The study of the human heart with conventional radionuclide techniques is largely confined to assessments of the relative distributions of regional myocardial blood flow and of global and regional myocardial contractile function. Positron emission tomography (PET) extends these capabilities because it offers assays for probing and defining regional functional processes that span from blood flow to biochemical reaction rates, substrate fluxes, membrane receptor density and function, and neuronal activity. Newly developed assays, still in the investigational stage, can target the expression of transfected and endogenous genes, visualize cell trafficking, or probe molecular processes. The many positron emitting, biologically active tracers, the quantitative imaging capability, and the in vivo application of tracer kinetic principles account for these capabilities. The recent addition of structural imaging with in-line PET/computed tomography (CT) hybrid systems is likely to further refine these noninvasive assays for regional functional processes because they now can be accurately localized and related to structure. Aspects of the human heart's physiology and pathophysiology can thus be assessed more comprehensively. Novel insights into the function of the human heart can be gained while, at the same time, findings with PET can decisively impact patient diagnosis and management. This chapter reviews the key ingredients of PET and the tools for the evaluation and/or quantification of local functional processes in the human heart. It continues by examining how these tools are applied to the diagnosis and characterization of coronary artery disease and its effects on regional myocardial tissue function and reviewing the impact of PET findings on patient management.
Fundamental to PET are (1) the quantitative imaging and high temporal resolution capability; (2) the in vivo application of tracer kinetic principles; and (3) the large number of physiologically active radiotracers.
Imaging with Positron Emitting Radiopharmaceuticals
The quantitative imaging capability of PET results from physical properties unique to positrons. After losing their kinetic energy, positrons combine with an electron and “annihilate.“ The annihilation represents the conversion of mass into energy, that is, the combined mass of the positron and the electron converts into two 511-keV photons that leave the site of the annihilation in diametrically opposed directions. If both strike at the same time, two scintillation detectors connected by a coincidence circuitry, an annihilation event is registered. Its location in space can be defined by circular arrays of scintillation detectors. The near simultaneous arrival of two 511-keV photons at the two scintillation detectors positioned at opposite directions allows the use of tomographic reconstruction algorithms analogous to those used with x-ray CT. Accordingly, the spatial resolution throughout the image plane is rather homogeneous, which differs from that obtainable with conventional single-photon emission CT (SPECT) approaches, where the spatial resolution declines as the distance of the imaged object to the scintillation detectors increases. Furthermore, images of the tissue radiotracer concentrations are corrected for photon attenuation so that regional radioactivity concentrations can be measured in millicuries or megabecquerels per volume or mass of tissue. With modern ...