From thallium scan to molecular imaging

The diagnostic and prognostic evaluation of cardiovascular diseases has been improved considerably by the application of imaging procedures. Among many, scintigraphic procedures have emerged as important diagnostic tools to assess extent, severity, and prognosis in patients with coronary artery disease (CAD). For more than the last 30 years, however, the application of perfusion imaging has been extended to allow the combined evaluation of perfusion, perfusion reserve, and ventricular function. With positron emission tomography (PET), quantitative assessment of perfusion has become possible. In combination with pharmacological stress agents, the coronary flow reserve (CFR) can be quantitatively assessed as an early marker of endothelial dysfunction. PET in combination with metabolic tracers has added the evaluation of cardiac substrate metabolism, which has become an important clinical marker for ischemically jeopardized myocardium. The PET information is widely considered as the gold standard for tissue viability in the management of patients with advanced CAD and impaired left ventricular function (LVF). New tracer approaches include the assessment of cardiac innervation, which plays an increasingly recognized role in the pathophysiology of cardiac diseases. Radiolabeled catecholamine analogues provide visualization of sympathetic nerve terminals that are functionally altered in patients with diabetes mellitus and cardiomyopathy. In addition, this scintigraphic information allows the monitoring of physiological processes such as reinnervation of the transplanted heart. New methods, such as imaging of apoptosis and gene expression, are of interest in cardiology. Combining the therapeutic gene with a reporter gene, the transfection of cardiac tissue can be monitored noninvasively. First results employing the herpes simplex virus thymidine kinase reporter gene (HSV1-tk) are encouraging and represent an attractive approach for the use of PET imaging in the control of cardiac gene therapy.