Radionuclides for PET and SPECT Molecular Imaging Probes

Gamma (y) rays are photons deriving from isomeric transitions. Isomeric transitions occur when a nucleus remains in an excited state after a particle emission or a decay by electron capture. These intermediate levels are referred to as isomeric states (or metastable states), and each decays to a lower state (either the ground state or another intermediate state) with lifetimes from picoseconds to years. Gamma ray emissions are characteristic of the radionuclide, and the energies of the emitted photons depend on the energy differences between the initial excited state and the next one. 

Radionuclides are primarily produced in cyclotrons or reactors, depending on the nuclear reaction required. Very short-lived radionuclides such as C, and are available only in institutions that have a cyclotron facility, and this limits their widespread use. Remote facilities rely on commercially available long- and medium-lived radionuclides In, Ga, etc.) and radionuclides produced by generators (e.g., Tc). The most commonly used radionuclides in PET and SPECT imaging are listed in Table 1.1. A complete discussion about radionuclide production, labeling conditions, and recent progresses in radiochemistry can be found elsewhere (18-20). 

T /2 Decay modes (%) Main y keV (%) /3max MeV (/lave) Production (18) 

The chemistry of iodine radionuclides used in nuclear medicine has been extensively explored. Isotopic exchange procedures, direct iodination approaches (those using chloramine-T and iodogen), and indirect approaches (the Bolton-Hunter method) are but a few of the well-known synthetic strategies to label biomolecules with iodine (24). Some of the most important 1-labeled imaging tracers [1-labeled iodoazomycin arabinoside ( lAZA), 1-labeled vasoactive intestinal peptide ( 1-VlP), etc.] will be discussed in the following sections. 

Copper radionuclides are promising candidates for both PET imaging 

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