Nitrogen-13-labeled ammonia for myocardial imaging

Cyclotron-produced nitrogen-13 (half-life 10 min), as labeled ammonia ( 13NH 4 +), has been evaluated as a myocardial perfusion imaging agent. The regional myocardial uptake of 13NH 4 + has been shown to be proportional to regional tissue perfusion in animal studies. Intravenously administered 13NH 4 + is rapidly cleared from the circulation, being extracted by the liver (15%), lungs, myocardium (2%—4%), brain, kidney, and bladder. Myocardial ammonia is metabolized mainly to glutamine via the glutamine synthetase pathway. Pufmonary uptake is substantial, but usually transient, except in smokers where clearance may be delayed. The positron annihilation irradiation (511 keV) of 13N may be imaged with a scintillation camera, using either a specially designed tungsten collimator or a pinhole collimator. After early technical problems with collimation and the production method of 13NH 4 + were overcome, reproducible high quality myocardial images were consistently obtained The normal myocardial image was established to be of a homogeneous “doughnut” configuration. Imaging studies performed in patients with varying manifestations of ischemic and valvular heart disease showed a high incidence of localized perfusion defects, especially in patients with acute myocardial infarction. Sequential studies at short intervals in patients with acute infarction showed correlation between alterations in regional perfusion and the clinical course of the patient. It is concluded that myocardial imaging with 13NH 4 + and a scintillation camera provides a valid and noninvasive means of assessing regional myocardial perfusion. This method is especially suitable for sequential studies of acute cardiac patients at short intervals. Coincidence imaging of the 511 keV annihilation irradiation provides a tomographic and potentially quantitative assessment of the regional myocardial uptake of 13NH 4 +.