Chapter 7 - Ionising Radiation and Tissue

Abstract

This chapter reviews the interaction between tissue and all forms of ionizing radiation, both particles and photons. The values observed for muscle can be taken to closely characterize the behavior of most soft tissues when studying tissue mass attenuation coefficient, μ/ρ, and mass energy absorption coefficient, μ en /ρ, for photons. Factors affecting photon attenuation are photon energy, atomic number, temperature, tissue variability, and fixation and storage. The definition for mass attenuation coefficient for photons may also be applied to neutrons, and similarly the attenuation coefficient may by separated into absorption and scatter components. It is convenient to quantify the interaction of neutrons and tissue using the kerma, K. The kerma can be defined as dE tr /dm, where dE tr is the sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged ionizing particles in a material of mass m, the unit used for kerma is joule per kilogram (J kg −1 ) and is also referred to as gray (Gy). For neutrons of energy E, the relationship between energy fluence, Ψ, and kerma, K, can be represented by the equation K = ψ [ μ t r ρ ] = Φ [ E [ μ t r ρ ] ] , where μ tr is the mass energy transfer coefficient,Φ is the particle fluence, and [E(μ tr /ρ)] is termed the kerma factor. The kerma factor has units gray per metre 2 (Gy m −2 ). It has been observed that two naturally occurring radionuclides reside in the body tissues, namely, Potassium-40 and Carbon-14.