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Presentation & Cause
Since the 1960s, coronary artery disease (CAD) has been the leading cause of death in the United States, and myocardial perfusion imaging (MPI) agents have been researched and developed to aid in early diagnosis and treatment planning. A myocardial perfusion imaging test is used to assess the blood flow through the heart muscle and is useful in patients experiencing chest discomfort. It is commonly known as a nuclear stress test.
When the heart doesn’t receive enough oxygen-rich blood, CAD symptoms and signs develop. Reduced blood flow to the heart in cases of CAD can result in angina, shortness of breath, and even a heart attack if blood flow is completely blocked. Fatal circumstances can be prevented by using MPI for early detection of such risks.1
MPI using single photon emission computed tomography (SPECT) and, more recently, positron emission tomography (PET) have diagnostic and prognostic value for the management of patients with known or suspected CAD.1 MPI can now be customized to the patient and clinical question at hand due to the fundamental changes in the acquisition, processing, and interpretation the new technology has brought to MPI.
The key principles for the use of a myocardial perfusion imaging test include performing the appropriate MPI test on the appropriate patient at the appropriate time. Another main principle incorporates patient comfort and convenience, cost, and especially image quality and radiation dose. Appropriate use of a myocardial perfusion imaging test is eased by appropriate use criteria.2
Myocardial Perfusion Imaging Diagnostic Workup
Widely used to diagnose and treat patients with known or suspected CAD, MPI using SPECT provides vital data on myocardial perfusion and function. The development of camera systems using solid-state crystals and novel collimator designs tailored for cardiac imaging has resulted in significant advancements in SPECT imaging.3 The main part of a SPECT system is the detector, which gathers the photon events, interprets the photon energy and position of detection, and produces data for subsequent image reconstruction.2
While maintaining high diagnostic accuracy, solid-state tomographic camera systems have allowed for significantly shorter imaging sessions and lower radiation doses. Fast or low-dose MPI has been made possible by improved photon sensitivity, and early dynamic imaging has emerged as a method for evaluating myocardial blood flow with SPECT.3 A SPECT detector’s performance is primarily determined by its sensitivity, energy resolution, and spatial resolution.2
Although MPI with SPECT has traditionally been the clinical diagnostic tool, PET MPI has seen growth over the past 20 years due to improved image quality that produces higher diagnostic accuracy than SPECT. Furthermore, routine quantification of myocardial blood flow (MBF) and myocardial flow reserve (MFR) in absolute units is done by dynamic PET imaging of the tracer distribution process from the time of tracer administration to tracer accumulation in the myocardium. Over MPI alone, MBF and MFR incrementally enhance the accuracy of diagnosis and prognosis. MPI, MBF, and MFR may be acquired simultaneously in some circumstances without added expense, radiation exposure, or processing time.1
Based on recent developments in PET, cardiac-specific SPECT with dynamic image acquisition has also sparked research into the creation and validation of SPECT MBF imaging protocols.1 PET and SPECT are particularly suited for molecular imaging because of their high sensitivity and well-established quantitative methodologies. Molecular imaging, as opposed to the detection of “defects” in MPI, is primarily based on “hot-spot” imaging.4 The development of hybrid SPECT/CT systems and general-purpose solid-state camera systems has also raised the possibility of significant clinical applications in cardiac imaging.3
A thorough understanding of the artifacts that can appear during the acquisition and processing of image data is necessary for accurate interpretation of SPECT/CT myocardial perfusion imaging studies, as well as working knowledge of possible abnormalities. Incorrect alignment of the raw SPECT/CT image datasets and patient motion are two factors that can cause artifacts to propagate onto the final images.5
To further understand CAD, it is also possible to combine MPI and cardiac CT angiography into one test to examine the anatomy and physiology of the coronary arteries as well as the structure and composition of plaque. However, further clinical studies are necessary to establish the added value of a combined assessment of biology by molecular imaging as well as anatomy and physiology.4 With a significant reduction in radiation exposure and procedure costs, hybrid imaging modalities allow for evaluation of myocardial perfusion and coronary artery calcium (CAC) quantification as a part of the same examination.6
Population Considerations
