Final answer:
The IR size or collimation for AP/PA axial projection of SI joints includes the entire sacrum and both hip joints. Proper patient positioning is essential for accurate imaging of the SI joints.
Step-by-step explanation:
The IR size or collimation for AP/PA axial projection of SI joints depends on the specific imaging equipment and techniques used. However, in general, the collimation for this projection should include the entire sacrum and both hip joints. The field of view should also include the area from the posterior superior iliac spines (PSIS) to the ischial tuberosities.
It is important to ensure proper alignment and positioning of the patient to obtain accurate images of the SI joints. The patient should be lying supine on the radiographic table with both legs extended straight. The central ray should be directed perpendicular to the center of the IR.
Overall, the collimation and positioning should allow for clear visualization of the entire sacroiliac region without any cut-off or overlap.