Anode heel effect and focus-film distance.
b. The intensity percentages given in figure 3-13 are specifically for a 20 o target.
As the target angle becomes smaller, the difference in the intensity percentages
becomes greater, which in effect is further non-uniformity of the x-ray beam. Hence,
when using tubes with small target angles, extra care should be taken to avoid the
unbalanced density that the anode heel effect can cause on radiographs.
c. The target angle also affects the total area of x-ray coverage. As the angle is
reduced, so is x-ray coverage, as seen in figure 3-14. The 10 target has less coverage
than the 20 target at an equal distance from the tube. X-ray coverage usually is of no
consequence, but with small target angles it may interfere with certain examinations.
For instance, a 12 target at a 40-inch focus-film distance (FFD) will only cover an area
with an a 8 inch radius and a 10 target at a 40-inch FFD will only cover area with a
7-inch radius. Clearly, these two targets will not cover a 14 x 17 film at 40 inches since