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MED 107- MEDICAL PHYSICS II > 33. CT: COLLIMATORS > Flashcards

33. CT: COLLIMATORS Flashcards

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1
Q
  1. What are Collimators?
A
  • they are used in X-Ray imaging
  • they are found at the exit of the X-Ray Tube
  • they are used to find the size and the shape of the
    X-Ray beam
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2
Q
  1. What do CT units make use of?
A
  • they make use of Collimators
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3
Q
  1. What does the interaction of X-Rays with tissue create?
A
  • it creates randomly scattered photons
  • these create image noise
  • this clouds the image
  • this decreases the image quality
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4
Q
  1. What role do Collimators play when it comes to X-Rays?
A
  • they are found at the front of the detector
  • they act as an anti-scatter grids
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5
Q
  1. What do these anti-scatter grids do?
A
  • they eliminate X-Ray photons
  • these deviate from a straight source-detector path
  • they reduce the beam size
  • they limit the beam thickness
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6
Q
  1. What does the presence of Collimator on the front of the detector result in?
A
  • it results in a small apparent focal spot
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7
Q
  1. What are Artefacts?
A

THEY ARE THE DISCREPANCIES THAT ARE FOUND BETWEEN:
- the CT numbers represented in the image
- AND the expected CT numbers (in HU units)

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8
Q
  1. What are 5 types of Common Artefacts?
A
  1. Beam Hardening
  2. Partial Volume Effect
  3. Bad Detectors
  4. Metal
  5. Patient Motion
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9
Q
  1. What 2 types of Collimators do we have when it comes to CT Scanners?
A
  1. THE ONE THAT IS PLACED AT THE FRONT OF THE
    X-RAY TUBE:
    • this is known as the Pre-Collimator
    • this reduces the beam size
    • this limits the beam thickness
    • this results in a smaller focal spot
    • results in better image quality
  2. THE ONE THAT IS IN CONTACT WITH THE DETECTOR:
    • this is known as the Post Collimator
    • this one acts as an Anti- Scatter Grid
    • it absorbs the Scatter Radiation
    • results in better image quality
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10
Q
  1. What is the Beam Hardening Effect?
A
  • this is when the X-Ray beam is hardened by passing
    through an object
  • this happens specifically in denser objects
  • specific areas of the image will then appear darker
    than they should be
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11
Q
  1. What happens in extreme Beam Hardening cases?
A
  • a detector element may fail
  • this results in a constant output signal
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12
Q
  1. What is a primary source of image noise?
A
  • detectors
  • their associated signal amplifiers
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13
Q
  1. What reduces the amount of image noise?
A
  • Electronic Noise Suppression Filters
  • Software Noise Suppression
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14
Q
  1. Who can reduce the noise in an image?
A
  • the operator
  • provided that this loss of detail is acceptable
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15
Q
  1. What kind of beams can X-Rays not produce?
A
  • Monochromatic beams
  • this is why an artefact related to beam hardening is
    common
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16
Q
  1. What does Beam Hardening result in?
A
  • the image can have streaks and shading
  • the image can be interpreted inaccurately

THE STREAKS:
- will be specifically seen in regions that are very dense
- such as the bone

17
Q
  1. When are lower energy X-Rays absorbed?
A
  • when the energy peak will shift towards higher
    energies
  • the absorption values will be underestimated
18
Q
  1. How can we reduce Beam Hardening?
A
  • by reducing the pre-hardening of the beam
  • this is done by placing a thin metal plate in front of the
    tube
19
Q
  1. What materials can the thin metal plate made out of?
A
  • Molybdenum
  • Tungsten
20
Q
  1. What can compensate for the Beam hardening in the projections?
A
  • applying appropriate algorithms in the reconstruction
21
Q
  1. What does this image show?
A
  • The Beam Hardening Effect
22
Q
  1. When does the Partial Volume Effect occur?
A
  • it occurs whenever a pixel represents more than one
    kind of tissue
  • it can also occur when a single voxel contains a variety
    of tissue
  • this occurs in CT and Nuclear Imaging
23
Q
  1. When is the Partial Volume Effect particularly relevant?
A
  • when a tissue boundary lies within a CT splice
  • this means that one splice contains information from
    the previous splice
24
Q
  1. What does the Partial Volume Effect result in?
A
  • it blurs the intensity distinction
  • this happens between the adjacent tissues
25
Q
  1. What can reduce the Partial Volume Effect?
A
  • higher resolutions
  • repositioning of the patient
  • we can use thinner slices
26
Q
  1. What do these images show?
A
  • the Partial Volume Effect

MED 107- MEDICAL PHYSICS II (108 decks)

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