial res in CT = ? mammo = ?
1 lp/mm 11lp/mm
42
Effect on spatial res focal spot
Smaller better
43
Effect on spatial res Mag
mag blurs, worsens spatial res
44
Effect on spatial res Detector aperture size
**Larger aperture improves Z axis spatial res** NO EFFECT ON X, Y
45
Effect on spatial res Pixel size, DFOV, matrix size
pixel size = DFOV/matrix bigger matrix, smaller DFOV = smaller pixel size **smaller pixel size = BETTER SPATIAL RES**
46
CT X-Y spatial res Z spatial res
**X-Y** determined by **focal spot** **Z** determined by **detector size**
47
pixels, Matrix, FOV and contrast vs spatial res
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
CT dose distribution body vs head
Body middle gets 50% of skin Head All the same
49
CTDI "weighted" "volume"
radiation dose, normalized to beam width "weighted" = 1/3 central CTDI + 2/3 Peripheral CTDI **"volume" = Weighted CTDI / PITCH** **CTDI vol**
50
Pitch and dose
proportional 2.0 halves the dose
51
DLP = ?
**CTDI Vol x length of scan**
52
CT effective dose unit?
k x DLP k = body part constant **effective dose unit = Sv**
53
Body phantom size what if patient bigger or smaller
32 cm ## Footnote **size 32** **bigger patient = OVER estimated dose** **smalle patient = UNDER estimated dose**
54
'average' dose and effective dose (CTDI) for CT head CT abd Peds belly
CT head = 58 mGy (1-2 mSv) CT abd = 18 mGy (8-11mSv) Peds belly = 15 mGy
55
ACR Established reference values (75th percentile) Head adult belly peds belly
Head = 75 mGy Adult belly = 25 mGy Peds belly = 20 mGy above these doses, "should be investigated and reduced if possible"
56
Risk of radiation induced cancer per dose? adult per Sv child per Sv
adult = 5% per Sv Child = 15 % per Sv 0.5% per Sv over 50 yo
57
2 view chests per CT radiation wise
100
58
embryo dose in CT abd/pel ?
30mGy
59
CT artifacts Cupping
Dark center of image 2/2 beam hardening | (center sees higher average energy)
60
CT artifacts beam hardening dark bands/streaks
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
CT artifacts How to fix beam hardening
**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
Partial volume 2 patterns how to fix?
**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
Photon starvation
typically shoulders or other high attenuating areas result in streaking when beam travels horizontally through shoulders Fix = automatic modulation, more mA there
64
Under sampling
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
Metal artifact fixes
Beam hardening, partial volume, aliasing worse with higher Z (fe, platinum) FIX = increase kVp, **THINNER SLICES**
66
Incomplete projection
body parts or IV contrast outside field but still attenuating xrays
67
Ring artifact
calibration error of **defective detector** recalibrate or replace
68
helical artifact in axial plane
top of skull, anatomy changing rapidly in Z direction worse with higher pitch lower pitch, use thinner sections
69

physics factoids (8 decks)

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