Physiology and physics of nuclear cardiology

This chapter is a primer on the physics of radionuclide detection, flow physiology, and methods of in vivo evaluation of myocardial metabolism and intercavitary flow by noninvasive methods of intravenous isotope injection. This summary presents key concepts for the application of currently available instrumentation as well as future directions of nuclear cardiology. 1. Quantitative information is obtained in nuclear cardiology at the cost of high resolution imaging for two reasons: (a) the intrinsic resolution of the detecting systems is limited by available technology, and (b) the statistics required to achieve a high resolution image necessitate doses and imaging times far in excess of those which can be tolerated. Image resolution for both projection images as well as transverse sections are limited to the range of 5 to 20 mm, depending upon the configuration and instrument involved. 2. The second important concept is the fact that nuclear cardiology gives quantitative information regarding the amount of radiopharmaceutical which has accumulated in or is flowing through the cardiovascular system. This information allows one to deduce the dynamics of flow as well as actual metabolic rates. 3. The major emphasis for future work might well lie in the multiple transverse section imaging of the myocardium using both rotating and static devices. The key feature of this approach is the fact that the volume of interest can be localized and actual concentrations of radiopharmaceuticals can be measured by external detection using reconstruction tomography images. 4. Quantitative data on the distribution of a metabolite which accumulates in the myocardium is of little value if regional blood flow is not also known. 5. Finally, it is shown in this chapter that specific volume flow can be evaluated using short half-life isotopes and equations derived from the principle of conservation of mass. In principle it is now possible to obtain quantitative values delineating endo- and epicardial flow for the heart of man without invasive catheterization or high radiation doses. These procedures involve constant inhalation of carbon dioxide labeled with 15O which converts to labeled water and can be used for evaluating myocardial perfusion; bolus injection of 82Rb, a short half-life analogue of potassium, for repeated (every 5 min) imaging of the evolution of myocardial infarction size; evaluation of the accumulation of labeled fatty acids, amino acids, and sugars in the myocarium; presentation of images which reflect the magnitude of ejection fraction; and noninvasive evaluation of cardiac shunts. It is now possible to perform on the same patient during a few hours the following studies of myocardium: cation perfusion evaluation; water perfusion; uptake of fatty acids, amino acids, and glucose; oxygen utilization of the myocardium; and even measurement of the quantity of lung water. We now have the tools and methods to evaluate the in vivo biochemistry of the ischemic, 128 infarcting, repairing, and hypertrophic myocardium.