It is generally accepted that an x-ray
beam's intensity is not uniform throughout its entirety. As
x-radiation is emitted from the target area in a conical shape,
measurements have determined that the intensity in the direction of
the anode (AC) is lower (over and above the difference caused by the
Inverse Square Law) than the intensity in the direction of the
cathode (AB). The fact that the intensities vary in such a manner
causes visible differences in the density produced on the
radiographs. This phenomenon is called heel effect and is
illustrated below.
NOTE:
A = 100-percent intensity
AB = consists of a slight increase over 100-percent intensity and
then a general decrease in intensity as B is approached
AC = consists of a considerable decrease in intensity as C is
approached
The decreased intensity at C results from emission which is nearly
parallel to the angled target where there is increasing absorption
of the x-ray photons by the target itself. This phenomenon is
readily apparent in rotating anode tubes because they utilize
steeply angled anodes of generally 17 degrees or less. Generally,
the steeper the anode, the more severe or noticeable the heel effect
becomes.
The effects of focus film distance on the visualization of heel
effect are illustrated below:
Figure G shows the film plane as having a shorter focus film
distance than the film plane in Figure H. Looking at Figure G, you
can readily see that the x-ray beam's involvement in exposing the
film runs from C to B (the full cone of radiation). Heel effect
causes a greater decrease in x-ray beam intensity as one travels
from the central ray to the cathode (A to B).
As you look at Figure H, note that a long focus film distance is
used which results in the involvement of the x-ray beam at the film
plane which does not utilize the full cone of radiation (C to B).
Hence, the extremities of the beam (C and B) are not used in
exposing the film. Because of this, heel effect is greatly reduced. |
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