Where does the central ray enter for an AP projection in the RPO position of the lumbar spine?

In an AP projection of the lumbar spine in the RPO (Right Posterior Oblique) position, the central ray is directed to an area in relation to specific anatomical landmarks. The correct entry point is 2 inches medial to the left anterior superior iliac spine (ASIS) and 1.5 inches superior to the iliac crest.

This positioning helps ensure that the X-ray captures the lumbar vertebrae appropriately while minimizing superimposition of structures. When the patient is in the RPO position, the left side is being targeted for imaging, which necessitates centering the ray more towards the left to adequately visualize the lumbar spine structures on the right side due to the angle of obliquity.

This angle is key in distinguishing the vertebral anatomy and showcases intervertebral foramina and other structures effectively. The measurement given ensures that the site of interest—the lumbar region—is properly illuminated and clear for diagnostic evaluation.