Final answer:
The AP projection of the L5-S1 interspace is typically achieved through the Ferguson technique where the patient lies on their back with the higher side's hip and knee flexed and with a slight elevation of the pelvis.
Step-by-step explanation:
The position that is typically used to provide an AP projection of the L5-S1 interspace is known as the Ferguson technique. In this position, the patient lies on their back on the X-ray table with the hip and knee of the higher side (usually the left side) flexed. A cushion or foam pad may be placed under the lower side to elevate the pelvis slightly. The central ray is directed perpendicular to the film at a point 1.5 inches superior to the pubic symphysis and centered to midline of the body. This position is used because it opens the interspace for better visualization.