What is the CR angled for an AP projection of the knee on a patient with an ASIS to tabletop measurement of 26 cm?

For an anteroposterior (AP) projection of the knee, the central ray (CR) needs to be angled appropriately based on the patient's anatomy, specifically the distance from the anterior superior iliac spine (ASIS) to the tabletop. When the measurement is 26 cm, this indicates that the patient's thighs are relatively thick.

In this case, it is appropriate to angle the CR 3 to 5 degrees cephalad. This angling is done to properly align the CR with the intercondylar plane of the knee, which is essential for obtaining a correct image of the knee joint. The cephalad angle compensates for the increased thickness of the soft tissue and helps avoid any potential superimposition of the joint structures, allowing for clearer visualization of the bony anatomy and any pathological conditions present.

Thus, angling the CR cephalad ensures that the beam enters at the correct angle to demonstrate the joint space adequately and minimize distortion of the knee structures on the radiograph.