Heel effect
phenomenon in X-ray tubes
In X-ray tubes, the heel effect or, more precisely, the anode heel effect is a variation of the intensity of X-rays emitted by the anode depending on the direction of emission along the anode-cathode axis. X-rays emitted toward the anode are less intense than those emitted perpendicular to the cathode–anode axis or toward the cathode. The effect stems from the absorption of X-ray photons before they leave the anode in which they are produced. The probability of absorption depends on the distance the photons travel within the anode material, which in turn depends on the angle of emission relative to the anode surface.
Factors
Source-to-image distance (SID)
The distance from the anode (the source of X-rays) to the image receptor influences the apparent magnitude of the anode heel effect. At short SID, image receptor captures a wider range of X-ray intensities than the same size image receptor at larger source–image distances (SID).
Beam width and receptor size
Wide X-ray beams can be cropped to narrow X-ray beams by use of a beam restricting device (variable-aperture X-ray "collimator", fixed aperture, or cone). A beam that is wide along the cathode–anode axis contains a wider range of X-ray intensities than a narrow beam. In a wide beam, a large image receptor captures a wider range of X-ray intensities than a small receptor (at the same SID). Both of these factors influence the visibility of the anode heel effect.
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