Advances in myocardial perfusion imaging.

The ability to image myocardial perfusion at rest and following stress using noninvasive imaging techniques began in the early 1970s. Initially performed using radioactive potassium as a tracer and planar imaging, substantial advances in perfusion tracers and imaging techniques have culminated in today’s noninvasive approaches. A literal explosion of information followed the introduction of thallium-201 as a perfusion tracer later in the 1970s. Examinations of its property of redistribution led to adoption of protocols that discriminated stress-induced ischemia from infarct following a single injection and following a second injection. In the early to mid-1990s, the approval of the Tc99m-based agents sestamibi and later tetrofosmin allowed improved image quality in more challenging patients, and enabled the widespread use of gated techniques to capture functional information simultaneously with perfusion. The early approval of rubidium-82 for imaging myocardial infarction and later stress perfusion led to its use in a relatively small number of centers that had access to the tracer. Planar imaging had for the most part given way to single-photon emission computed tomography (SPECT), which although subject to more artifacts, found widespread use as imagers were comfortable with the tomographic depiction of organs. Cardiologists in particular were familiar with the tomographic approach from echocardiography. Positron emission tomography (PET) technology evolved to a point where it was accessible beyond major academic centers due to the availability of generators, and a small number of clinical practices availed themselves of the advantages of that technique. The technical evolution of tracers and imaging technology was accompanied by a steady expansion in our understanding of the diagnostic and physiological information contained in the stress and rest perfusion data, and most importantly, by the powerful prognostic information provided by noninvasive imaging. Perhaps more than any other imaging modality, the clinical use of contemporary SPECT and PET perfusion imaging is supported by a very strong evidence base on the relation of the imaging findings to outcomes. This has contributed to its widespread use as a technique to assess risk of major cardiovascular events, and thus drive patient management. This large evidence base and clinical use has evolved despite the fact that the tracers that are used several million times per year are somewhat suboptimal as perfusion tracers. The SPECT tracers do not track flow well in the upper range of flow due to the ‘‘roll off’’ phenomenon, leading to reduction in diagnostic sensitivity for less than severe coronary stenoses and moderate reductions in flow reserve, as well as defect severity that often underestimates true perfusion differences. SPECT tracers are also subject to artifacts produced by substantial soft-tissue attenuation. In everyday reading life we use gated SPECT information and attenuation correction techniques and also incorporate clinical data into our interpretation of images to minimize the impact on diagnostic specificity. While these approaches have validity according to publications that generally emanate from expert centers, ‘‘real world’’ registries imply a disturbing proportion of patients who have normal coronary arteries at catheterization. While PET imaging overcomes some of the problems noted above, very widespread use has been limited by barriers to entry that particularly limit the growth in clinical practices. PET imaging technology is generally more expensive to purchase, and the use of rubidium-82 requires monthly purchase of a generator, engendering a fixed cost and requiring very steady and consistent patient volume to avoid financial loss. Moreover, rubidium-82, like the SPECT tracers, does not track higher levels of myocardial blood flow very well. For many years, the Nuclear Cardiology community has desired a tracer that has more ideal properties of tracking flow to improve sensitivity, and that is subject to less attenuation to enhance specificity. A tracer that is able to track flow at upper ranges of flow reserve could in theory also enable more widespread application of quantitation of flow reserve, also potentially improving sensitivity and allowing better identification of ‘‘balanced’’ ischemia and multivessel disease. From the Division of Cardiology and the CardioVascular Center, Tufts Medical Center, Boston, MA. Reprint requests: James E. Udelson, MD, Division of Cardiology and the CardioVascular Center, Tufts Medical Center, 800 Washington St., P.O. Box 70, Boston, MA 02111; JUdelson@tuftsmedicalcenter. org. J Nucl Cardiol 2012;19:S1–2. 1071-3581/$34.00 Copyright 2011 American Society of Nuclear Cardiology. doi:10.1007/s12350-011-9492-7