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term test 2 imaging > Alot Of CT > Flashcards

Alot Of CT Flashcards

1
Q

What are the factors that affect quality of a CT image

A
  • mA
  • scan time
  • FOV
  • reconstruction algorithm
  • kV
  • pitch
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2
Q

What are image quality metrics

A

High contrast (detail) spatial resolution
Low contrast resolution (contrast detectability)
Temporal resolution
CT number uniformity
Accuracy
Noise
Artifacts

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3
Q

What is spatial resolution

A

Ability to resolve small, high contrast objects that are very close together

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4
Q

What is spatial resolution

A

Ability to resolve small, high contrast objects that are very close together

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5
Q

How do we measure spatial resolution

A

Directly (line pairs)
MTF modulation transfer function

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6
Q

What factors affect spatial resolution

A

Matrix size
Pixel size
DFOV
Slice thickness
Reconstruction algorithm
Interactive reconstruction
Focal spot size
Pitch
patient motion

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7
Q

Pixel size formula

A

DFOV / matrix size

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8
Q

What is sampling frequency

A

Directly related to spatial resolution
More sampling = more image detail

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9
Q

How do we increase sampling frequency

A
  • increase sampling rate
  • increase speed at which detectors are read
  • increase time during which PET radio tracer emissions are read
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10
Q

Image noise increases with

A
  • increased spatial frequency
  • ## increased image detail
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11
Q

How does FBP work in relation to image resolution

A
  • increases noise and dose
  • can offer high spatial resolution or high contrast resolution but not at the same time
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12
Q

How does iterative reconstruction affect image resolution

A

Enhances image quality with lower dose

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13
Q

What are the drawbacks for iterative reconstruction

A

Image artifacts
Artificially enhanced edges (becomes sharper, soft tissue becomes smoother)

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14
Q

How does focal spot size effect resolution

A

Large focal spots cause more geometric unsharpness which reduces spatial resolution. (Release of heat and photons)

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15
Q

What is low contrast resolution

A

Ability to differentiate objects with slightly different densities

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16
Q

Between film/screen and CT, which is better in spatial resolution and resolving differences in tissue density (LCD)

A

Film/screen - superior to CT in spatial resolution
CT is superior - in resolving small differences in tissue density

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17
Q

When is LCD used

A

Discussing the ability to see an object that is nearly the same density as its background

18
Q

What 3 factors does visible object size depend on

A

Level of contrast in object
Image noise
Window setting

19
Q

How would someone decrease noise by 1/2? How does this effect dose

A
  • mAs is increase by 4x
  • dose is increased by 4x
20
Q

How is slice thickness related to noise

A
  • as thickness decreases, noise increases
  • thicker slice, more photons hits detector but you cannot see smaller objects as well anymore
21
Q

If window level is the same but window width is increased, how does the image change

A
  • less black and whites
  • more greys
22
Q

How do bone algorithm and soft tissue algorithm differ

A

Bone - produces lower contrast resolution but better spatial resolution
Soft tissue - produces better contrast resolution but lower spatial resolution

23
Q

How do larger patient effect contrast resolution

A

more photons attenuated
Fewer will reach detectors
Higher noise
Lower contrast resolution

24
Q

What is temporal resolution

A

Indication of the CT scanner’s ability to freeze motion of an object

25
Q

How is temporal resolution controlled

A

gantry rotation speed
# of detector channels in the system

26
Q

How is temporal resolution achieved

A

Prospective gating (x ray tube is turned off during acquisition times)

Balance between dose and image quality

Retrospective gating (beam on entire time)

27
Q

What is the advantage and disadvantage of prospective gating

A
  • reduce dose to patient
  • reliance of regular heart beat
28
Q

What is the fastest rotation time available in CT scans

A

0.275s (275ms)

29
Q

What is the CT number of water and air

A

Water 0
Air -1000

30
Q

What is CT number uniformity

A
  • ability of scanner to have the same CT number regardless of where it is in an object
31
Q

What are the sources of image noise

A
  • quantum noise / mottle
  • inherent physical limitations
  • reconstruction parameters
32
Q

What is quantume noise / mottle and how is it influenced

A
  • x ray flux (number of detected photons)

scanning parameters
Scanner efficiency
Patient size

33
Q

What are some physical limitations that cause electronic noise

A
  • detector in photodiode
  • data aquisition system
  • scatter radiation
34
Q

What are high resolution kernels

A
  • increase noise level
  • increase high frequency signals `
35
Q

How do we verify external to internal lasers and ISO

A

Option 1 - incorporate into QA phantom (take and check image)
Option 2 - use separate phantom

36
Q

How is CT pet done

A

Separately then together , CT first
- warm up
- air calibration
- phantom checks

37
Q

How is PET QA done

A
  • use water phatom mixed and calibrated with solid geranium (68 phatom)
  • check coincidence
38
Q

What is a Jaszcak / ACR phatom

A
  • water phatom with sphere that can be filled with contrast or F-18 solution mix
39
Q

How is MRI QA done

A
  • phantom with anterior receiver coil
  • slices taken through phatom for noise, spatial linearity, flood field uniformity, slice profile and spatial resolution tests
40
Q

How is MV and MR iso checked

A
  • zirconium ceramic balls are surrounded by an MR visible fluid separated by a thin layer of phantom plastic
41
Q

Why are tests from MRI to CT different

A
  • effect of machine geometry and magnetic field on radiation beam measurements
  • effect of magnetic field on measuring tools and devices
  • consortium has put out some recommendations

term test 2 imaging (3 decks)

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