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Understanding
the Image – 2
Attenuation
Attenuation
The most
important factor is the presence of ‘heavy’ elements in the tissues. The term
‘heavy’ refers to the atomic mass (as in the periodic table of elements), not density or
specific gravity.
Most body
tissues contain abundant amounts of carbon (12), hydrogen (1), oxygen (16) and
nitrogen (14). The common heavier elements are calcium (40) and iron (56).
Bone has a great concentration of calcium. Muscle tissue has a fair amount of
calcium; and blood contains calcium and iron.
Remember,
this does not make all bone or blood opaque to X-rays! The thickness of the
tissue and the relative abundance of heavy elements also matters. Thus, a
thick mass of muscle or blood may be more opaque than a thin plate of
bone.
Key Points :
X-ray attenuation depends
largely on the average atomic mass in a tissue, though thickness and density do
have a role to play.