3-12. ANODE HEEL EFFECT
Because .the anode is angled, the intensity of the x-ray beam along the
longitudinal axis of the tube varies. This variation in intensity results from absorption of
some photons by the target itself. Consider figure 3-12 where several photons are
given off at a point within the target. Those photons which make up the anode side of
the x-ray beam stand a greater chance of being absorbed because they have to travel
through more target material than those which make up the cathode side of the beam.
(Notice the different distances from the point where the photons are given off to the
edge of the target.) Consequently, the intensity of the x-ray beam is greater on the
cathode side than on the anode side. This nonuniformity is known as anode heel effect.
Figure 3-12. Anode heel effect.
a. The anode heel effect can be used to advantage when x-raying parts of
uneven thickness and/or density, such as the lower leg. By placing the proximal end of
the leg under the cathode side of the tube, the finished radiograph would have balanced
density. On the other hand, a disadvantage of the heel effect can be experienced when
parts of even density and thickness are x-rayed.
(1) Figure 3-13 shows intensity percentages caused by the heel effect with
a 20 target at various emission angles. Note how focus-film distance affects the
percentage variations. If a radiograph were made of part number one using film number
one a, the intensity spread over the film would be 95 percent to 104 percent, a
difference of 9 percent, which would be negligible. If a radiograph were made of part
number two, using film number two a, the intensity spread would be 31percent to 95
percent, a difference of 64 percent, which is not tolerable for interpretation.
(2) From the above information, one can see that the focus-film distance
and, therefore, the area of the x-ray beam to be used, must be considered in connection
with the heel effect. In other words, the anode heel effect can cause an exposure
problem at short focus-film distances while at long focus-film distances, the problem is
less likely to exist.