Myocardial scintigraphy by infarct-avid radiotracers

Abstract

Specific radioindicators are sequestered by acute myocardial infarctions, and their uptake is detectable by external detection systems, such as the Anger scintillation camera. The resultant scintigraphic image may be used to estimate infarct size, although inferior and subendocardial infarcts may pose difficulties. Infarct localization as to anatomic area of the heart is also reasonably accurate. The majority of the clinical experience has been with technetium chelates, particularly 99mTc-tetracycline and 99mTc-pyrophosphate. Optimal imaging with 99mTc-tetracycline is within the first 3 days after infarction, with gradual return to normal of the scintigraphic appearance after this time. While larger infarcts remain positive for longer periods of time, significant uptake or reappearance of uptake after the initial period may be helpful in the identification of reinfarction or extension after an initial infarct. Tetracycline appears to be sequestered only by acutely infarcted myocardium, and therefore is a sensitive agent for distinguishing normal, previously infarcted, and ischemic myocardium from acutely infarcted myocardium. The major clinical experience has been with 99mTc-pyrophosphate, a bone-seeking radionuclide. The major advantage of 99mTc-pyrophosphate over 99mTc-tetracycline is the earlier imaging interval. Optimal scans are obtained at 1–2 days after infarction and only 90 min after the administration of 99mTc-pyrophosphate (as opposed to 24 hr with 99mTc-tetracycline). While 99mTc-pyrophosphate is a quite sensitive indicator of infarction, there is suggestive evidence that ischemic as well as infarcted myocardium sequesters the agent. In addition, various other conditions, including cardioversion, rib fractures, left ventricular aneurysms, and breast tumors may cause uptake of 99mTc-pyrophosphate and lead to false positive myocardial infarct scintigrams. Thus, while only a few patients with negative scans who have been imaged at the appropriate time will turn out to have clinically detectable infarcts, a somewhat larger number without infarction will have positive scans, particularly those patients with unstable angina pectoris but without clinical infarction. While the final role of acute myocardial scintigraphy remains to be determined, its contribution to the further understanding of the pathogenesis of ischemia and infarction, as well as its clinical utility, has been significant.