Computed Tomography (Diagnostic Radiology) Flashcards Preview

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

Flashcards in Computed Tomography (Diagnostic Radiology) Deck (65)
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1

Fill in the following logic flow.

Projections --> _______ --> Recon Slice

Sinogram

(raw data)

2

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

Eliminates superposition

Superior contrast

Fast image acquisition

 

Various acquisition modes

3

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)

Larger cone angle

4

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

The SFOV

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

5

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

Slip Rings

6

Main benefit to Z-dimension oversampling

Improves spatial resolution in z-dimension

7

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

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

8

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

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

9

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

For uniform image quality (N)

10

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

Central

11

Where is beam width defined at?

Isocenter

12

What is the slice width determined by?

Detector binning

13

Where is the slice width measured?

Isocenter

14

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

Better resolution

higher noise level

15

Mathematical equation for CT number in hounsfield units.

HU(x,y,z) = 1000 (µ(x,y,z) - µw) / µw

16

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

Compton

17

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

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

18

Why is PA preferred over AP?

Reduced breast dose

19

What is the purpose of mA modulation?

Use lowest mA to get a present SNR

Reduces dose

20

What is the main purpose of the helical scan?

To increase the scan speed

21

Equation for pitch during a helical scan

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

22

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

Shorter scan times

Lower dose

23

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

Longer distance between interpolation points

Worse image resolution in z-dimension

24

What is the most commonly used type of reconstruction algorithm?

Filtered back projection

25

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?

Back projection

 

(You're literally working backwards)

26

What is the main issue of simple back projection?

1/r image blurring from geometry of the back projection

27

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

Filtered removes the blurring using convolution before back projecting them

28

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

Pros:

Enhanced image quality, reduced dose

 

Con:

Longer computational time

29

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

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

30

What affect does dual source have on

  • Acquisition time
  • Noise
  • Dose
  • Scatter cross talk
  • SFOV of tubes A and B
  • Registration

  • Acquisition time - cuts it in half
  • Noise - Makes it lower
  • Dose - no change
  • Cross scatter radiation - increases
  • Registration - additional registrations needed for moving structures