When performing the AP lumbar projection, where is the upper edge of the collimation typically marked?

In the AP lumbar projection, the upper edge of the collimation is typically marked at one inch above the iliac crest. This position is chosen because it ensures that the entire lumbar spine is included in the radiographic field while also minimizing exposure to surrounding tissues. The iliac crest serves as a reliable anatomical landmark for this positioning, as it is located at the level of L4-L5 vertebrae, which corresponds well with the lumbar region we aim to image. Positioning the collimation at this point also adheres to radiation safety standards by limiting the exposure area, focusing on the area of interest.

Other potential landmarks, such as the xiphoid process, the lower rib margin, or the umbilicus, do not accurately reflect the optimal field for imaging the lumbar spine specifically and could result in compromising image quality or unnecessary radiation to other areas.