Heel Effect

There is a variation in the intensity of an x-ray beam due to the angle at which the x-rays emerge from within the target material.  Those x-rays traveling closer to parallel with the target tend to have longer, more absorbing paths in the target material. In the diagram, the beam intensity increases as it goes from left to right.  The beam intensity diminshes quite rapidly from the mid beam, toward the ANODE side of the tube (to the left). 

This phenomenon is not necessarily bad.  It can be used to obtain balanced densities in exposing parts of the body that vary in absorption of x-ray. 

The target of the angle plays greatly in this effect.  An anode target of 15°, gives a broader beam, therefore greater variation in density, than would a 10° target.  So the comprimise would be film coverage versus.  For example, I once put a replacement tube on an GE AMX-2 portable with to low a target angle.  The result was that the techs could no longer expose a 14 X 36 inch film, used for scoliosis diagnosis of the entire spine.  I had to put on another tube with a greater target angle. 

smaller degree of anode target angle= less heel effect=less effective film coverage 

higher degree of anode target angle= more heel effect=greater effective film coverage