Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

A plot on a lineat axis of activity as a function of time results in a curvilinearexponential relationship in which the total activity asymptotically approaches zero(Fig. 18-1). If the logarithm of the activity is plotted versus time (semilog plot), thisexponential relationship appears as a straight line (Fig. 18-2).As mentioned previously, when the atomic nucleus undergoes the spontaneoustransformation, called radioactive decay, radiation is emitted. If the daughternucleus is stable, this spontaneous transformation ends. If the daughter is unstable(i.e., radioactive), the process continues until a stable nuclide is reached. Mostradionuclides decay in one or more of the following ways: (a) alpha decay, (b) betaminusemission, (c) beta-plus (positron) emission, (d) electron capture, or (e) isomerictransition.Alpha (ex) decay is the spontaneous emission of an alpha particle (identical to ahelium nucleus consisting of two protons and two neutrons) from the nucleus.Alpha decay typically occurs with heavy nuclides (A> 150) and is often followed bygamma and characteristic x-ray emission. These photon emissions are often accompaniedby the competing processes of internal conversion and Auger electron emission.Alpha particles are the heaviest and least penetrating form of radiation considerin this chapter. They are emitted from the atomic nucleus with discreteenergies in the range of 2 to 10 MeV. An alpha particle is approximately four timesheavier than a proton or neutron and carries an electronic charge twice that of theproton. Alpha decay can be described by the following equation:~v A 4 Y 4 +2 ••U"- ~ Z - 2 + 2H e + tranSitiOn energy(alpha particle)Alpha particles are not used in medical imaging because their ranges are limitedto approximately 1 cm/MeV in air and typically less than 100 Ilm in tissue.Even the most energetic alpha particles cannot penetrate the dead layer of the skin.However, the intense ionization tracks produced by alpha particles make them aserious health hazard should alpha-emitting radio nuclides enter the body via ingestion,inhalation, or a wound. Research efforts are underway to assess the clinicalutility of alpha-emitting radio nuclides chelated to monoclonal antibodies directedagainst various tumors as radioimmunotherapeutic agents.