Myocardial Perfusion Imaging Utilizing Single Photon Emission Computed Tomography Techniques

Abstract

The major clinical applications of single photon computed tomography (SPECT) myocardial perfusion imaging (MPI) include detection of CAD among patients presenting with symptoms such as chest pain or dyspnea, assessing prognosis in patients with suspected or known CAD, identifying which CAD patients may benefit most from coronary revascularization rather than medical therapy and determining extent of myocardial viability in patients with ischemic cardiomyopathy. Patients with normal SPECT MPI studies have an excellent prognosis with <1% annual rate of cardiac death or nonfatal myocardial infarction (MI). The annual event rate increases to 6 % in those patients with abnormal scans. Compared to nondiabetic patients, patients with diabetes with abnormal SPECT scans have a higher event rate with female diabetic patients with abnormal scans having a >10 % annual cardiac death or MI rate. Gated SPECT imaging permits the assessment of regional wall motion and measurement of the LV ejection fraction, which adds further prognostic information to the perfusion findings. Stress-only SPECT imaging in low risk patients eliminates the need for rest imaging when the stress scans are normal. This reduces radiation exposure to the patient as well as reduces cost. Vasodilator SPECT imaging is currently most frequently performed in patients unable to exercise adequately using regadenoson, a specific adenosine A2a receptor agonist. SPECT MPI can be combined with CT coronary angiography (CTA) for hybrid multimodality imaging of anatomy and physiology. CTA is most often performed when the SPECT images are non-diagnostic or equivocal. Conversely, if the CTA is done first, SPECT imaging may be useful as the second test in the instance of finding intermediate coronary stenosis (50-70 % narrowing) on CTA. Resting SPECT imaging with either thallium-201 or one of the Tc-99m-labeled perfusion agents (sestambi or tetrofosmin) can be used to determine the extent of viability in dysfunction myocardial segments. The greater the extent of viability in dyssynergic segments, the greater the chance of improved regional and global function after revascularization. Finally, new solid state gamma cameras employing cadmium-zinc-telluride (CZT) crystals with new software algorithms have emerged in recent years. This new technology permits more rapid acquisition of SPECT images at a lower radiation dose to the patient.