Which of the following projections would NOT take advantage of the so-called "heel effect" if the cathode end was aimed toward the head?

The correct choice indicates a scenario where the "heel effect" is not utilized effectively. The heel effect occurs in radiographic imaging primarily due to the geometry of the x-ray tube, where the intensity of the beam is greater on the cathode side compared to the anode side. This phenomenon is beneficial for certain projections, particularly when the thicker part of the anatomy is positioned toward the cathode end to take advantage of the higher radiation dose.

In the case of the AP thoracic spine projection, the anatomy being imaged is more evenly distributed in thickness, and the projection is made with the patient lying supine. When the cathode side is directed toward the head, the varying thickness of the thoracic spine does not create an optimal scenario for using the heel effect, as the intended benefit of increased intensity toward the thicker parts of the spine is not markedly significant.

For the lateral projections of the femur and thoracic spine and the AP lower leg, there is a distinct advantage to positioning the cathode toward the thicker parts of the anatomy, making the selection of these projections ideal for harnessing the benefits of the heel effect. Consequently, the AP thoracic spine projection does not take full advantage of this effect, as the exposure remains quite uniform