ReviewAssessment of myocardial perfusion and viability by Positron Emission Tomography
Introduction
Positron Emission Tomography (PET) is a nuclear medicine modality, which for many years has been employed as a highly sophisticated research tool only in centers with access to cyclotron facilities. A shift in the use of PET imaging from the research to clinical setting has taken place recently, as a result of the greater availability of PET scanners (mainly to support cancer services) and the increasing documentation of PET's clinical efficacy. PET scanners are currently available in a hybrid form, combining PET with computed tomography (CT) or magnetic resonance imaging (MRI) offering a unique opportunity for a comprehensive noninvasive evaluation of the consequences of atherosclerosis both in the coronary arteries and the myocardium and translating advances in molecular imaging into humans. This review covers the current developments and future directions of PET imaging, emphasizing the role of PET on the assessment of myocardial perfusion and viability. It discusses the radiopharmaceuticals which are used for performance of clinical studies and also the stress tests and imaging protocols as well as the modality's usefulness in the evaluation of coronary artery disease (CAD), from its early detection to heart failure. Issues around cost effectiveness and comparisons with other imaging modalities are also discussed.
Section snippets
Selection of patients for PET myocardial perfusion imaging
Both the ACC/AHA/ASNC clinical guidelines and the position statement on advanced noninvasive [1] cardiac imaging produced jointly by the Canadian Cardiovascular and Imaging societies [2] recommend PET myocardial perfusion imaging (MPI) for diagnosis and/or risk stratification of CAD patients who have had nondiagnostic, noninvasive imaging tests, or when there is discrepancy between a test's results and clinical diagnosis (Class I and evidence level B). Patients with left bundle brunch block
Physiology and tracers
The myocardium has one of the highest energy demands of any tissue. It uses a variety of metabolic substrates, but under physiological conditions most of the energy required for contraction comes from the oxidation of free fatty acids. Glucose, lactate, pyruvate, ketone bodies and amino acids are also used depending on availability, hormonal factors, and myocardial oxygen demand. Under conditions of ischemia, oxidation of fatty acids is suppressed and there is increased glycolysis and glycogen
Radiation exposure from PET studies
The main advantage of the positron emitting tracers is the shorter physical half-life, compared to SPECT tracers. Therefore, the radiation burden to the patient and the nuclear medicine personnel is lower. The estimated effective radiation dose to the patient is 3.8 mSv for 82Rb (rest and stress) and less than 3 mSV for 13N-ammonia and 15O-water (rest and stress) [83], [84], [85], [86]. Even when the additional burden of 0.04 mSv for scout/localizing CT and 0.35 mSv for CT attenuation correction is
Economic evaluation
There is limited information on the cost effectiveness of PET based strategies for the diagnosis and management of stable CAD. Patterson et al. [88] have compared the cost-effectiveness and cost-utility of exercise ECG, SPECT, PET and coronary angiography, to diagnose obstructive CAD. Effectiveness was defined as the number of patients with diagnosed CAD, and utility as the number of quality-adjusted life years (QALY) extended by therapy after the diagnosis. They observed that in patients with
Hybrid PET/CT and PET/MRI imaging
Combined assessment of coronary anatomy and function is an attractive option that is feasible with the new PET/CT systems. A number of reports have demonstrated improved specificity and positive predictive value (PPV) in the detection of angiographically significant stenoses when perfusion data from PET and angiographic data from CT angiography were combined. The largest of these studies has included 107 patients with chest pain and intermediate likelihood of CAD. The study compares hybrid 15O-H
Conclusion
Cardiac PET is currently undergoing a paradigm shift. Being originally a highly versatile research tool, it has gradually become a valuable imaging modality that is currently used in a variety of clinical scenarios ranging from early detection of coronary atherosclerotic disease to heart failure. Experience with PET was accumulated over decades and there is a wide body of evidence, including cost effectiveness data, to support its integration into investigative strategies of CAD. The modality's
Acknowledgments
The authors of this manuscript have certified that they comply with the Principles of Ethical Publishing in the International Journal of Cardiology.
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2020, Academic RadiologyCitation Excerpt :However, the evaluation of hibernating (and not scarred) myocardium in CMR is not possible without the application of stress (low-dose dobutamine) and, consequently, the assessment of improved wall motion under stress conditions (19). Dual-tracer [13N]NH3/[18F]FDG PET, however, enables the identification of perfusion/viability match and mismatch patterns, which are commonly used to define the myocardium as nonviable, viable, or hibernating (20). In patients with low EF, an improvement of left ventricular function was demonstrated when PET was used to guide revascularization therapies (1,2).
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