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MED 107- MEDICAL PHYSICS II > 21. X-RAY DETECTORS: UNSHARPNESS > Flashcards

21. X-RAY DETECTORS: UNSHARPNESS Flashcards

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1
Q
  1. What aspects is the unsharpness of an image related to?
A
  1. The geometry of the imaging system
  2. Focal spot size
  3. The distance between the focus, patient and detector
  4. The fundamental sharpness of the detector
  5. Subject Contrast
  6. The quality and resolution of the imaging system
  7. Beam Quality (kV)
  8. Scatter
  9. Movement Unsharpness
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2
Q
  1. What is necessary to determine the image quality?
A
  • the image must be reasonably sharp
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3
Q
  1. What does Blurring do?
A
  • it reduces the image quality
  • it reduces the diagnostic quality of the image
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4
Q
  1. How is the sharpness of a system characterised?
A
  • it is best characterised in terms of its Modulation
    Transfer Function (MTF)
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5
Q
  1. What is the MTF?
A
  • it is a way to incorporate the resolution and contrast
    into a single specification
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6
Q
  1. What kind of exposure should always be used?
A
  • the smallest exposure
  • that is compatible with the diagnostic quality
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7
Q
  1. What should the practitioner be aware of when determining the quality required in an image?
A
  • they should be aware of what structures need to be
    defined
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8
Q
  1. What can be said about the resolution of images needed to diagnose fractures vs. the images needed to determine bone position?
A

AN IMAGE TO DETERMINE THE POSITION OF BONES
- in a plaster cast
- following an orthopaedic reduction needs less
resolution
- than the original image to diagnose the bone fracture

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9
Q
  1. What should ideally be used to achieve fine focus?
A
  • the smaller foci of the X-Ray tube
  • this should be able to determine a short exposure time
    to be used on a patient that is likely to move

IF IT DOES NOT:
-the practitioner may need to use a broad focus

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10
Q
  1. What does Spatial Resolution refer to?
A
  • it refers to the ability of the imaging system to
    represent distinctive anatomic features within the
    object that is being imaged
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11
Q
  1. How else can we define Spatial Resolution?
A
  • the ability of the system to distinguish the
    neighbouring features of an image from each other
  • this is related to sharpness
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12
Q
  1. What is the maximum spatial resolution of an image defined by?
A
  • the pixel size
  • the spacing

PIXEL SIZE:
- affects the resolution
- it varies between systems

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13
Q
  1. Does this diagram make sense?
A
  • yes
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14
Q
  1. Does this diagram make sense?
A
  • yes
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15
Q
  1. Does this diagram make sense?
A
  • yes
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16
Q
  1. What is the Brightness defined as?
A
  • this is defined as the intensity of light that represents
    the individuals pixels in the image
17
Q
  1. What is Brightness controlled by?
A
  • it is controlled by the processing software
  • it can be adjusted following the processing of the
    image
18
Q
  1. What can be said about all Digital Images?
A
  • they are auto-windowed
  • the computer will automatically apply an algorithm to
    the detected data
  • it will then provide the best range of contrast and
    brightness

THIS MEANS THAT:
- under and over exposure is not visually apparent
- other checks are necessary to determine if the
optimum exposure has been used

19
Q
  1. What are the checks that are needed to determine if the Optimum Exposure has been used?
A
  1. The Evaluation of the Exposure Indicator (EI)
  2. The Evaluation of the Noise within the image due to
    underexposure
  3. The Evaluation of the Burn Through due to gross
    overexposure
20
Q
  1. What is the Evaluation of the Exposure Indicator?
A
  • there are numbers allocated to the image
  • these numbers should be in the range that is expected
    by the system

CR SYSTEM RANGE:
- is given as 1700-2300

DR SYSTEM RANGE:
- is given as 200-800

21
Q
  1. Which evaluation do these images show?
A
  • Evaluation of Noise
22
Q
  1. What evaluation do these images show?
A
  • Evaluation of Burn Through
23
Q
  1. What happens if the exposure indicator is consistently higher than recommended?
A
  • the patient is being overexposed to radiation
  • they are receiving very high radiation doses
24
Q
  1. Define Contrast?
A
  • a measure of the relative brightness difference
    between two locations (area or pixels) in an image
25
Q
  1. What does the X-Ray beam attenuation in a patient depend on?
A
  • it depends on the energy of the X-Ray beam (quality)
  • it depends on the Total Exposure (mA and kVp)
  • these factors are critical when using film technology
26
Q
  1. Why are the factors of contrast, X-Ray beam quality and total exposure less important in Digital X-Ray systems?
A
  • the images taken at 60 and 110 kV may have similar
    brightness and contrast levels when they are displayed
  • and then viewed on a monitor
27
Q
  1. What must the practitioner do if a range of exposures exists for producing a diagnostic image?
A
  • the practitioner should always select the total
    exposure with the lowest radiation dose
28
Q
  1. What happens when there are imaging variations within the human body?
A
  • there is a natural subject contrast
  • these variations depend on the thickness of the patient
  • they depend on the area of the patients body that is
    being examined
29
Q
  1. Why does the thorax have a high inherent contrast?
A
  • this is because the adjacent areas have large
    differences in their density of structures
30
Q
  1. Why does the abdomen have a low inherent contrast with?
A
  • this is because the adjacent areas have similar atomic
    numbers
31
Q
  1. Can subject contrast ever be altered?
A
  • NO

HOWEVER:
- Auto-winding can modify the image
- this can improve the brightness and the contrast
- these processes can help to create a diagnostic image
from poor raw data

32
Q
  1. What reduces the contrast of any image?
A
  • scattered radiation reaching the detector
  • this is because this radiation does not carry any useful
    information
  • this radiation creates a signal on the detector
33
Q
  1. How can the effect of Scattered Radiation be minimised?
A
  1. Reducing the production of scatter
  2. Preventing the scatter from reaching the detector
34
Q
  1. How do we reduce the production of scatter?
A
  1. We close collimation to the area of interest

COLLIMATION:
- to make rays of light or particles that are accurately
parallel

  1. We displace the body part
    - EG: Mammography
35
Q
  1. How do we prevent the scatter from reaching the detector?
A
  1. Use a grid or Bucky assembly
  2. We use an air gap

NB:
- scattered radiation should be minimised by the
practitioner

MED 107- MEDICAL PHYSICS II (108 decks)

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