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MED 107- MEDICAL PHYSICS II > 20. X-RAY IMAGE QUALITY > Flashcards

20. X-RAY IMAGE QUALITY Flashcards

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1
Q
  1. Why are poor quality images easier to define?
A
  • they have a poor signal-to-noise ratio
  • they have poor spatial resolution
  • they detract from the process of extracting information
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2
Q
  1. How must radiological images be produced?
A
  • with the lowest radiation dose that is consistent with the
    diagnostic quality
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3
Q
  1. What does an optimum quality image enable the user to do?
A
  • it enables them to make an accurate diagnosis
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4
Q
  1. What can happen to specific characteristics of an image?
A
  • they can be evaluated
  • this enables us to determine the diagnostic quality of
    the image
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5
Q
  1. List the characteristics that can be evaluated.
A
  1. Patient Positioning
  2. X-Ray beam and detector positioning
  3. Collimating and centring the beam to the area of
    interest
  4. Minimising the patient movement
  5. Detector data that is acquired
  6. Quantity and Quality of photons collected
    (brightness and contrast)
  7. Scattered Photons
    (noise)
  8. Image viewing display
  9. Monitor and image matrix size
  10. Software processing applied to the raw data
  11. Viewing conditions
    (background illumination)
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6
Q
  1. What can be said about the sizes of the radiographic images?
A
  • all radiographic images are larger than the objects
    being X-Rayed
  • the magnification is due to the geometry of the
    imaging
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7
Q
  1. What is the ideal image geometry situation?
A
  1. THE OBJECT THAT IS BEING IMAGED
    - is parallel to the X-Ray beam and the image receptor
  2. THE RADIATION BEAM
    • is at right angles to the object

3.THERE IS A LONG FOCUS TO RECEPTOR DISTANCE
- there is a small object to receptor distance

NB:
- these conditions minimise the distortion of the image
- they minimise the magnification of the unsharpness in
the image

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8
Q
  1. What can be said about the distance between the object and the image receptor?
A
  • the object should be as close as possible to the image
    receptor

AS THE OBJECT MOVES AWAY FROM THE IMAGE RECEPTOR
- the magnification will increase
- this makes the object bigger
- this magnifies any unsharpness in the image

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9
Q
  1. How do we mathematically describe the relation between the object distance and the magnification?
A

NB:
- the positioning of the patient to produce the image
has a direct effect on the quality of the image

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10
Q
  1. In practice, how much Geometric Sharpness (Ug) are we dealing with?
A
  • the amount of Geometric Sharpness is small
  • it may be much less than 0.4 mm

AT 0.4 MM:
- is the point at which we begin to perceive
unsharpness in an image
- this is due to the geometric unsharpness

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11
Q
  1. How do we work out the Geometric Unsharpness?
A
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12
Q
  1. Name 2 other factors that increase the level of Unsharpness?
A
  1. Movement
  2. The resolution of the monitor
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13
Q
  1. When will an object be magnified?
A
  • if the object is not parallel to the image receptor
  • different aspects of the object will be magnified
    differently
  • this causes distortion
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14
Q
  1. What are 2 examples of Distortion?
A
  1. Elongation of the image
  2. Foreshortening of the image
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15
Q
  1. What are deliberately elongated images used for?
A
  • they are used to aid in the diagnosis of a fracture
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16
Q
  1. What can be said about the distance between the patient and the image receptor?
A
  • this distance is abbreviated as ORD
  • this distance should be as short as possible
  • the object should be in contact with the image receptor
    if possible
17
Q
  1. How long is the FRD usually?
A
  • it is about 110 cm for techniques on the X-Ray table
  • it is about 180 cm for erect chest and cervical spine
    work
18
Q
  1. How can we assess the image quality?
A
  • we can assess it using the signal-to-nose ratio
  • this is abbreviated to: SNR
19
Q
  1. What is the SNR?
A
  • it is the useful information from the patient
  • this information is being imaged
  • the noise is anything that detracts from accessing the
    information
20
Q
  1. Where is the useful information derived from?
A
  • from the Photoelectric Interactions within the patient
21
Q
  1. Where is the noise derived from?
A
  • Compton Scatter
22
Q
  1. What does the image receptor not have the ability to do?
A
  • it does not have the ability to determine the origin of
    scattered photons
  • there is also electrical noise from the system that
    interferes with this process
23
Q
  1. What can be said about the signal level of Radiographic images?
A
  • they have a signal level that is high compared to the
    noise
  • this enables structures to be seen clearly
24
Q
  1. When will the structures disappear?
A
  • when the signal level is similar to or less than the noise
25
Q
  1. What can be said about the images produced by
    X-Rays?
A
  • they are often a compromise between obtaining a
    perfect signal and reducing the noise
26
Q
  1. What exists when we image radiosensitive areas?
A
  • there is a constraint of minimising the radiation dose
    to that area
27
Q
  1. What can be said about the extremities of the human body?
A
  • they are less radiosensitive
  • images with higher doses and better definition will be
    required for X-Ray purposes
28
Q
  1. What is the purpose of a Bucky (grid)?
A
  • it will enhance the contrast
  • it does this by removing the scattered photons before
    they reach the detector
29
Q
  1. Why do we need to justify the increased contrast by the use of the grid?
A
  • this is because we are using a grid that will increase the
    dose
30
Q
  1. What happens when there is an air gap left between the patient and the image receptor?
A
  • there are fewer scattered X-Rays that will be
    intercepted
31
Q
  1. Why are there fewer scattered X-Rays that are intercepted?
A
  • the scattered X-Rays emerge from the body with a
    greater range of angles than the transmitted photons

MED 107- MEDICAL PHYSICS II (108 decks)

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