What is the correct central ray (CR) angle for an anteroposterior (AP) axial projection of the sacroiliac (SI) joints?

The central ray (CR) angle for an anteroposterior (AP) axial projection of the sacroiliac (SI) joints is typically set at 30 to 35 degrees cephalic. This specific angling is essential for properly visualizing the sacroiliac joints, as the joints are oriented obliquely in the body. A cephalic angle directs the x-ray beam upwards towards the head, which aligns the beam with the anatomy of the sacroiliac joints for improved visualization and clearer images.

When using this angle, the CR is positioned at the midline, a couple of inches below the level of the anterior superior iliac spines (ASIS), which also helps center the beam on the SI joints and reduces the potential for distortion. This positioning and angling help in visualizing the joints without superimposition from surrounding structures, making it a critical technique in radiographic procedures involving the pelvis and lower spine.

Other angles, such as 20-25 degrees caudal or 10-15 degrees cephalic, do not adequately align the beam with the orientation of the SI joints, leading to potentially suboptimal images. Likewise, a perpendicular angle may not provide the necessary separation and detail