Single photon-emission computerized tomographic (SPECT) myocardial perfusion imaging (MPI) remains the dominant noninvasive functional imaging perfusion method for the diagnosis as well as prognosis of epicardial coronary artery disease (CAD). The advent and advances of other methods used for similar purposes (cardiac positron emission tomography [PET], stress echocardiography, coronary computerized tomography [CTA], and magnetic resonance imaging [MRI]) have all contributed to recent re-examination of traditional MPI protocols to optimize its use.1 This task was facilitated by introduction of high-efficiency cadmium zinc telluride (CZT) nuclear cameras and innovative software. With changes in society and concern of radiation exposure, emphasis has shifted from “one size fits all” to patient-centered imaging with individualized approach to each patient’s unique constellation of reasons and urgency of testing, comorbidities, age, body habitus, physical ability, and results of previous tests and procedures.2 In-depth knowledge of the advantages and disadvantages of available radionuclide tracers and stressors by those who perform stress testing and imaging is paramount. Patient participation in decision making becomes desirable, as well.
With an acknowledgment of possibly harmful effects of low radiation doses used at times frequently over an extended period of time due to the chronic nature of CAD, attention has shifted to dose reduction3 and potentially mandatory tracking of all received radiation doses.4
In view of competing noninvasive imaging modalities, cost effectiveness has also been addressed. The length of “traditional” MPI is almost ½ a day, which poorly compares to on average 1 hour to completion and diagnosis using CTA, stress echo, or PET. Many of the newer imaging protocols therefore address the need for increased throughput and improved productivity of a Nuclear Cardiology Laboratory. This chapter will describe protocols for SPECT MPI for the two primary imaging agents, thallium-201 and technetium-based products.
The primary indication for a SPECT MPI study is the assessment of the relative distribution of coronary flow in patients with suspected or known CAD. Since this distribution of coronary flow both at rest and stress is equal in all segments of the left ventricle, the presence of perfusion defects suggests intraluminal coronary obstruction, and if worse at stress than rest, ischemia. An increase in coronary flow is needed for the detection of significant coronary artery stenosis (>50% of luminal narrowing) since rest flow distribution is even unless prior infarction is part of the history. Coronary flow can be increased most physiologically with physical effort (treadmill exercise), or in patients who are unable to exercise adequately, using coronary vasodilators (adenosine, dipyridamole, and regadenoson) or dobutamine (for a more complete description, see Chapter 8).
Evaluation of left ventricular size and function became possible with the development of gating algorithms used in conjunction with MPI. The combination of perfusion and function data improved both the diagnostic and the prognostic value of SPECT studies. ECG-gated SPECT imaging is a powerful tool for evaluating fixed attenuation artifact. Ventricular function assessment and interpretation is described in Chapter 11.
The availability of more than one perfusion tracer and different modes of stress provides a multitude of imaging protocols. Ideally, the imaging protocol should be tailored for the individual patient, taking into account the patient’s age, gender, size, physical ability, various comorbidities, and particularly the clinical question to be answered. Laboratory logistics, test urgency, and cost effectiveness also dictate imaging sequences. Knowledge of tracer and stressor characteristics is critical for the right choice and best results.
Tl-201 (clinically used since the 1970s) is a monovalent cation, analogous to potassium, with a physical half-life of 73.1 hours. Decay is by electron capture to Hg-201, with principal emission of 68- to 80-keV x-rays. First-pass extraction is high (approximately 85%). The tracer is actively transported to the myocyte as well as to other organs and washed out (redistributed) beginning 10 to 15 minutes after an IV injection. The relationship between flow and uptake is almost linear at physiologic flows and even during vasodilator-induced hyperemia. Recommended Tl-201 dose, injected at peak stress is 2.5 to 3.5 mCi. Lower Tl-201 doses are recommended with use of high-efficiency cameras (as low as 1 mCi). The standard effective radiation dose for Tl-201 is approximately 4.4 mSv per 1 mCi of Tl-201, or 10.9 to 15.3 mSv per patient. For a more detailed description, refer to Chapter 3.
Tl-201 is injected approximately 1 minute prior to termination of the exercise or at peak effect of a coronary vasodilator. SPECT imaging must begin within 10 to 15 minutes (Fig. 9-1). The delay is needed for poststress monitoring, patient positioning in the camera, and avoidance of “upward creep,” which is caused by cranial motion of the diaphragm due to hyperventilation during stress. Delay in imaging beyond this time may lead to missed ischemia as redistribution begins within 10 to 15 minutes. Tl-201 is by design “stress-first” imaging. Stress images should be reviewed, and, if there are no perfusion defects, rest imaging is unnecessary. The purpose of rest imaging is to ascertain reversibility (redistribution) of perfusion defects seen on stress images. The mechanism of Tl-201 “redistribution” is for the most part due to differential washout from the myocardium. In segments with high initial tracer uptake, that is, in segments supplied by nonobstructed coronary flow and with functional myocytes, washout rate is high. At the time of initial equilibrium (shortly after peak stress tracer injection), intravascular tracer concentration is negligible. In segments supplied by an obstructed epicardial coronary artery, coronary flow is limited and initial tracer uptake is decreased; the intracellular:intravascular tracer gradient is lower and washout rate is slower. After 3 to 4 hours of injection, a second SPECT imaging is obtained (rest scan). If there are no abnormalities, the study is considered normal. If a stress defect appears less prominent or absent on rest, the defect is considered due to ischemia and is consistent with viable but hypoperfused myocardium. If no stress defect reversibility is seen on rest images, the defect is assumed to be scar, possibly due to prior myocardial infarction. If there is concern for missed ischemia, additional imaging can be done either with additional delay (up to 24 hours) or after 1-mCi Tl-201 reinjection. (See strategies as outlined by ASNC guidelines in Fig. 9-1.)
Apart from using Tl-201 for diagnostic or prognostic purposes, Tl-201 is also indicated for detection of myocardial “viability” (Fig. 9-2). The testing should be limited to patients with resting LVEF ≤35% and who are also candidates for myocardial revascularization (surgical or percutaneous). In this case, a rest injection is performed and imaging at 15 to 20 minutes and 3 to 4 hours later. The presence of viability is judged by 50% or greater uptake in the region or vascular territory under consideration (left anterior descending, right or circumflex arteries). If viability is not demonstrated on 3- to 4-hour imaging, a 24-hour image can be obtained (Fig. 9-2).
Figure 9-2
Thallium-201 viability protocols from ASNC imaging guidelines for SPECT. The primary protocol is rest (15 minutes) followed by 3- to 4-hour delayed imaging. A secondary protocol is delay imaging 24 hours later. (Reproduced with permission from Henzlova MJ, Duvall WL, Einstein AJ, et al. ASNC imaging guidelines for SPECT nuclear cardiology procedures: Stress, protocols, and tracers. J Nucl Cardiol. 2016;23(3):606–639.)
Extensive experience, evidence-based data.
No need for delay after stress injection, possible “stress-only” protocol.
Good flow-uptake linearity, high first-pass extraction.
Absence of liver uptake.
Assessment of myocardial viability.
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