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Cardiac Positron Emission Tomography: Integrating Myocardial Perfusion Imaging, Computed Tomography, and Myocardial Blood Flow Assessment

Guest Bloggers: Emily S. Lau, MD and Sanjay Divakaran, MD, member of the ASNC Social Media Task Force - 

In the era of high value care, the use of cardiac positron emission tomography (PET) is rapidly growing and is fast becoming one of the mainstays of cardiac imaging. Like myocardial perfusion single photon emission computed tomography (SPECT), cardiac PET has been widely adopted by cardiologists to provide high-quality, patient-centered care. PET can aid in the assessment of known or suspected coronary artery disease (CAD), and can provide information regarding the presence, extent, and severity of CAD.
Basics of Image Acquisition
Cardiac PET imaging is typically performed using a whole-body PET/computed tomography (CT) scanner. Most protocols require that imaging is obtained after at least four hours of fasting and at least 24 hours since last caffeine intake. After transmission imaging, list-mode images are acquired at rest after intravenous (IV) administration of the perfusion tracer (typically Rubidium-82 or 13N-ammonia. Maximal coronary hyperemia is induced with IV infusion of a vasodilator or dobutamine. At peak stress, a second dose of the same tracer is injected, and images are obtained in the same manner. The patient is in the PET/CT scanner during IV infusion of the radiotracer (for both the rest and stress imaging) which allows for the acquisition of dynamic images. Rest and peak stress myocardial blood flow are computed from these dynamic images.1

Advantages of PET compared with SPECT
While SPECT remains the most widely used modality for the assessment of CAD, PET boasts several advantages to SPECT. PET has higher spatial and contrast resolution as well as a higher signal-to-noise ratio. Both of these characteristics, among others, produce consistently higher quality images. Quantification of regional and global myocardial blood flow provides uniquely helpful data in patient management. PET/CT scanners allow for attenuation correction, which can improve image quality in patients who are prone to have low quality SPECT studies. Coronary artery calcium can be identified using PET/CT, which can aid in the diagnosis and management of CAD.

Clinical Considerations for PET
There are many clinical scenarios and indications for which PET may be the preferred stress imaging test.2 PET should be considered in cases of suspected coronary microvascular disease. Many of these patients may have clinical symptoms of CAD, but noninvasive and/or coronary angiography have not confirmed the diagnosis. The assessment of global myocardial blood flow and myocardial flow reserve allows PET to diagnosis microvascular dysfunction and identify diffuse, nonobstructive atherosclerosis. Obtaining a CT coronary artery calcium score at the time of PET is also very valuable in these scenarios. The quantification of regional and global myocardial blood flow also improves the sensitivity to detect multivessel CAD. This can be very informative in scenarios where there is a concern for balanced ischemia. Segmental quantification of myocardial blood flow in CAD can be used to help map the physiologic contribution of focal stenoses and diffuse, nonobstructive disease.3 Additionally, global stress myocardial blood flow data can be paired with global myocardial flow reserve data in patients with CAD to further stratify risk of cardiovascular events.4,5 The higher quality images and myocardial blood flow quantification can be particularly helpful for high-risk patients (those with diabetes or advanced chronic kidney disease, for example) for whom diagnostic uncertainty or error carries greater downstream risk and who would benefit from additional data for risk stratification from myocardial blood flow assessment. Finally, PET can provide high utility in patients who have had prior testing that yielded inconclusive results. Patient characteristics that can be associated with inconclusive results include obesity, large breasts, and/or prominent central adiposity.

Underutilization of Cardiac PET
Despite the advantages of cardiac PET relative to alternative imaging modalities, it is significantly underutilized. For small practices, the economic barriers to starting a PET program are often high, and therefore, cardiac PET imaging is limited to larger practice sites. However, even for larger programs, the uptake of cardiac PET can often be slow. While imaging technology and PET radionuclides are expensive, the added efficiency of PET over SPECT has the potential to significantly reduce costs. Furthermore, the costs of a myocardial perfusion PET are largely fixed while costs related to SPECT imaging rise proportionally to volume. Finally, the greater diagnostic value of PET reduces the need for downstream testing, further adding economic value to this technology. For some practices, PET scanners are shared across departments/divisions (oncology and cardiology, for example), which can lead to limitations on scanner access. However, many centers have been successful at navigating cross-department/division collaborations and may serve as models for similar centers seeking to start a cardiac PET program. Finally, there is a natural learning curve that comes with the introduction of a new technology. Team members including physicians, technologists, and nurses all require advanced training to become facile with PET tracers and operations, as the protocols do differ significantly from SPECT. For nuclear cardiologists, the American Society of Nuclear Cardiology (ASNC) Cardiac PET Intensive Workshops are an excellent source for learning as they “are designed specifically for physicians who are just stating or planning to start a cardiac PET lab.” ASNC has also curated extremely helpful resources in the Cardiac PET Resource Center. The inherent advantages of PET imaging have led both ASNC and the Society of Nuclear Medicine and Molecular Imaging to release strong recommendations for increased utilization of cardiac PET imaging. 

Summary
As displayed in the Figure, cardiac positron emission tomography (PET) integrates myocardial perfusion imaging, myocardial blood flow assessment, and coronary artery calcium assessment to provide patient-centered care. It can be of high-value in cases of known or suspected obstructive coronary artery disease (CAD), the assessment of coronary microvascular dysfunction, in patients at high-risk for cardiovascular events (such as those with diabetes mellitus (DM) and chronic kidney disease (CKD), and in patients with prior inconclusive single photon emission computed tomography (SPECT) imaging.
 
Emily S. Lau, MD is a clinical cardiology and advanced echocardiography fellow at Massachusetts General Hospital/Harvard Medical School and a research fellow in the Cardiovascular Research Program at Brigham and Women's Hospital/Harvard Medical School in Boston, MA.
 
Sanjay Divakaran, MD is an associate physician in the Division of Cardiovascular Medicine and the Cardiovascular Imaging Program at Brigham and Women's Hospital/Harvard Medical School in Boston, MA. He is a member of the ASNC Social Media Task Force Committee and the Education Committee. He is also a member of the Entering Class of 2020 ASNC Leadership Development Program.

References
1.         Dilsizian V, Bacharach SL, Beanlands RS, et al. ASNC imaging guidelines/SNMMI procedure standard for positron emission tomography (PET) nuclear cardiology procedures. J Nucl Cardiol. 2016;23(5):1187-1226.
2.         Bateman TM, Dilsizian V, Beanlands RS, DePuey EG, Heller GV, Wolinsky DA. American Society of Nuclear Cardiology and Society of Nuclear Medicine and Molecular Imaging Joint Position Statement on the Clinical Indications for Myocardial Perfusion PET. J Nucl Cardiol. 2016;23(5):1227-1231.
3.         Gould KL, Nguyen T, Johnson NP. Integrating Coronary Physiology, Longitudinal Pressure, and Perfusion Gradients in CAD: Measurements, Meaning, and Mortality. J Am Coll Cardiol. 2019;74(14):1785-1788.
4.         Dorbala S, Di Carli MF, Beanlands RS, et al. Prognostic value of stress myocardial perfusion positron emission tomography: results from a multicenter observational registry. J Am Coll Cardiol. 2013;61(2):176-184.
5.         Gupta A, Taqueti VR, van de Hoef TP, et al. Integrated Noninvasive Physiological Assessment of Coronary Circulatory Function and Impact on Cardiovascular Mortality in Patients With Stable Coronary Artery Disease. Circulation. 2017;136(24):2325-2336.

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