Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

When they lose all (or most) of their kinetic energy, they may be captured by anatom or absorbed into the free electron pool. Positrons undergo a similar process ofenergy deposition via excitation and ionization; however, when they come to restthey react violently with their antiparticles (electrons). This process results in theentire rest mass of both particles being instantaneously converted to energy andemitted as two oppositely directed (i.e., 180 degrees apart) 51l-keV annihilationphotons (Fig. 18-4). According to Einstein's mass-energy equivalence formula, E =mc 2 , 511 keV is the energy equivalent of the rest mass of an electron (positron ornegatron). Therefore, there is an inherent threshold for positron decay equal to thesum of the annihilation photon energies (i.e., 2 X 511 keY; or 1.02 MeV). The transitionenergy between the parent and daughter nuclide must be greater than orequal to 1.02 MeV for positron decay to occur. Medical imaging of annihilationradiation from positron-emitting radiopharmaceuticals, called positron emissiontomography (PET), is discussed in Chapter 22.Electron capture (E) is an alternative to positron decay for neutron-deficient radionuclides.In this decay mode, the nucleus captures an orbital (usually a K- or L-shell)electron, with the conversion of a proton into a neutron and the simultaneous ejectionof a neutrino. Electron capture can be described by the following equation:AX - AyZ + e ~ Z-J + V + energy(neutrino)The net effect of electron capture is the same as positron emission: the atomicnumber is decreased by 1, creating a different element, and the mass numberremains unchanged. Therefore, electron capture is isobaric and results in an increasein the N/Z ratio.