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Medical Physics 2nd Semester > Computed Tomography (Diagnostic Radiology) > Flashcards

Computed Tomography (Diagnostic Radiology) Flashcards

1
Q

Fill in the following logic flow.

Projections –> _______ –> Recon Slice

A

Sinogram

(raw data)

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2
Q

What are some of the key advantages of CT (3 and a half of them)

A

Eliminates superposition

Superior contrast

Fast image acquisition

Various acquisition modes

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3
Q

Which of these is not a way to increase the scan FOV?

Larger fan beam angle

Larger cone angle

More detectors

(not a trick question. One of them is right)

A

Larger cone angle

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4
Q

Display FOV (DFOV) is the reconstructed FOV with a mixed matrix size. That being said, what is the maximum DFOV possible?

A

The SFOV

(You can’t reconstruct what you never scanned!)

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5
Q

What is the moving component of the CT that enables helical CT scan mode, thus reducing the scan time?

A

Slip Rings

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6
Q

Main benefit to Z-dimension oversampling

A

Improves spatial resolution in z-dimension

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7
Q

What effect does the bow tie filter have on the x-ray fluence profile along the fan angle?

A

Reduces fluence at the ends, does not affect the middle. Makes things closer to uniform

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8
Q

Why does removing peripheral x-ray intensity not result in poor image quality?

A

Overall noise is unaffected because it is dominated by the central intensity anyway (minumum N)

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9
Q

Why do we need a uniform X-ray fluence/intensity at the detector?

A

For uniform image quality (N)

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10
Q

What area of the beam is noise dominated by? Central or peripheral?

A

Central

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11
Q

Where is beam width defined at?

A

Isocenter

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12
Q

What is the slice width determined by?

A

Detector binning

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13
Q

Where is the slice width measured?

A

Isocenter

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14
Q

For the same scan parameters (same dose), what is the change in resolution and noise when using thinner slice?

A

Better resolution

higher noise level

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15
Q

Mathematical equation for CT number in hounsfield units.

A

HU(x,y,z) = 1000 (µ(x,y,z) - µw) / µw

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16
Q

What is the dominant photon interaction in the energy range of CT (80-140 kVp)

A

Compton

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17
Q

What is the use of the localizer/scout? What is it?

A

A digital radiographic image used to set up the scan region, check patient centering and decide other scan parameters

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18
Q

Why is PA preferred over AP?

A

Reduced breast dose

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19
Q

What is the purpose of mA modulation?

A

Use lowest mA to get a present SNR

Reduces dose

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20
Q

What is the main purpose of the helical scan?

A

To increase the scan speed

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21
Q

Equation for pitch during a helical scan

A

Pitch = Ftable/nT

Where Ftable is the table feed distance per rotation of gantry (how much the table travels in 1 gantry rotation)

nT is the collimated beam width

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22
Q

What is the benefit of having a higher pitch? (two answers)

A

Shorter scan times

Lower dose

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23
Q

What is the main drawback of having higher pitch (two answers)?

A

Longer distance between interpolation points

Worse image resolution in z-dimension

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24
Q

What is the most commonly used type of reconstruction algorithm?

A

Filtered back projection

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25
Q

What is the method in which intensity values in each projection are smeared over the image matrix along the direction of the original rays to which they respond, then summed to produce the image?

A

Back projection

(You’re literally working backwards)

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26
Q

What is the main issue of simple back projection?

A

1/r image blurring from geometry of the back projection

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27
Q

What is the difference between filtered back projection and simple back projection?

A

Filtered removes the blurring using convolution before back projecting them

28
Q

What are the pros and cons to Model based iterative reconstruction?

A

Pros:

Enhanced image quality, reduced dose

Con:

Longer computational time

29
Q

Why is 90 kVp used when contrasting vessels using an iodine solution?

A

Iodine has a photopeak of about 30 keV. Average keV in a 90 kVp beam is about 33 keV.

30
Q

What affect does dual source have on

  • Acquisition time
  • Noise
  • Dose
  • Scatter cross talk
  • SFOV of tubes A and B
  • Registration
A
  • Acquisition time - cuts it in half
  • Noise - Makes it lower
  • Dose - no change
  • Cross scatter radiation - increases
  • Registration - additional registrations needed for moving structures
31
Q

For fast kV switching dual energy CT’s, what affect does it have on

  • mA
  • SFOV
  • Registrations
  • Scatter crosstalk
A
  • mA - higher
  • SFOV - uses full SFOV
  • Registrations - No issues
  • Scatter crosstalk - No issues
32
Q

For dual-layer detector dual energy CT’s, what affect does it have on

  • SFOV
  • Dose
  • Energy separation
A
  • SFOV - full SFOV used
  • Dose - higher dose since the design is optimized for 120 kVp
  • Energy separation - Poor high-low E separation
33
Q

Is spatial resolution measured at high or low dose level?

