The central ray entry for AP projection of the lumbar spine in an RPO position is 2 inches medial to the left ASIS and 1.5 inches superior to the iliac crest. Which of the following describes this entry?

• A. 2 inches medial to the left ASIS and 1.5 inches superior to 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)

The entry point for an AP lumbar spine oblique is described using palpable landmarks: the anterior superior iliac spine and the iliac crest, with directions toward or away from the midline and above or below the crest. In this case, the central ray is 2 inches medial to the left ASIS and 1.5 inches superior to the iliac crest. Medial from the left ASIS means toward the midline from that landmark, and 1.5 inches above the iliac crest places the ray at about the L3 region. This combination is exactly how you describe the appropriate entry for this projection, aligning the beam to open the intended lumbar joints. Other placements—more lateral, or positioned at a different vertical level relative to the crest—would not target the same anatomy or open the same joints.