For a thin patient undergoing an AP lumbo-pelvic x-ray, where should the collimation be focused?

In an AP lumbo-pelvic x-ray for a thin patient, the collimation should be focused to the iliac wings. This is because the iliac wings represent the upper limits of the pelvis and encompass the area that needs to be included in the imaging. Proper collimation helps to limit the radiation exposure to surrounding tissues while ensuring that all necessary anatomy is captured in the image, allowing for an accurate assessment of both the lumbar spine and pelvic structures. By centering the collimation at the iliac wings, the radiologist can visualize relevant areas without unnecessary exposure or loss of critical anatomy.

Collimation to other areas like the diaphragm, pelvis only, or lumbar spine would not adequately include all necessary structures for a complete assessment while also risking increased radiation to areas that do not need to be imaged.