Chapter 6

### INTRODUCTION

Single photon emission computed tomographic imaging (SPECT) myocardial perfusion imaging (MPI) is a >40-year-old diagnostic method used for the functional evaluation of the presence of obstructive epicardial coronary artery disease (CAD). With the accumulation of a large body of supporting evidence, MPI is currently considered the best non-invasive test for the diagnosis and risk stratification of flow obstructing CAD in patients with intermediate to high pretest probability of the disease. It is also the most appropriate test for therapeutic decisions—triage between medical therapy and mechanical revascularization (percutaneous coronary interventions [PCI] or coronary artery bypass grafting [CABG] surgery). Lastly, but very importantly, MPI is an excellent test for risk stratification for future (1–2 years) cardiac events. The following chapter describes currently accepted protocols for the performance of high-quality SPECT imaging studies. More detailed and continuously updated description of imaging protocols can be found on the American Society of Nuclear Cardiology web page.1

Two isotopes are commercially available for SPECT MPI: thallium (Tl-201) and technetium (Tc-99m). Tl-201 was used for the first time for planar myocardial imaging in 1973 by H.W. Strauss and E. Lebowitz. After introduction of the SPECT camera in 1976 (J. Keyes and R. Jaszczak), SPECT imaging became the prevailing acquisition technique for cardiac imaging in the early 1990s (Fig. 6-1). Tc-99m sestamibi was approved by the FDA for MPI in December 1990 and Tc-99m tetrofosmin was approved in 1996. More than 9 million SPECT studies were performed in the USA in 2008 using approximately 12,000 SPECT cameras, with the majority being performed with Tc-99m radiopharmaceuticals.

###### FIGURE 6-1

Stress-redistribution–reinjection thallium 201 protocol.

The primary indication for a MPI study is the assessment of the relative distribution of coronary flow in patients with suspected or known obstructive CAD. Since the distribution of coronary flow both at rest and at stress is almost even in all segments of the left ventricle, the presence of perfusion defects suggests intraluminal coronary obstruction. An increase in coronary flow is needed for the detection of significant coronary artery stenosis (>50% of luminal narrowing) since rest flow distribution is even unless high-grade stenosis is present. Coronary flow can be increased most physiologically with physical effort (treadmill exercise), or in patients who are unable to exercise adequately, using coronary vasodilators (adenosine, dipyridamole, and regadenoson) or dobutamine (Chap. 5).

Evaluation of left ventricular size and function became possible with the development of gating algorithms used in conjunction with MPI. The combination of perfusion and function data improved both the diagnostic and the prognostic value of SPECT studies.

The availability of more than one perfusion tracer and different modes of stress allows for a multitude of imaging protocols. Ideally, the imaging protocol should be tailored for the individual patient, taking into account ...

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