A

High dose level

(low noise)

34
Q

What is the name of the 2-D function that describes the response of a imaging system to a point input?

(Volume under = 1)

35
Q

What is the name of the 1-D function that describes the response of a imaging system to a line input?

(Volume under = 1)

A

Line Spread Function

PSF

36
Q

What is the name of the 1-D function that describes the response of a imaging system to a sharp edge input?

(Volume under = 1)

A

Edge Square Function

ESF

37
Q

In general, blurring in the spatial domain leads to reduced ________ and reduced __________

A

contrast

wave amplitude

38
Q

General mathematical relationship between LSF and PSF

What about ESF and PSF?

A

LSF(x) = PSF(x,y) * LINE(y)

ESF(x) = PSF(x,y) * EDGE(y)

39
Q

When you fourier transform a 1D LSF to the frequency domain, what do you get out?

A

A 1D MTF (modulation transfer function)

40
Q

In general, when you increase spatial frequency, what happens to MTF?

41
Q

What value is the Limiting spatial resolution (LSR) defined as in terms of MTF?

42
Q

Physically, why does higher mA raise the focal spot size?

A

Higher mA means more electrons going towards the anode target. More electrons mean they repel off each other and spread the beam, making the actual focal spot size larger than it should be.

43
Q

In general, raising pitch will ___________ FWHM and make resolution ____________

A

widen

worse

44
Q

What are the literal definitions of MTF(f) and NPS(f)?

A

MTF: how well an imaging system processes signal

NPS: how well an imaging system processes noise

45
Q

What is the affect of slice thickness on noise level?

A

Thicker slices –> lower relative noise

(more photons per slice)

46
Q

What is the relationship between iterative recon and FBP in reconstruction method for noise level of the same dose

A

Iterative recon Noise level < FBP noise level

47
Q

If you increase the rotation speed, then for the same mA and pitch, what affect does it have on relative noise level?

A

Higher relative noise level

Because lower mAs per slice

48
Q

Anytime you make a beam to view area between bones you will experience ________________ because the lower energy photons are removed by the bone. The intensity drops and there are less attenuations in the tissue itself.

Your machine has no idea ____________________ occured, because of this it assumed the thickness is reduced and it leads to dark area/bands.

A

Beam hardening

hardening/attenuation

49
Q

What can you raise to decrease likelihood of beam hardening wedge artifacts occuring?

A

Increase kVp

50
Q

Equation of MTF?

A

Contrastoutput / Contrastinput ???

Maximum is 1

51
Q

Equation for CT contrast?

A

C = Meantarget - MeanBKG

52
Q

What are two ways to reduce streak artifacts?

A

Higher kVp or use a metal artifact reduction (MAR)

53
Q

What type of artifact is caused when one detector is out of calibration on a third generation scanner or MDCT scanner?

A) Ring artifact

B) Streak artifact

C) Wedge Artiface

D) Cone beam artifact

A

A) ring artifact

54
Q

What is the geometry cause of cone beam artifacts?

A

For a beam, adjacent points have different Z travel in the peripheral beam.

Result: can’t resolve adjacent material

55
Q

If you increase the beam width, does CTDI increase or decrease?

56
Q

Equation for CTDIvol for a helical scan

A

CTDIvol = CTDIw / pitch

Where, if you recall, pitch is inverse proportional to dose

57
Q

Equation for dose length product using CTDI

A

CTDIvol x L

58
Q

Effective dose (ED) for risk accessment using dose length product

A

ED = DLP x K

K is some constant which depends on organ sensitivity and distribution

59
Q

Equation for size specific dose estimation (SSDE)

A

SSDE = f x CTDIvol

f is affected by the CTDI phantom size and effective diameter.

When you use f factor, K factor doesn’t apply anymore, so you can’t calculate ED anymore.

60
Q

If you what to reduce dose, what do you need to do to

kVp for ped

mAs for large patient

Pitch

Recon

Scan range

A

kVp for ped - reduce

mAs for large patient - reduce

Pitch - increase

Recon - use a different one

Scan range - don’t go beyond

61
Q

f depends on CTDI phantom size and Dw where Dw is the water equivalent diameter. How do you get Dw?

A

Fine Aw by using a summation of the attenuation percent increase between the material of a pixel multiplied by the pixl. Do a weighted sum of all voxels and associated pixel.

62
Q

What is equilibirum dose defined as?

A

The cumulative dose as the scan length goes to infinity

63
Q

In order to get f factor for SSDE calculations, you need to know what type of phantom is being used to get CTDIvol and what the effective diameter of the patient is from the scout image.

Given AP and lateral diameters, what is the effective diameter of the patient?

A

Effective Diameter = sqrt(AP x LAT)

64
Q

True or false

Number of photons per slice is directly proportional to the slice thickness

65
Q

Where is field size defined at for CT scan?

A

Isocenter on the fan angle side

Medical Physics 2nd Semester (7 decks)

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