In this chapter, the labeled compounds will be referred to as tracer, radiotracer, or radiopharmaceutical. The term tracer implies that the injected compound, including both labeled and unlabelled molecules, is present in the tissue at negligible mass concentrations, so that little or no change in the saturation of relevant enzymes or receptors occurs. For this discussion, we assume that tracer levels are appropriate, except where explicitly noted.
Figure 6.2 provides an overview of the various paths that a tracer X may follow after delivery by intravenous injection. Arterial inflow delivers X to the region of interest and venous outflow carries it away. The tracer may cross the capillary membrane and enter the tissue. From the tissue, it may be bound irreversibly or re-versibly to intra- or extracellular sites, or may be metabolized into one or more chemical forms. The original labeled tracer or the metabolites may exit the tissue to the blood.
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