The primary reason for having the patient flex the knees for an AP projection of the thoracic spine in the supine position is to decrease OID.

• A. Decrease OID
• B. Shorten exposure time
• C. Improve contrast
• D. Increase image sharpness

Answer: (A)

Flexing the knees in a supine AP thoracic spine study reduces the curvature of the spine and positions the vertebrae closer to the image receptor. When the hips and pelvis are less lordotic, the thoracic spine sits flatter against the table, bringing the spine nearer to the IR and decreasing the object-to-image distance. Lower OID minimizes magnification and improves geometric sharpness of the vertebral bodies, which is the primary reason for this positioning. Shorter exposure time and improved contrast aren’t direct results of knee flexion, and while better sharpness can follow from reduced OID, the key point is reducing the distance between the spine and the receptor.