What is the proper CR placement for an AP oblique projection of the sacroiliac (SI) joint in a right posterior oblique (RPO) position?

The proper central ray (CR) placement for an AP oblique projection of the sacroiliac (SI) joint in a right posterior oblique (RPO) position is 1 inch medial to the anterior superior iliac spine (ASIS). This positioning is crucial as it ensures that the SI joint is accurately visualized and centered in the image.

In an RPO position, the right SI joint is of primary interest, and this technique requires an oblique orientation of the patient to allow for optimal imaging of the joint space without superimposition from adjacent structures. Positioning the CR 1 inch medial to the ASIS effectively targets the area of interest, ensuring that the joint is illuminated correctly and the desired anatomical details are captured.

Other placement options would not effectively visualize the SI joint as intended. For instance, lateral or excessive medial positioning may cause the joint to shift out of the field of view, leading to a suboptimal image. Thus, it's this precise measurement of 1 inch medial to the ASIS that achieves the best results and adheres to standard imaging protocols for examining the sacroiliac joints.