Week 3 SID OID 122

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.

Impact of SID and OFD on Image Quality

• When we can’t get the part close to the film, the SID should be increased.

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.

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

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

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.

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.

Magnification Distortion

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

films the same?

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?

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..

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

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

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.

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

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.

The End

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