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MED 107- MEDICAL PHYSICS II > 18. X-RAY DETECTORS > Flashcards

18. X-RAY DETECTORS Flashcards

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1
Q
  1. What are various Radiation Detectors designed for?
A
  • different purposes within Medical Imaging
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2
Q
  1. List the 5 types of Radiation Detectors.
A
  1. Film-Based Detectors
  2. Computed Tomography Detectors
  3. Detectors used in Fluoroscopic Imaging
  4. Automatic Exposure Devices
  5. Detectors used within General Radiography
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3
Q
  1. What is the function of the Image Receptor used in forming a radiograph?
A

IT PLAYS AN ESSENTIAL ROLE IN DETERMINING:
- contrast
- noise level of the image
- spatial resolution
- the patient’s radiation dose

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4
Q
  1. What was the standard choice for recording X-Ray images for decades after the early days of Radiography?
A
  • Film
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5
Q
  1. List the 5 advantages of Film Based Radiography?
A
  1. Extremely good spatial resolution
  2. Good sensitivity
  3. Low Price
  4. Widely available
  5. Permanency for record keeping
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6
Q
  1. What parts does the Film used in Radiography consist of?
A
  1. A FILM BASE
    • this can be made of transparent polyester
    • or it can be made from an acetate sheet
  2. EMULSION COATINGS
    • these capture the images
    • the emulsion is a suspension of tiny silver bromide
      grains in gelatine
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7
Q
  1. How does Film Based Radiography Work?
A

WHEN THE FILM IS EXPOSED:
- the photons will interact with the grains
- this sensitises the grains
- these grains form a latent image
- this means the image exists but has not yet been
developed

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8
Q
  1. What happens to the sensitised grains?
A
  • they are retained
  • they are blackened during the process of development
  • this leaves those regions to be opaque
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9
Q
  1. What happens to the Unsensitised Grains?
A
  • they are removed
  • this leaves behind a transparent base
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10
Q
  1. How is the Radiographic image formed when it comes to Film Based Radiography?
A
  • it is formed by the resulting pattern of the transparent
    and opaque regions of film
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11
Q
  1. Is photographic film sensitive to X-Rays by itself?
A
  • no
  • even though it has basic processes that allow it to
    create a radiograph

NB:
- this sensitivity can be increased

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12
Q
  1. How can the sensitivity of photographic film to
    X-Rays be increased?
A
  • the film can be placed close to one or two of the thin
    intensifying screens of Fluorescent Material
  • this forms a Film-Screen combination
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13
Q
  1. What do the Intensifying Screens do?
A
  • they convert the X-Ray photons to visible photons
  • these interact with the motion of the film
  • this helps to form the image

NB:
- an increase in the intensifying screens leads to an
increase in the details shown in the photo
- it also leads to an increase in the sensitivity of the
photographic film to the X-Rays

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14
Q
  1. How does the Intensifying Screen system work?
A
  • the X-Ray photons hit the intensifying screen
  • this emits visible photons
  • these photons hit and expose the film emulsion
  • Phosphors in the screens will absorb the X-Rays
  • they then re-emit their energy as visible light
  • the entire system will then be encased in a light-tight
    cassette for handling
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15
Q
  1. How can we improve the detection efficiency of Film further?
A
  • the X-Ray film can be made with an emulsion on both
    sides
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16
Q
  1. Do these images make sense?
A
  • yes
17
Q
  1. Can X-Ray film images be manipulated?
A
  • not really
  • there are very few manipulations that can enhance the
    quality of the images
  • other than magnification
18
Q
  1. What are some issues of Film Based Radiography?
A
  1. FILM IS LIMITED
    - in its ability to distinguish between subtle variations
    in the absorption of X-Rays
    - this restricts its ability to have contrast in the image
19
Q
  1. Who typically views the X-Ray films?
A
  • Radiologists
  • they are viewed in transmission
  • they are illuminated from behind with a light box
20
Q
  1. What does Optical Density measure?
A
  • it measures the perceived extent of film transparency
    and opaqueness
21
Q
  1. What does Optical Density do?
A
  • it measures the extent of film transparency and
    blackness in the image

AN INCREASE IN OPTICAL DENSITY:
- leads to the film being more black
- the film is more exposed to the x-Ray
- the film is more transparent

22
Q
  1. What do the Dynodes in this image allow for?
A
  • they allow for the attraction of negative electrons
23
Q
  1. What happens in the Photomultiplier tube?
A
  • the photon is converted to a photo electron
  • this photo electron needs to be multiplied
  • this will convert it into an electrical signal
  • this will form the digital image
24
Q
  1. What does the Scintillator detector do?
A
  • it uses a crystal
  • this crystal absorbs X-Rays
  • it then re-emits their energy as Visible Light Photons
25
Q
  1. What happens to these Visible Light Photons?
A
  • they enter the Photomultiplier Tube
26
Q
  1. What happens to these Visible light photons inside the Photomultiplier Tube?
A
  • they hit a Photocathode
  • this is a device that converts light into photoelectrons
  • it does this via the photoelectric effect
27
Q
  1. What can be said about these Photoelectrons in the Photomultiplier Tube?
A
  • they are too few to produce an appreciable electrical
    signal

THE OVERCOME THIS PROBLEM BY:
- accelerating at a large voltage towards a positive
electrode
- this is known as the Dynode

28
Q
  1. What happens when the energetic incoming electrons and the Dynode metal collide?
A
  • they free many more electrons
  • these newly freed electrons are accelerated to the next
    dynode
  • this further multiplies the signal
  • this causes these electrons to collide again
  • this then frees more electrons
29
Q
  1. What happens after many such multiplications between the electrons and the Dynodes?
A
  • a very large electrical signal is produced from each
    original X-Ray

MED 107- MEDICAL PHYSICS II (108 decks)

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