In the AP projection of the lumbar spine with an RPO position, the midsagittal plane forms what angle with the image receptor?

• A. 45 degrees
• B. 0 degrees
• C. 90 degrees
• D. 30 degrees

Answer: (A)

When you position the lumbar spine obliquely, you rotate the patient so that the midsagittal plane sits at a specific angle to the image receptor. For an AP projection in an RPO position, the standard rotation is about 45 degrees. This places the midsagittal plane roughly 45 degrees to the IR, which is ideal for opening the lumbar facet joints and reducing overlap so the anatomy of interest is best visualized. The other angles correspond to different setups: zero rotation is a straight AP with no oblique view, ninety degrees would be a true lateral, and thirty degrees is an under-rotated oblique not typically used for lumbar foramina visualization.