ABSTRACT
Both the linear and mass attenuation coecients are functions of the x-ray energy.
Several physical processes contribute to the attenuation coecients. At medical diagnostic energies (from about 20 to 140 keV), the two most important processes are
1. Photoelectric absorption 2. Compton scatter
e attenuation coecient of an arbitrary material is the sum of these two processes:
m m m r r= + = ´ = ´ +( )P C P Cm m m , (5.2)
where the subscripts stand for the two processes and each has its own dependence on energy. If the elemental formula is known for the material such as H2O, the mass attenuation coecient of water can be found by
m N A m
N A
, (5.3)
where the sum is over the elements and Ni and Ai are the number of atoms of the ith element and its atomic weight. If the fractional weight of a tissue is known, such as fat with 12% hydrogen, 77% carbon, and 11% oxygen [2,3], then the mass attenuation coecient is given by
m w mi i
= å (5.4)
with wi the fractional weight of element i or one of several composite materials that make up the tissue. Values for mi for the elements and composite materials can be found or calculated on the NIST website [4].
