The Myocardial Perfusion Scan (MPS) is a widely used, non-invasive imaging test that evaluates blood flow to the heart muscle (myocardium). This procedure helps physicians determine if areas of the heart receive sufficient blood supply at rest and during physical or chemical stress. The MPS is primarily used to detect or rule out Coronary Artery Disease (CAD). Its accuracy is measured using distinct statistical metrics that reflect how often the test result matches the patient’s true condition.
Understanding Medical Test Metrics
The reliability of any medical test, including the MPS, is quantified using specific metrics. Sensitivity measures the test’s ability to correctly identify the presence of disease in those who have it (the true positive rate). A high sensitivity means the test is unlikely to miss a case of CAD, making it useful for ruling out the condition.
Specificity measures the test’s ability to correctly identify the absence of disease in healthy individuals (the true negative rate). High specificity means a positive result is more likely to be correct, reducing the chance of false alarms and unnecessary follow-up procedures.
The most practical measures for patients are the Positive Predictive Value (PPV) and the Negative Predictive Value (NPV). PPV is the probability that a person has CAD given a positive test result, while NPV is the probability that a person does not have CAD given a negative result. These predictive values are directly influenced by the prevalence of CAD in the tested population, known as the clinical pre-test probability. For instance, a positive result in a low-risk patient is less likely to be a true positive than the same result in a high-risk patient.
General Accuracy Rates
The Myocardial Perfusion Scan (MPS), typically performed as a Single-Photon Emission Computed Tomography (SPECT) scan, demonstrates high diagnostic performance for CAD. Clinical studies show that the sensitivity of the MPS often ranges from 82% to 91%. This indicates the scan successfully identifies most people with flow-limiting coronary artery disease.
Specificity is generally reported to be slightly lower, typically ranging between 70% and 90%. This reduction is often due to non-disease-related factors that mimic a perfusion defect. Pharmacological stress testing, which uses medication to simulate exercise, tends to achieve slightly higher sensitivity than a standard exercise-based MPS.
These figures represent broad averages derived from diverse populations and scanning environments. Some meta-analyses have reported lower average specificity, sometimes as low as 52%, reflecting technical variability and patient selection challenges. Despite this variability, the Negative Predictive Value (NPV) frequently remains high, often above 80%, indicating that a negative scan strongly suggests a good prognosis and low likelihood of disease.
Patient and Technical Factors Affecting Reliability
The accuracy of an MPS is affected by physical and technical issues that introduce imaging artifacts. A common challenge is attenuation artifact, where non-cardiac tissue absorbs the tracer’s energy, creating the appearance of a false blood flow defect. For example, breast tissue in women or the diaphragm and liver in men can obscure parts of the heart, leading to false positive results.
Body size and composition also play a role, as a larger body habitus can degrade image quality and complicate interpretation. Modern SPECT scanners use attenuation correction software, often integrated with a CT scanner, to compensate for these effects and substantially improve specificity.
Patient movement during the long imaging acquisition time can blur the image and simulate a perfusion defect, though motion-correction algorithms help mitigate this. The choice of stress method also impacts reliability. While exercise is preferred, inability to reach the target heart rate can cause an indeterminate or false negative result. In these cases, pharmacological stress agents are used to ensure maximum coronary vasodilation, often yielding a more sensitive result.
A complex issue is balanced ischemia, which occurs when severe disease affects multiple coronary arteries. Blood flow is reduced equally across the entire heart, making the scan appear falsely normal because there is no relative difference in tracer uptake. Interpretation of the scan must always be performed by an experienced physician who considers the patient’s full clinical profile alongside these technical factors.
Integrating Scan Results into Patient Care
The MPS guides clinical decision-making by integrating results with a patient’s symptoms, medical history, and established risk factors for CAD. Physicians utilize the MPS results to determine the probability of disease. A negative MPS result strongly indicates a low risk for future cardiac events, often leading to a recommendation for continued medical management and lifestyle adjustments.
A positive MPS, which shows an area of reduced blood flow, is a significant finding that prompts further action. For instance, a clear perfusion defect suggesting a flow-limiting blockage might lead to more invasive diagnostic tests, such as coronary angiography. The scan’s ability to provide prognostic information—predicting the likelihood of future major adverse cardiovascular events—is highly valued by clinicians.
The MPS rarely stands alone as the sole piece of evidence for a diagnosis. It provides incremental value beyond a simple medical history and physical examination. By visualizing the functional impact of potential blockages on the heart muscle, the scan helps physicians distinguish between stable disease and patterns of ischemia requiring aggressive intervention. This comprehensive approach ensures that patients receive the most appropriate and personalized care.