T.O. 33B-1-16-136.2.4.4 IntensityDistributionofanX-RayBeam.6.2.4.5 HeelEffectandBeamCoverage.For simplicity's sake, most literature states that the intensity of radiation of the primary beam is constant. This is notquite 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.6.2.4.5.1The intensity of the beam diminishes fairly rapidly from the central ray toward the anode side and increases slightlytoward the cathode side. In general practice the heel effect is not evident, provided the maximum lateral dimension ofthe object to be radiographed is less than half the source-to-film distance (SFD). In other words, coverage of a 14 by17inch film requires an SFD of approximately 36 inches to provide a field intensity of plus or minus 12 percent overthe 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 SFDmust be increased. For example, to determine the SFD to cover an area that fits within a circle with a diameter of 56inches, do the following calculation:SFDRTan=qqequals the half-angle of the cone = 15 degreesTan 15 = 0.268R = one-half the diameter = 28 inchesTherefore, SFD inches= 2802681045..
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