MPI can now be customized to the patient and clinical question.2 A well-validated index of atherosclerosis known as the CAC score is used to classify asymptomatic patients at an intermediate risk of developing CAD. To improve the diagnostic and prognostic power of radionuclide cardiac imaging, coronary perfusion data may be combined with anatomical data from CAC measurements. The addition of a CAC score may change the classification of MPI scans.6
The risk of cardiovascular disease in patients with chest pain varies significantly. Acute angina is associated with changes in electrocardiography (ECG) or an increase in cardiac enzymes in patients with atypical chest pain. The evaluation of atypical chest pain can be difficult because ECG results can be negative in a large number of patients with ischemic heart disease (IHD) who are experiencing chest pain. An MPI SPECT scan is a method used to increase the speed and accuracy of acute coronary syndrome (ACS) diagnosis. It can also be used to determine long-term prognosis.7
Differential Diagnosis for Myocardial Perfusion Imaging
The majority of patients with ischemia in non-obstructive coronary artery disease (INOCA) exhibit coronary microvascular dysfunction (CMD), which is linked to poor outcomes and abnormal MPI. CMD is also hardly identified by routine coronary angiography, as seen in nearly 60% to 70% of women and 30% of men undergoing diagnostic coronary angiography. A new non-invasive method to evaluate CMD is the coronary angiography-derived index of microvascular resistance (caIMR).8
IMR, independent of hemodynamic perturbations, enables direct and repeatable quantitative measurement of the minimal microcirculatory resistance in a particular coronary artery territory. IMR’s use in clinical practice is constrained by the need for a dedicated pressure-temperature sensor wire and hyperemic agents, which add to the complexity of the procedure. An offline evaluation of the microcirculatory system without the use of specialized wires or hyperemic agents has been proposed for the novel angiographic index known as caIMR.8
In patients with ST-segment elevation myocardial infarction (STEMI) and myocardial infarction with non-obstructive coronary arteries (MINOCA), caIMR has been shown to have a positive correlation with invasive IMR and to provide predictive implications for abnormal results. However, the value of the prognosis of caIMR-derived CMD in INOCA patients is still unknown.8
Relevant Measures & Metrics
In order to identify regional myocardial ischemia and infarction as the underlying cause of patient symptoms, radiolabeled tracer imaging in tandem with physiologic (exercise) or pharmacological (vasodilator) stress has proven to be an effective combination. These imaging biomarkers are frequently used to guide medical and interventional therapies intended to alleviate symptoms and lower the risk of serious adverse cardiac events like myocardial infarction (MI), heart failure, and death from cardiovascular causes.1
A noninvasive myocardial perfusion imaging test is used as a gatekeeper test before invasive coronary angiography in North America, Europe, and many other industrialized nations, helping to restrict the costs and risks of embolic stroke and MI to patients who need interventional treatment after receiving an image-guided diagnosis.1
Myocardial Perfusion Scan Risks & Complications
A risk of using MPI is radiation exposure to the patient and staff performing the scan. Clinically, stress-only imaging with a solid-state camera system enables the lowest total radiation exposure with current tomographic MPI (1 mSv). Stress-only MPI can cut patient radiation exposure by up to 60% and significantly cut radiation exposure for staff as well.3
References
1. Klein R, Celiker-Guler E, Rotstein BH, et al. PET and SPECT tracers for myocardial perfusion imaging. Seminars in Nuclear Medicine. 2020 (Vol. 50, No. 3, pp. 208-218). WB Saunders.
2. Dorbala S, Ananthasubramaniam K, Armstrong IS, et al. Single photon emission computed tomography (SPECT) myocardial perfusion imaging guidelines: instrumentation, acquisition, processing, and interpretation. Journal of Nuclear Cardiology. 2018;25(5):1784-1846.
3. Slomka PJ, Miller RJH, Hu L, Germano G, et al. Solid-state detector SPECT myocardial perfusion imaging. Journal of Nuclear Medicine. 2019;60(9):1194-1204.
4. Sadeghi MM. Beyond perfusion imaging. Journal of Nuclear Cardiology. 2022;29(4):1485-1486.
5. Dvorak RA, Brown RKJ, Corbett JR. Interpretation of SPECT/CT myocardial perfusion images: common artifacts and quality control techniques. RadioGraphics. 2011;31(7):2041-2057.
6. Zampella E, Assante R, Acampa W. Myocardial perfusion imaging and CAC score: Not only a brick in the wall. Journal of Nuclear Cardiology. 2021:1-3.
7. Taban Sadeghi M, Mahmoudian B, Ghaffari S, et al. Value of early rest myocardial perfusion imaging with SPECT in patients with chest pain and non-diagnostic ECG in emergency department. The International Journal of Cardiovascular Imaging. 2019;35(5):965-971.
8. Liu L, Dai N, Yin G, et al. Prognostic value of combined coronary angiography-derived IMR and myocardial perfusion imaging by CZT SPECT in INOCA. Journal of Nuclear Cardiology. 2022:1-8.
Sydney Murphy is the Associate Editor of HealthDay Physicians Briefing and a freelance science writer based in New York City. You can follow her on Twitter @SydneyLiz_Murph.
