Download - Week 3 SID OID 122
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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.
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Impact of SID and OFD on Image Quality
• When we can’t get the part close to the film, the SID should be increased.
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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.
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Experiment
• Film # 4 is taken @ 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
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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
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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.
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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.
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Magnification Distortion
• 1. Compare the images on films 4 and 5.– Evaluate the image size and detail. Are both
films the same?
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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?
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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..
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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 DistanceMagnification =Source to Object Distance
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Magnification Distortion
• Film 4 SID=34” Object size= 1.5” Sponge height= 6.75”
• Image A MF= 34 / 34-1.5 = 1.046• Image B MF=34/ 34-1.5-6.75 = 1.32• Note that even with the Object on the film
there is some magnification distortion
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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.
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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
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Magnification Distortion
• If a long SID produces the least magnification distortion, why don’t 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.
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The End
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