Current-day nuclear cardiology consists predominantly of radionuclide myocardial perfusion imaging (MPI) with single photon emission computed tomographic (SPECT) acquisition techniques, using thallium-201 and/or technetium-99m compounds (sestamibi or tetrofosmin) as radiotracers. Protocols in use include stress/rest imaging to assess the presence and severity of coronary artery disease (CAD), scar, and myocardial ischemia, and rest/delayed imaging to determine the presence and extent of myocardial viability, as well as various techniques to assess ventricular function. SPECT radionuclide MPI has served patients well with high diagnostic and prognostic utility, and importantly a demonstrated ability to help guide patient management for improvement of outcome and well-being.1–6
At the same time, there are significant limitations of SPECT MPI as it is currently performed, including soft tissue attenuation, perfusion tracer roll-off at high flow rates, and balanced ischemia, all of which can lead to decreased accuracy and underestimation of CAD severity.7–9 Improved attenuation correction methods using computed tomography (CT), quantitation of absolute coronary blood flow and flow reserve, and development of new tracers (e.g., the positron emission tomography [PET] radionuclide BMS 747158) are underway to address these issues,10–12 beyond the scope of this chapter.
Another key limitation of SPECT MPI is the inability to specifically show myocardial ischemia. While there have been investigations of agents that specifically image hypoxic myocytes, the most promising impending technique for clinical use is imaging of the consequences of ischemia, specifically changes in myocyte metabolism. Among the tracers for this are the PET tracer F-18 fluorodeoxyglucose (18FDG) that can be imaged with specially designed SPECT systems, and the fatty acid compound 123-iodine-beta-methyl-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (123I-BMIPP). Such tracers may allow detection of a prior ischemic episode, that is, "ischemic memory."
Finally, a condition that is assessed poorly with SPECT MPI is congestive heart failure (CHF), largely affecting cardiac myocytes and the neurohormonal state of the heart rather than coronary perfusion. CHF has a high morbidity and mortality, as well as imposing a heavy financial burden on our healthcare system. Better imaging techniques are needed, particularly to help guide medical management and the use of expensive device treatments. This review will discuss the potential role of the cardiac neuronal imaging agent 123-iodine meta-iodobenzylguanidine (123I-mIBG) in this regard.
At the time of publication, the aforementioned radiotracers and/or techniques, although used clinically in some areas of the world, are considered investigational in the United States (Table 12-1). Nevertheless, large-scale studies are underway, some completed, and clinical use appears imminent. The accompanying table summarizes the tracers to be discussed here.
Table 12-1Cardiac Radionuclide Tracers Under Investigation (For SPECT Imaging) |Favorite Table|Download (.pdf) Table 12-1 Cardiac Radionuclide Tracers Under Investigation (For SPECT Imaging)
|Tracer ||Myocardial Uptake ||Development Stage ||Imaging Role ||Potential Clinical ...|