Where does the central ray enter for an AP projection, RPO position of the Lumbar spine?

• A. 2 inches medial to the left ASIS and 1.5 inches superior to the level of the iliac crest
• B. 1 inch medial to the left ASIS and 2 inches superior to the iliac crest
• C. 3 inches lateral to the left ASIS and 0.5 inches superior to the iliac crest
• D. 2 inches lateral to the left ASIS and 1.5 inches inferior to the iliac crest

Answer: (A)

In an AP projection with a right posterior oblique (RPO) position, the left side is the elevated (up) side. The central ray is directed to pass through the lumbar region at about L3, which is roughly 1.5 inches above the iliac crest. Because the elevated side is the one you center to, the entry point is 2 inches medial to the elevated left ASIS. This combination—2 inches medial to the left ASIS and 1.5 inches superior to the iliac crest—aligns the beam correctly to visualize the lumbar area in this oblique view without excessive pelvic overlap. Other entry points would misalign the beam, either placing it too far lateral or at the wrong vertical level to capture the intended portion of the lumbar spine.