Hussein Ali Madlool Dhahi<br />The Anode Heel Effect in Radiographic <br />The heel effect refers to the reduced intensity of the X-ray beam towards the anode side of the X-ray field as shown in figure below. The X-ray photons that are emitted on the anode side of the field must pass through a greater thickness of the anode than those directed toward the cathode side. One unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x-ray field is greater than that on the anode side. Electrons interact with target atoms at various depths into the target. The x-rays that constitute the useful beam emitted toward the anode side must traverse a greater thickness of target material than the x-rays emitted toward the cathode direction. The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target and therefore increased absorption. This is the heel effect.<br />The difference in radiation intensity across the useful beam of an x-ray field can vary by as much as 45%. The central ray of the useful beam is the imaginary line generated by the centermost x-ray in the beam. If the radiation intensity along the central ray is designated as 100%, then the intensity on the cathode side may be as high as 120%, and that on the anode side may be as low as 75%.<br />The heel effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density. In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor. The cathode and anode directions are usually indicated on the protective housing, sometimes near the cable connectors.<br />For better balance of the transmitted X-rays, the cathode side of the tube is oriented over the thicker parts and anode over the thinner parts of the patient.<br />