Regional myocardial perfusion assessed with N-13 labeled ammonia and positron emission computerized axial tomography

Abstract

The usefulness of N-13 ammonia as an indicator of regional myocardial perfusion suitable for positron emission tomography has been suggested. However, the relation between myocardial blood flow and uptake of N-13 ammonia has not been examined quantitatively. To quantitate the relation of myocardial N-13 ammonia tissue concentration to myocardial blood flow and examine its suitability for positron emission tomographic imaging, 12 open chest dogs were studied. In eight of the dogs, 25 imaging procedures with N-13 ammonia and positron emission tomography were performed; in the remaining four dogs the relation between flow and myocardial N-13 ammonia concentration was assessed with in vitro techniques. Positron emission tomography provided high quality cross-sectional images of the distribution of N-13 ammonia in left ventricular myocardium. No significant redistribution of N-13 ammonia in the myocardium occurred with time. Alterations in regional myocardial blood flow resulted in changes in the regional distribution of N-13 ammonia that were visible in the cross-sectional images. Myocardial N-13 ammonia concentrations measured with positron emission tomography were closely related to myocardial blood flow, although in a nonlinear fashion. The nonlinear relation determined with positron emission tomography was identical to that observed with in vitro techniques. The nonlinearity of this relation of flow to N-13 ammonia concentration was probably due to the inverse relation between flow and the N-13 ammonia single pass extraction fraction. The uptake of N-13 ammonia was studied at myocardial blood flows ranging from 0 to 500 ml/min per 100 g, whereas physiologically occurring flows in man range only from near 0 to approximately 300 ml/min per 100 g. Within this physiologic flow range, the relation between flow and N-13 ammonia concentration was relatively linear, and a 100 percent increase in flow produced an approximately 70 percent increase in N-13 ammonia concentration. It is concluded that N-13 ammonia is a good indicator of myocardial blood flow and compares favorably with conventional monovalent perfusion imaging agents. Positron emission computerized axial tomography permits quantitative measurements of the distribution of this agent in myocardium and therefore should prove useful in measuring regional myocardial perfusion and blood flow.