Читать книгу Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology - Rachel A. Powsner - Страница 48

When a beam contains photons of different energies such as an X‐ray beam, it is termed polychromatic. As a polychromatic beam penetrates a material, lower energy photons are extinguished or scattered preferentially over higher energy photons and the result is that, while the overall intensity is diminished, the average energy of the transmitted fraction of the beam is increased. This phenomenon is known as beam hardening. A hardened beam is more penetrating and so a second HVL or TVL will be slightly thicker than the first.

Figure 2.6 The amount of attenuation of a photon beam is dependent on the photon energy and the thickness (and/or atomic number) of the attenuator.

Table 2.2 HVL and TVL of lead for photons of common medical nuclides

Nuclide	Gamma energy (keV)	Half‐value layer (cm)	Tenth‐value layer (cm)
99mTc	140	0.03	0.09
67Ga	93, 185, 300, 393	0.07	0.41
123I	159	0.04	0.12
131I	364	0.3	1
18F	511	0.39	1.3
111In	172, 245	0.023	0.2

Figure 2.7 Penetrating radiation and nonpenetrating radiation.

The term penetrating radiation may be used to describe X‐ray and gamma radiation, as they have the potential to penetrate considerable thickness of a material. Although we have just described some of the many ways photons interact with matter, the likelihood of any of these interactions occurring over a short distance is small. An individual photon may travel several centimeters or farther into tissue before it interacts. In contrast, charged particles (alpha, beta) undergo many closely spaced interactions. This sharply limits their penetration (Figure 2.7).