What is the correct central ray angle for an AP axial projection of the sacroiliac joints?

• A. 30-35 degrees cephalic
• B. 10-15 degrees caudal
• C. 0 degrees
• D. 45 degrees cephalic

Answer: (A)

The central idea is to align the X-ray beam with the orientation of the sacroiliac joints by using a cephalad angle. The sacroiliac joints run obliquely in the pelvis, roughly from posterior-superior to anterior-inferior, so angling the central ray cephalad about 30 degrees places the beam along the joint plane. This opens and visualizes the joint spaces clearly while reducing superimposition from surrounding structures. A shallower angle (or none) wouldn’t line up with the joints and would obscure or foreshorten them, while a much steeper angle (like 45 degrees) could distort or misproject the anatomy. Therefore, a cephalad angle in the vicinity of 30–35 degrees is the best choice to demonstrate the sacroiliac joints on an AP axial projection.