Radiotracer properties
PET studies range from standard image displays that provide indices of physiological function to complex kinetic analysis methods for absolute quantification. Depending on their biochemical properties, PET radiotracers may be divided into two categories.
The first category includes non-specific radiotracers following a biochemical pathway and allowing for measurement of tissue extraction or metabolism. Radiotracers from the first category include: 15O-water which is a freely diffusible inert tracer used for cerebral blood flow measurement; 18FDG which follows the initial phases of glucose metabolism but does not enter the Kreb's cycle after phosphorylation and therefore is effectively trapped in the cells to allow tissue glucose metabolism measurement; and 18F-fluoromisonidazole which is a bio-reductive drug that in vivo follows an intracellular reduction pathway and can be used for viable hypoxic tissue measurement. Radiopharmaceuticals from this category may be assessed using a single or two-compartmental plasma-tissue model.
The second category includes specific radioligands involved in an interaction with a receptor transporter or a receptor site. Radiotracers from the second category include: 11C-flumazenil which is an antagonist with high affinity and selectivity for central benzodiazepine receptors; and 11C-SCH23390 which is an antagonist with high affinity and selectivity for dopamine D1 receptors. Both radiotracers are used to study changes in density and affinity of central receptors. Radiopharmaceuticals from this category may be assessed using a three-compartment model: unmetabolised free ligand in plasma; free ligand in tissue; and specifically bound ligand in tissue.
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