Answer
The nuclear myocardial scan is the best initial imaging study for the detection of myocardial ischemia; however, in some locales, stress echocardiography is performed more often because it is more readily available and because local clinicians are better trained in echocardiography and are more comfortable with the technology. Nonetheless, the wealth of research into nuclear scanning makes a strong case for its role as the best and preferred test for detecting CAD. Nuclear cardiology studies are used to assess myocardial blood flow, evaluate the heart's pumping function, and visualize the size and location of a myocardial infarction. Among the techniques of nuclear cardiology, myocardial perfusion imaging is the most widely used. [3, 4, 6, 7, 8]
The assessment of myocardial perfusion and function using PET and hybrid positron emission tomography (PET)/CT imaging is becoming more available as the cost of the technology decreases and as positron-emitting radiopharmaceuticals become more available. Cardiac PET has the advantage of higher spatial resolution, temporal resolution, and, in many cases, a decreased radiation exposure to patients. Newer gamma cameras using simultaneous 180° acquisition appear to have the potential of offering similar benefits as PET technology but are able to use the less costly technetium (TC)-99m-based radiopharmaceuticals and thallium-201 (Tl-201). [9, 10, 11, 12, 13, 14, 15]
One potentially important physiologic parameter obtained by these newer technologies is the myocardial perfusion reserve (MPR). In patients with ischemic heart disease who undergo revascularization based on PET viability assessment with fludeoxyglucose F-18 (F-18 FDG), those with a low myocardial perfusion reserve were at an increased risk of adverse cardiac events. [16, 9]
Currently, nuclear myocardial scans include both perfusion and gated wall motion images. [17, 8] Coronary artery blood flow can be assessed, and the scans can also be used to accurately determine the left ventricular ejection fraction, the end-systolic volume of the left ventricle, regional wall motion, and wall thickening. [18] In addition, solid evidence links these findings to clinical outcomes.
In the past, the field of nuclear cardiology encountered large variations in clinician training, quality control, and scan interpretation. In response, the American Society of Nuclear Cardiology (ASNC) was formed by a coalition of cardiologists, radiologists, and nuclear medicine physicians. Along with the American College of Cardiology and the Society of Nuclear Medicine, the ASNC established standard training guidelines for physicians, along with a minimum knoNuclear cardiology studies use noninvasive techniques to assess myocardial blood flow, evaluate the pumping function of the heart as well as visualize the size and location of a heart attack. Among the techniques of nuclear cardiology, myocardial perfusion imaging is the most widely used knowledge base. The Certification Board of Nuclear Cardiology has established rigorous training verification and a knowledge assessment test.
Because CAD is common and deadly but highly treatable, early diagnosis is critical. Nuclear imaging plays an important role in the diagnosis of myocardial ischemia as well as several other cardiovascular diseases. Quality control forms the backbone of nuclear myocardial imaging, and as a result, nuclear cardiology has developed into a medical specialty. With rigorous training standards and knowledge assessment, the Certification Board of Nuclear Cardiology has established a unique specialty in medicine dedicated to the use of nuclear scanning for the detection of cardiovascular disease. [19]
New hardware and software designs have been developed to optimize image quality with reduced radiation exposure. For example, improved software incorporates iterative reconstruction, resolution recovery, and noise compensation to maintain or improve myocardial perfusion single-photon emission computed tomography (SPECT) image quality with conventional sodium iodide cameras. Temporal correlation among the gated perfusion frames and higher resolution SPECT acquisitions hold promise to further improve image quality and diagnostic accuracy. Novel collimator designs, such as multipinhole or locally focusing collimators arranged in geometries that are optimized for cardiac imaging, enhance photon-detection sensitivity. [20, 21, 22, 23, 24, 8]
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The risk of coronary artery disease (CAD) can quickly be stratified by determining whether the patient's pain is nonanginal, atypical, or typical. For men in their 30s with nonanginal chest pain, the pretest probability of disease is approximately 5%; however, men in their 30s with typical chest pain have a 70% likelihood of disease (a 14-fold increase).
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In women, the risk of CAD can quickly be stratified on the basis of the type of chest pain. For example, a woman in her 50s with nonanginal chest pain has a pretest probability of disease of approximately 10%, whereas a woman in her 50s with typical chest pain has a pretest likelihood of approximately 80%; this rate is an 8-fold increase.
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An exercise stress test is inadequate for excluding CAD when the pretest probability of disease is intermediate to high. Of 1000 American men older than 60 years who have nonanginal chest pain, the exercise stress test causes clinicians to miss almost one third of CAD cases.
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Adding nuclear cardiac imaging to the stress ECG test greatly increases the clinical value of the test. Of 1000 American men older than 60 years who have nonanginal chest pain, CAD is misdiagnosed in 25 of the 281 men who have the disease; however, of these 25 patients, the annual rate of severe cardiac events (myocardial infarction or cardiac death) is still less than 1% per year.
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Normal perfusion on a single-photon emission computed tomography (SPECT) perfusion study.
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A SPECT perfusion study in a patient with a large degree of tracer redistribution affecting the left anterior descending and right coronary artery territories.
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This study in a patient with normal findings on myocardial perfusion scanning represents the value of gated imaging in nuclear cardiac scans. The gated images show akinesis of the distal anterior wall along with a reduced left ventricular ejection fraction and an elevated end-systolic volume.
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Polar maps can help identify abnormal findings on nuclear cardiac images. The polar map shows fairly good end-diastolic perfusion, a decreased regional ejection fraction and motion by the septum, and fairly normal thickening.
