An important evolutionary advancement in the field of nuclear cardiology has been the introduction of cardiovascular positron emission tomography (PET) into mainstream clinical care. An extensive peer-reviewed literature spanning almost 30 years virtually shouts out a role for cardiac PET in a substantial proportion of patients historically assessed with single-photon emission computed tomography (SPECT) or not previously evaluable by nuclear cardiology techniques. The substantial and growing evidence base has led to several pivotal multisocietal guidelines,¹ position statements,² and consensus documents^3,4 on the role of PET imaging in cardiovascular disorders. The theme of this Introduction is that a contemporary nuclear cardiology laboratory will not be SPECT-only or PET-only, but that these two modalities will coexist, at least for the foreseeable future.⁵ The published data and clinical need together are moving toward a concept in which SPECT or PET will be a preferred test for definable presentations and clinical descriptors.

In the coming chapters of this book, all of the various applications of cardiovascular PET will be thoroughly explored. The purpose of the current section is to provide a vision for how SPECT and PET should and can coexist in a culture of high-quality patient-centered imaging while achieving space and workforce efficiencies.

Cardiac radionuclide imaging had its birth in an era in which cardiologists were primarily focused on coronary artery disease (CAD). By the 1970s, multicenter studies had shown that patients with triple-vessel, left main, and proximal left anterior descending disease fared better if treated with revascularization compared to medical therapies available at the time. However, it was difficult to identify among the large number of patients with symptoms potentially indicating CAD those who needed invasive evaluation. Cardiac radionuclide imaging addressed that need, evolving newer and better approaches in hardware (first with rectilinear scanners,⁶ then with planar cameras,⁷ and by the mid-1980s with SPECT),⁸ tracers (thallium-201 followed by Tc-99m sestamibi and tetrofosmin), and quantitative software. With an almost singular focus on CAD, a voluminous literature established the value of SPECT myocardial perfusion imaging (MPI) as a gatekeeper to the catheterization laboratory for many patients with nonacute known or suspected CAD. SPECT MPI also found daily use for diagnosing CAD, for risk stratification, and for assessing likelihood of viability of dysfunctioning myocardium. Figure S1-1 illustrates schematically how SPECT became positioned as a pivotal test for almost all patients except those who could perform a diagnostic-level treadmill exercise test that was adequate for decision making or who were deemed to be appropriate for direct referral to coronary angiography. Levin et al⁹ used only Medicare data to show that SPECT MPI grew from 63.4 tests per 1000 beneficiaries in 2001 to 88 tests per 1000 beneficiaries in 2006, an impressive rate of growth that propelled SPECT MPI to become one of the top 10 annual Medicare expenditures. As expected, focus on SPECT MPI raised questions about the appropriateness and safety of so much testing. Table S1-1 highlights some of the major criticisms.

The past 15 years have seen increasing focus from payers, the public, from professional medical societies, and from the Institute of Medicine on the concept of patient-centered imaging.¹⁰ The Centers for Medicare and Medicaid Services defines patient-centered imaging as effective, efficient, equitable, and safe.¹¹ In this paradigm, it becomes challenging to advocate that a single imaging procedure and protocol for the wide array of patients (Table S1-2) presenting for CAD assessment will be optimal for all. The SPECT community has been slow to address the concerns expressed in Table S1-1. While SPECT MPI has proved to be highly valuable, the reader will appreciate that for many of the criticisms, there is a common theme centering on expense, risk (radiation exposure), and the length of the test.

1. 
   

   SPECT MPI is expensive relative to other tests for evaluating presence, extent, and severity of CAD. In fact, the 2018 Medicare fee schedule for the technical performance of a rest/stress SPECT MPI is more than 4 times that of coronary CT angiography, and almost as much as a rest/stress PET MPI (Table S1-3). For lower at-risk patients, SPECT now competes with several modalities that are less expensive to payers and to patients (Figure S1-2). Figure S1-2 also shows how PET has claimed superiority for higher-risk patients, such that if SPECT is to survive, it must embrace principles of faster throughput (eg, stress-only imaging when appropriate), less risk (lower radiation associated with stress-only studies; lower dosages when CZT cameras are used), and higher accuracy (fewer false-positives when attenuation correction is employed). To successfully compete for the lower-risk patients, the typical large field of view cameras that necessitate high tracer dosages and long imaging times will be at a major disadvantage.

   
   
2. 
   

   Studies show that the prevalence of an abnormal SPECT MPI has become low, suggesting that the test is being used in a progressively lower risk population^12–14 (Figure S1-3). As discussed previously, if SPECT is to compete with lower-cost alternatives for lower-risk patients, it must adapt to a value proposition that includes faster throughput, lower radiation, and fewer false-positive studies due primarily to bodily attenuation.

   
   
3. 
   

   Because of its frequent and widespread use, SPECT MPI became identified as a significant source of population-based radiation exposure.¹⁵ Despite efforts from major professional medical societies,¹⁶ the provider community as a whole has failed to adopt radiation-saving protocols such as stress-only imaging when appropriate.¹⁷ Newer CZT technology with the capability of major reductions in radiation exposure¹⁸ has been slow to be adopted, as providers choose to retain older Anger-based systems that require longer procedures and higher dosages of tracer and, hence, radiation exposure in order to provide sufficient count density for diagnostic quality images.¹⁹

   
   
4. 
   

   A major step forward for the SPECT MPI field was the introduction of intravenous vasodilators for “stressing” those patients unable to undergo a diagnostic level of treadmill exercise. Unfortunately, numerous studies and subsequent meta-analyses showed that such patients could never be categorized as being at low risk for events, even when the SPECT scan was normal.²⁰ Furthermore, for any MPI result, the events were higher after vasodilator stress than after exercise stress to an adequate end-point.²¹ In addition, the “warranty period” of a normal SPECT MPI was much shorter after vasodilator stress versus exercise stress.²² Many providers and clinicians began to look for more definitive assessments as focus changed from population-based to individual-based metrics.

   
   
5. 
   

   The spatially relative nature of SPECT MPI and the inherent limitations of the most often used radionuclides (Tc-99m sestamibi or Tc-99m tetrofosmin) carry limitations for identifying multivessel or left main disease.^23,24 This well-publicized reality has seriously limited the ability of SPECT MPI to be an optimal gatekeeper to the catheterization laboratory. All clinicians fear the possibility that SPECT MPI may be missing the most serious of high-risk CAD patients: those with multivessel or left main CAD. One study showed that fewer than 40% of patients with three-vessel disease had any more than a single vessel SPECT perfusion defect.²³ Another demonstrated that less than 10% of 99 patients with significant left main CAD had even a small localized perfusion defect.²⁴

   
   
6. 
   

   Finally, there is an inherent inefficiency to the traditional yet commonly utilized rest/stress protocol that requires some 4–6 hours from patient arrival until departure from the testing facility. This is a large inconvenience as well as societal cost to the elective outpatient. For hospital inpatients, it can often make for an extra day or two of hospitalization, limiting the ability to accommodate late-day add-ons or to complete testing early enough to allow same-day coronary angiography when indicated by MPI results.