avb
Attenuation Patterns
X-ray source
Film
Muscle
Skin
A
B
Imagine
that the region of the knee is being subjected to X-ray imaging. The X-ray
beam passes from the source to the film as shown, with the knee joint in
between. The cross section of the knee in the lower part of the picture shows
how X-rays may be attenuated.
Note the
two hollow bones (most long bones in the body are hollow). The large masses
are the muscles, with blood vessels and nerves scattered among them.
Most
significantly, note that X-rays passing through the region labelled ‘A’ face a
much larger thickness of bone compared to those passing through ‘B’.
The
muscles, though much thicker, still do not offer as much “opacity” as the
bones, the skin and the softer tissues even less.