Which projection would take advantage of the heel effect if the cathode end is directed toward the head?

• A. AP Thoracic Spine
• B. Lateral Femur
• C. AP Lumbar Spine
• D. Lateral Lumbar Spine

Answer: (B)

The beam intensity varies across the field because more photons are produced toward the cathode side (anode heel effect). When the cathode end is directed toward the head, the portion of anatomy near the head—the proximal part—receives the greater exposure.

In a lateral view of the femur, the proximal femur near the hip is the thickest portion and lies toward the head. Placing the cathode end toward the head increases exposure to that thick, highly attenuating region, helping penetration and image quality where it’s most needed. Other projections don’t align the thickest part of the anatomy with the cathode side as effectively, so they don’t gain the same benefit from the heel effect.