What Is Anode Heel Effect ?
The principle of anode heel effect is that, the intensity of the x-ray beam that leaves the x-ray tube is not uniform throughout all portion of x-ray beam. The x-ray beam attenuation is greater in anode anode direction than in the cathode direction because of difference in the path length within the target so the results in higher intensity at the cathode side and lower x-ray intensity at the anode side of x-ray beam. This variation is called heel effect.
The heel effect depends on the anode angle, focus to film distance and field size. It is less important at large FFD because the film subtended a smaller beam angle. To reduce heel effect increase the anode angle and field size decrease.
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 image receptor. The anode and cathode direction are usually indicated on the protective housing some time near the cable connection.
In chest radiography, the cathode side is inferior side of chest because the Lower thorax in region of the diaphragm is thicker than the upper thorax there for required higher radiation intensity. In abdomen radiography the cathode in superior side because the upper abdomen is thicker then the lower abdomen and pelvis. In mammography the cathode is positioned towards the chest wall because the chest wall is thicker then the nipples side of breast.
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