For a 30x35 cm image receptor in an AP projection of the lumbar spine, where should the central ray enter the patient?

In an AP projection of the lumbar spine, the proper centering point for the central ray is commonly located 1.5 inches superior to the iliac crest. This positioning is crucial as it accurately targets the lumbar vertebrae, specifically the area around L3, which is the central vertebra within the lumbar region.

The iliac crest, being a palpable anatomical landmark, serves as a reliable reference for centering the beam correctly. By placing the central ray 1.5 inches above this landmark, the resultant image effectively includes the pertinent structures without clipping important anatomical features. This technique ensures a comprehensive visualization of the lumbar spine that can assist in diagnostic evaluations.

In contrast, centering directly at the level of the iliac crest may not adequately encompass all necessary lumbar vertebrae in the image, while centering at L1 or at the L4-L5 interspace may misrepresent the lumbar alignment or omit crucial segments of the spine depending on the patient's anatomical variations. Thus, centering 1.5 inches superior to the iliac crest is the most effective method for obtaining a quality AP lumbar spine radiograph.