What is the SID for a lateral projection of the sacrum?

• A. 40 inches
• B. 72 inches
• C. 44 inches
• D. 60 inches

Answer: (A)

The main idea here is choosing the distance from the X-ray tube to the image receptor that gives a sharp, accurately sized image of the sacrum while keeping the exposure practical. For a lateral projection of the sacrum, 40 inches is the standard SID. This distance provides a good balance: it keeps the sacrum close enough to the IR to minimize magnification and blur, yet is practical for patient positioning and dose.

Using a longer SID, like 72 inches, is common for lateral chest radiographs to reduce magnification of the heart and mediastinum, but it isn’t needed for the sacrum and would require adjustments to technique and exposure. Shorter or nonstandard SIDs would either increase magnification (if the SID is too short) or complicate setup without notable benefit. So 40 inches is the typical, best-supported choice for a lateral sacrum projection.