Review ArticleThe future of SPECT MPI: Time and dose reduction
Introduction
Myocardial perfusion imaging (MPI) is the noninvasive method most commonly employed and used for the longest time for the evaluation of suspected and known coronary artery disease (CAD). MPI became possible due to the introduction of the Anger gamma camera in 1957 and the establishment of Thallium-201’s suitability for visualization of myocardial perfusion in 1973. Landmarks leading to the current applications of MPI include the transition from planar to SPECT imaging in 1976, the change from single- to dual-head gamma cameras, the introduction of coronary vasodilators for stress, the development of Tc-99m-based tracers in 1990s, the introduction of gating algorithms, the use of attenuation correction, and most recently the development of hybrid systems (SPECT/CT) and solid state detectors.
During this almost 50-year journey of nuclear cardiology, other noninvasive methods for similar indications became available: positron emission tomography (PET), stress echocardiography, CT angiography (CTA), and cardiac magnetic resonance imaging (MRI). As is often the case, competition points to the weaknesses and limitations of its competitors. Criticism of SPECT MPI focuses on the length of the test, radiation exposure to the patients (and to the personnel), inferior specificity (false positivity), and the inappropriate use and the non-sustainable increase in the number and cost of performed tests.
The general availability, accumulated expertise and unparalleled diagnostic and prognostic value of MPI SPECT have long outweighed these limitations. However, the accumulated flaws brought to light by decades of use are finally being addressed. Documented overuse has been addressed by Guidelines and Appropriateness criteria which are periodically updated.1,2 Perhaps, a more controversial approach to address MPI over-utilization is an attempt by payers to regulate the unsustainable volume of the tests by pre-test authorization and/or payment denials. Another unexpected challenge to the modality was the recent shortage of Tc-99m.3
The purpose of this review is to describe recently applied approaches to address the major procedural limitations of MPI: the length of the tests and the radiation exposure to the patients. As will be discussed, both challenges have similar solutions.
Section snippets
Challenges: Test Length
The “classic” Tl-201 protocol requires a 3-4-hour delay between the stress and the redistribution imaging. In addition, some patients may require a second set of delayed images after Tl-201 re-injection. Including the time needed for the stress test and at least two sets of images, often more than 6 hours are needed for study completion. Similarly, a traditional Tc-99m rest-stress sequence with 40-60 minute post-injection waiting, requires at least 4 hours for completion. A dual isotope
Challenges: Radiation Exposure
The use of Tl-201 and Tc-99m for diagnostic purposes involves internal radiation to the patient and external radiation to laboratory personnel. The doses commonly utilized for MPI (2-4 mCi of Tl-201 and 30-40 mCi of Tc-99m) are considered low, compared to therapeutic radiation or exposure from a nuclear bomb explosion or a nuclear accident. When expressed as effective doses, the average MPI study exposes the patient to 12-15 mSV (Figure 2). There is no definite proof of increased cancer risk
Summary and Future Trends
The modernization of SPECT imaging is feasible with progress in reducing both the length of the test and the radiation dose to the patient and to the personnel. Changes can be implemented without hardware or software upgrades with stress-only imaging or tracer choice. Greater changes can be made with migration to newer technologies such as ½ time acquisition software and solid state cameras.
Perhaps, the future belongs to hybrid imaging, which combines physiologic and anatomic information
Acknowledgment
We would like to thank Dr. Andrew Einstein for providing Figure 2.
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