Final answer:
The left sacroiliac joint is positioned perpendicular to the IR when the patient is positioned in a prone position.
Step-by-step explanation:
The left sacroiliac joint is positioned perpendicular to the IR when the patient is positioned in a prone position. In the prone position, the patient lies face down with their upper limbs, including palms, touching the sides and lower limbs touching at the sides. This positioning allows for a clear and accurate imaging of the left sacroiliac joint.