Should early post-stress imaging be performed on a routine clinical basis for myocardial perfusion studies?

Abstract

Establishing imaging protocols for radionuclide myocardial perfusion imaging (as with any other radiotracers used in nuclear medicine) always requires some sort of trade-off in order to obtain an ‘‘optimal’’ imaging protocol. Various considerations must be taken into account before finalizing recommendations for imaging with a specific agent. Even after general consensus in the medical literature, new clinical data or new type of equipment or softwares could and should modify the ‘‘standard’’ imaging protocol guidelines. Among those parameters, considerations must be applied to the ‘‘practicality’’ of the imaging protocol in a day-to-day clinical setting in order to answer the clinical question in a reasonable time-frame. Before obtaining the optimal protocol for a myocardial perfusion imaging agent, data on several parameters must be known such as, but not limited to, dosimetry (physical, biological, and effective half-lives), biological characteristics (biodistribution and pharmacokinetics, degree of myocardial uptake and retention, the presence or not of myocardial redistribution, level of activity in the adjacent organs to the heart, and the relationship between myocardial uptake and coronary blood flow). In addition, the impact of a given imaging protocol on the overall diagnostic accuracy (sensitivity, specificity, the ability to detect the true extent and degree of perfusion defects, and appropriate results using either pharmacologic or exercise stress testing) must be evaluated. When 99mTc-sestamibi (Cardiolite) was introduced into clinical research in the late 1980s (and later on 99mTc-tetrofosmin, Myoview), researchers noted that the pulmonary activity was negligible even immediately after its injection, the blood clearance had a half-life of 4 minutes at rest and 1.6 minutes at stress, the biological half-life for the liver was approximately 30 minutes after a rest or exercise injection. The myocardial uptake is approximately 1.5% of the injected 99mTc-sestamibi dose, while the liver and biliary uptake represents almost 30% of the injected dose. The effective half-life of clearance (which includes both the biological half-life and the physical half-life of 99mTc, or radionuclide decay) for the heart was approximately 3-4 hours, and for the liver was approximately 30 minutes after a rest or exercise injection. Therefore, the ideal imaging time should reflect the best compromise between high myocardial count rate and the lowest surrounding organ uptake (mostly liver and gallbladder). Considering the potential variability of hepatic clearance through the biliary tree (variable hepatobiliary transit time), and possible related ‘‘subdiaphragmatic contamination,’’ and considering that there is no clinically relevant myocardial redistribution within few hours after its administration, it was initially recommended to wait approximately 60-75 minutes after 99mTc-sestamibi intravenous injection before starting the image acquisition. Subsequently, different approaches have been proposed in order to decrease this relatively longtime interval between injection and imaging. Although initially the liver uptake was seen as a potential cause of image artifacts, it was soon recognized that the hepatobiliary excretion would more likely affect the imaging protocols (bowel activity). Some ‘‘recipes’’ such as the Reprint requests: Raymond Taillefer, MD, FRCP, ABNM, Department of Nuclear Medicine, Hopital du Haut-Richelieu, 920 Boul. du Seminaire Nord, St-Jean-sur-Richelieu, QC, Canda; rtaillefer@ hotmail.com J Nucl Cardiol 2014;21:1177–80. 1071-3581/$34.00 Copyright 2014 American Society of Nuclear Cardiology.