SPECT myocardial perfusion imaging (MPI) has been widely used for over 30 years for the detection and risk stratification of coronary disease. SPECT imaging has become the largest billing code for non-invasive imaging and its acceptance in the community has been very strong due to its reliability, ease of use, and availability. While the technology has improved throughout the years, there remain several important limitations including: long protocol times, high incidence of attenuation and other artifacts, use of suboptimal tracers, and frequent underestimation of the presence and severity of coronary artery disease.
Currently the protocol most often used in nuclear laboratories necessitates a separate rest and stress SPECT study, requiring 3–4 h for completion. Present technologies do not adequately address potential artifact including motion during the test, attenuation artifact, and errors in processing that can lead to a high percentage of false-positive studies. The available stress agents all have limitations of either image quality or accuracy such that multivessel disease is often missed. Active research is aimed at addressing some of these problems, but potential solutions are not clinically implemented.
An alternative to SPECT imaging is cardiac positron emission tomographic imaging (PET). PET imaging has been available for several years as a diagnostic modality in patients with oncologic diseases but only recently has become more widely used in the cardiac arena. As PET imaging has grown, so has come with it the recognition that the limitations found in SPECT imaging might be resolved with this more advanced technology. This has resulted in a growing interest in PET imaging for cardiac diseases. When compared to SPECT MPI, cardiac PET imaging has been shown to provide better image quality, better diagnostic accuracy, and faster acquisition times. This chapter will review the principles of cardiac PET and the differences between PET and SPECT imaging, and provide recommendations for when PET should be used clinically.
PRINCIPLES OF CARDIAC PET
PET imaging provides accurate temporal and spatial distribution of radioactive atoms as they decay. A radioactive tracer, which has been engineered to be taken up in to the organ of interest, is injected into the patient. After it reaches the target organ, the radioactive agent begins to decay and emits a positron (Fig. 11-1). This positron then collides with a nearby electron. The resulting collision causes annihilation of both an electron and a positron. The annihilation creates a high-energy discharge of 1.02 MeV. This energy is split into two gamma rays each 511 keV in energy, which are emitted 180° from each other. Multiple detectors circle the patient; absorption from both emissions simultaneously occurs. The processing of these simultaneous events forms the basis of PET imaging.1
Annihilation event during PET imaging.
CARDIAC PET INSTRUMENTATION