Читать книгу Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology - Rachel A. Powsner - Страница 46

As the result of the interactions between photons and matter, the intensity of the beam (stream of photons), that is, the number of photons remaining in the beam, decreases as the beam passes through matter (Figure 2.5). This loss of photons is called attenuation; the matter through which the beam passes is referred to as the attenuator. Specifically, attenuation is the ratio of intensity at the point the beam exits the attenuator, I_out, to the intensity it had where it entered, I_in. Attenuation is an exponential function of the thickness, x, of the attenuator in centimeters. That the function is exponential can be understood to mean that if half of the beam is lost in traversing the first centimeter of material, half of the remainder will be lost traversing the next centimeter, and so on. This resembles the exponential manner in which radioactivity decays with time. Expressed symbolically,

where μ, the linear attenuation coefficient is a property of the attenuator. When, as is usually the case, thickness is given in centimeters, the linear attenuation coefficient is expressed as “per centimeter” or “cm^–1”. As might be expected, the linear attenuation coefficient is greater for dense tissue such as bone than for soft tissue such as fat. In general, the linear attenuation coefficient depends on both the energy of the photons and on the average atomic number (Z) and thickness of the attenuator. The lower the energy of the photons or the greater the average atomic number or thickness of the attenuator, the greater the attenuation (Figure 2.6).

Figure 2.3 Angle of photon scattering.

Figure 2.4 Photoelectric effect.

A separate term, the mass attenuation coefficient (μ/ρ), is the linear attenuation coefficient divided by the density of the attenuator. When the density of a material is given in grams/cm³ the units of the mass attenuation coefficient are cm²/gram.

Absorption of radiation describes another aspect of the process of attenuation. Attenuation describes the weakening of the beam as it passes through matter. Absorption describes the transfer of energy from the beam to the matter.