Effective atomic numbers for photon energy absorption and photon attenuation of tissues from human organs

Abstract

Effective atomic numbers for photon energy- absorption (Z pea eff) and photon interaction (Z pi eff) of human organs and tissues such as cortical bone, ovary, eye lens, testis, breast tissue, adipose tissue, lung tissue, soft tissue, soft tissue, (4-component), blood (whole), brain (grey/white matter), and skeletal muscle have been calculated by a direct method in the energy region of 1 keV to 20 MeV. The Z pea eff and Z pi eff values steadily increase, up to 8–50 keV, and steadily decrease up to 1.25–2.0 MeV for all of the substances studied. From 2.0 MeV, the values rise with the increase in energy, up to 20 MeV. Significant differences exist between the Z pi eff and Z pea eff in the energy region of 20–400 keV and 3–20 MeV for cortical bone; 15–150 keV for soft tissue, ovary, testis, blood, brain, lung, and skeletal muscle; 15–100 keV for breast tissue, eye lens, and soft tissue (4-component); and 10–100 keV for adipose tissue. A maximum difference of 28.37% is observed at 100 keV for cortical bone, and 30.43% at 40 keV for adipose tissue. For ovary, eye lens, testis, breast tissue, lung tissue, soft tissue, soft tissue (4-component), blood (whole), brain (grey/white matter), and skeletal muscle, a maximum difference of 31.74%, 29.60%, 31.87%, 30.61%, 31.47%, 31.52%, 29.95%, 31.63%, 32.36%, and 31.42%, respectively, is seen at 50 keV. The energy positions at which the maximum of Z pea eff and Z pi eff occurs differ. The single effective atomic number directly obtained using the program XMuDat (Z xmudat eff) are found to be higher compared to those of Z pea eff and Z pi eff values. The effect of absorption edge on effective atomic numbers, and its variation with photon energy and the possibility of defining 2 set values of effective atomic numbers below the absorption edges of elements present in the organs and tissues, are discussed.