Final answer:
The anode heel effect happens in X-ray tubes when the intensity of X-rays emitted from the anode side is lower than the intensity emitted from the heel side.
Step-by-step explanation:
The anode heel effect occurs in X-ray tubes when the intensity of X-rays emitted from the anode side of the tube is lower than the intensity emitted from the heel side. This effect is caused by the geometry of the X-ray tube and the absorption of X-rays in the anode material.
The anode heel effect is more pronounced in X-ray tubes with a large anode angle, where the anode is tilted with respect to the X-ray beam. When X-rays are emitted from the anode, they undergo attenuation as they pass through the anode material. The X-rays emitted from the anode side experience more absorption compared to those emitted from the heel side. As a result, the X-ray intensity is higher on the heel side and lower on the anode side.
For example, in a chest X-ray, the anode side is positioned towards the patient's head, where the X-ray beam needs to penetrate through a larger thickness of body tissue. The higher X-ray intensity on the heel side compensates for the absorption in the anode material and ensures more uniform X-ray image quality across a large field of view.
This occurs due to the geometry of the X-ray tube and the absorption of X-rays in the anode material. The anode heel effect is more noticeable in tubes with a large anode angle, where the anode is tilted with respect to the X-ray beam.