Infographics / X-ray Physics

Anode heel effect

An x-ray tube with an angled anode target. Electrons from the cathode strike a focal spot; x-rays fan out to a field. X-rays directed toward the anode side must exit through more target material (the heel) and are self-absorbed, so beam intensity is higher on the cathode side and lower on the anode side. A plot shows relative intensity falling from cathode to anode across the field, and a patient cross-section shows the thicker body part placed under the cathode side for more even exposure.

The anode heel effect An x-ray tube with an angled anode target. Electrons from the cathode strike a focal spot; x-rays fan out to a field. X-rays directed toward the anode side must exit through more target material (the heel) and are self-absorbed, so beam intensity is higher on the cathode side and lower on the anode side. A plot shows relative intensity falling from cathode to anode across the field, and a patient cross-section shows the thicker body part placed under the cathode side for more even exposure. Anode heel effect: beam intensity falls from the cathode side to the anode side cathode electrons anode θ steeper target angle (smaller θ) → larger heel effect anode-side x-rays exit through more target (the heel) and are self-absorbed → lower intensity, harder beam relative beam intensity across the field higher lower cathode side anode side effect is greatest at small anode angle, short focus–film distance and large field; anode side is also harder (higher mean energy). patient — thicker anatomy under the cathode side thick thin clinical use: higher cathode-side intensity offsets the thicker part (e.g. AP femur/spine; mammography cathode to chest wall).
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