Intensity Distribution of an X-Ray Beam.
Heel Effect and Beam Coverage.
For simplicity's sake, most literature states that the intensity of radiation of the primary beam is constant. This is not
quite correct. There is a variation in intensity due to the angle at which the X-rays are emitted from the focal spot.
This is called the heel effect (see Figure 6-10).
Figure 6-10. Variation of Intensity in the Primary Beam Due to the Heel Effect.
The intensity of the beam diminishes fairly rapidly from the central ray toward the anode side and increases slightly
toward the cathode side. In general practice the heel effect is not evident, provided the maximum lateral dimension of
the object to be radiographed is less than half the source-to-film distance (SFD). In other words, coverage of a 14 by
17inch film requires an SFD of approximately 36 inches to provide a field intensity of plus or minus 12 percent over
the whole film. This is based upon using part of the radiation field within a cone having a 30-degree included angle.
(The source for an X-ray tube is the focal spot.) For a single exposure of larger areas requiring multiple films, the SFD
must be increased. For example, to determine the SFD to cover an area that fits within a circle with a diameter of 56
inches, do the following calculation:
q equals the half-angle of the cone = 15 degrees
Tan 15 = 0.268
R = one-half the diameter = 28 inches