Myocardial perfusion imaging (MPI) is a well-established noninvasive method of assessing coronary blood flow.1–5 MPI is capable of identifying regional abnormalities in coronary artery blood flow and determining their physiological relevance to myocardial function and viability. MPI requires the intravenous injection of a radioactive blood flow marker followed by imaging of regional myocardial uptake.6–10 Imaging is most frequently performed by tomographic techniques (ie, SPECT [single-photon emission computerized tomography]) with electrocardiographically synchronized image gating. Used frequently in conjunction with either physical exercise or pharmacological stress, MPI is useful in unmasking myocardial perfusion deficits not evident at rest. A thorough discussion of clinical indications for MPI is beyond the scope of this review, but the reader is referred to recently published consensus committee documents.11,12 Three radioactive blood flow markers (RBFMs) are clinically available and in widespread use: (1) thallium Tl 201 chloride, (2) technetium Tc-99m sestamibi (Cardiolite; Lantheus Medical Imaging, North Billerica, Mass), and (3) technetium Tc-99m tetrofosmin (Myoview; GE Healthcare, Princeton, NJ). In the present review, these agents will be referred to by their nonproprietary names. The oldest of the 3 RBFMs, thallium 201, has been in use since approximately 1980, and the original experimental and clinical validation studies of MPI were performed with this agent.1,13–16 More recently, 99mTc-labeled sestamibi, approved in 1990, and tetrofosmin, approved in 1995, have been developed and validated clinically.6–8,17,18 In comparative imaging studies analyzed for accuracy (sensitivity, specificity, and normalcy rates), no obvious differences have been found among these 3 agents. Each RBFM has unique properties that determine the logistics of its use and its test performance characteristics. The purpose of the present review is to compare and contrast these 3 RBFMs. It is hoped this detailed overview will assist clinicians in choosing which agent(s) to use and in understanding …