In medicine, gold standard techniques are rare, may be quite invasive or expensive and more currently accepted than perfect. In some cases the gold standard is unknown, and a result is plotted against that of another more accepted method. Cardiac imaging techniques are used for estimating different parameters such as flow and function, and single photon emission tomography (SPECT) is a nuclear cardiology multipurpose technique used in several cardiovascular fields. Each field has different standards, and SPECT is frequently used as a standard itself. This paper will compare the current accuracy of SPECT with respect to coronary anatomy, starting from significant lesions up to intermediate coronary stenoses and “normal” vessels.
Background
The principle behind SPECT is impairment between myocardial supply and demand. Since flow tracers have a good relationship with myocardial blood flow up to moderate hyperemia, ischemia or simply flow misdistribution result in a reversible perfusion defect.
Significant coronary lesions
Since its introduction in 1976, taking as a standard a coronary lesion > 70%, myocardial perfusion scintigraphy has been characterized by a continuous increase in sensitivity up to 85-90% after the introduction of Gated SPECT.
In the era of planar imaging, specificity also increased from qualitative to quantitative Thallium 201 analysis. However, SPECT resulted in a loss of specificity mostly due to a high incidence of false positives caused by artefacts in image reconstruction or soft tissue attenuation. In the last decade, the diffusion of Technetium tracers with higher photons flux resulted in a continuous increase in specificity up to 70-75% or even >80% with Gated SPECT (1). Thus, using significant coronary lesions as standard, SPECT has a high accuracy among stress imaging techniques. If we shift the standard from significant angiographic lesions to PET quantification of regional blood flow, SPECT continues to identify most segments with a reduced coronary flow reserve (2). However, a minority of PET areas with intermediate reduction of hyperemic coronary blood flow are not detected by SPECT. The good overall accuracy of SPECT is associated with several added values such as the possibility of separating single and multivessel diseases, identifying high risk patients after myocardial infarction and detect tissue viability.
Intermediate lesions
These kind of lesions are rather complex because their angiographic degree of stenosis does not predict the occurrence of stress ischemia, and thus additional information is needed for clinical decision making. There are several studies demonstrating that even quantitative angiography does not predict ischemia while Doppler measurements of flow reserve are able to differentiate between “innocent” or “culprit” lesions (3). In these vessels, SPECT is in close agreement with fractional flow reserve rather than with quantitative angiography, and this is true for normal as well as for previously infarcted segments (4). Thus, beyond invasive Doppler measurements, SPECT is a technique that shows a good correlation with flow reserve, and thus may be clinically used to assess whether an intermediate lesion should be treated or not by percutaneous coronary interventions or by-pass surgery.
“Normal” vessels
Recent observations obtained by intravascular ultrasounds showed that SPECT reversible defects may occur even in areas perfused by “normal” coronary arteries with unrecognised atherosclerotic plaques (5). These plaques may be not detected through angiography, but may result in flow misdistribution as evidenced by a reduced coronary flow reserve. Once again, the angiographic standard gives imperfect results.
Gated SPECT
Beyond improving specificity by reducing the number of attenuation artefacts, Gated SPECT has its own standards in terms of volume and regional wall motion. Several studies demonstrated that Gated SPECT accurately measures end diastolic and end systolic volumes providing a quantitative evaluation of regional wall motion and thickening at the same time (6). Finally, the combined flow/function analysis provides additional diagnostic and prognostic power to SPECT.
Conclusion
SPECT has been compared with different standards. Significant lesions are detected with high accuracy in the majority of patients, and intermediate lesions with a positive scan are undoubtedly to be considered as flow limiting stenoses. For these reasons, in the absence of a detectable coronary artery disease, a positive SPECT deserves more attention than the generic label of false positive scan, although different conditions such as diabetes, hypertrophy and cardiomyopathies that may reduce “angiographic” specifity mostly through microvascular mechanisms should also be considered.
The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.
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