Impact of SID and OFD on Image Quality

1
Impact of SID and OFD on Image Quality

• We usually want to get the body part as close to
  the film as possible to reduce magnification
  distortion.

2
Impact of SID and OFD on Image Quality

• When we cant get the part close to the film, the
  SID should be increased.

3
Impact of SID and OFD on Image Quality

• When the SID is increased, we use the less
  divergent rays in the center of the beam.
• This reduces magnification.

4
Experiment

• Film 4 is taken _at_ 34 SID.
• The wrist is placed on the film for Image A.
• The wrist is placed on two sponges to increase
  the OID for Image B

5
Technique adjustment

• If 2 mAs was used at the 34 SID, what mAs would
  be used at 68?
• New mAs Old mAs x new distance 2
• Old distance2
• New mAs 2 x 682 / 342 8
• If the distance is doubled, the mAs will be the
  square of the difference or 4 x 2 8

6
Experiment

• Film 5 was taken with an increased SID.
• Image B was taken with the wrist resting on the
  film.
• Image A was taken with the wrist resting on the
  sponges.

7
Magnification Distortion

• When the Object to Film Distance increases we
  get magnification distortion. The part will be
  larger than actual size.
• When the Source to Image Distance is increased,
  the magnification distortion is reduced.

8
Magnification Distortion

• 1. Compare the images on films 4 and 5.
• Evaluate the image size and detail. Are both
  films the same?

9
Magnification Distortion

• 1. The images are not the same. The films taken
  at 68 are smaller and sharper.
• 2. What observations can be made when the image
  is close to the film and when it is away from the
  film?

10
Magnification Distortion

• 2. The amount of magnification is determined by
  the Object to Image Distance.
• The Source to Image Distance impacts the
  magnification distortion. Longer SID will produce
  less magnification..

11
Magnification Distortion

• If we know the size of the object, the SID and
  the OID we can compute the magnification factor
  using this formula
• Source to Image Distance
• Magnification Source to Object Distance

12
Magnification Distortion

• Film 4 SID34 Object size 1.5 Sponge height
  6.75
• Image A MF 34 / 34-1.5 1.046
• Image B MF34/ 34-1.5-6.75 1.32
• Note that even with the Object on the film there
  is some magnification distortion

13
Magnification Distortion

• Image 5 SID 68 Object 1.5 Sponge height
  6.75
• Image B MF 68/ 66.5 1.02
• Image A MF 68/ 59.75 1.13
• If you want actual size, a long SID must be used.
  If the Object is away from the film, a long SID
  will reduce magnification.

14
Magnification Distortion

• 4. Compare the 68 films to the 20 films.
• The films taken at 20 have more magnification
  distortion.
• When we want the least amount of magnification
  distortion, a 72 SID is used. Example P-A
  Lateral Chest x-ray

15
Magnification Distortion

• If a long SID produces the least magnification
  distortion, why dont we take all films with a
  long SID?
• Radiation dose and tube limits. At 72 it takes
  3.5 times more radiation to produce the same
  image. This would result in longer exposure times
  and shorter tube life.

16
The End

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