A myocardial perfusion scan (MPS), often called a nuclear stress test, is a non-invasive medical imaging procedure that provides functional information about the heart. The scan measures blood flow, or perfusion, to the heart muscle (myocardium) both at rest and during physical or pharmacological stress. This test involves injecting a small, safe amount of a radioactive tracer, such as Technetium-99m, into the bloodstream. A specialized Single-Photon Emission Computed Tomography (SPECT) camera detects the energy emitted by the tracer to create detailed, three-dimensional images of the heart.
Why This Scan Is Performed
Physicians order a myocardial perfusion scan primarily to diagnose or assess Coronary Artery Disease (CAD), a condition where the arteries supplying the heart muscle become narrowed or blocked. The test is an effective tool for evaluating patients experiencing chest pain (angina) to determine if symptoms are related to insufficient blood flow. The scan is also used to evaluate heart function following a previous heart attack, helping to assess the extent of muscle damage. For patients already diagnosed with CAD, the test helps determine the severity of the disease and measures the risk for future cardiac events. It can also monitor the effectiveness of previous interventions, such as bypass surgery or stent placement, by assessing if adequate blood flow has been restored.
The Complete Procedure: Rest and Stress Phases
The MPS procedure is conducted in two distinct parts: the rest phase and the stress phase. This structure allows for a comparison of myocardial blood flow under different physiological conditions. The entire process often takes between three and four hours to complete, though the actual imaging time is much shorter.
The rest phase begins with the injection of the radioactive tracer into a vein. After a waiting period of 30 to 60 minutes, which allows the heart muscle to absorb the tracer, the patient lies on a table while the SPECT camera rotates around the chest. These initial images establish a baseline of blood flow to the myocardium when the heart is not under strain.
The stress phase is designed to increase the heart’s workload and reveal any hidden blood flow limitations. The preferred method for inducing stress is physical exercise, typically walking on a treadmill or using a stationary bicycle. If a patient is unable to exercise adequately, a pharmacological stress agent is used instead. Common agents include Regadenoson, Adenosine, or Dipyridamole, which act as vasodilators to temporarily widen the coronary arteries, simulating exercise. At the peak of stress, a second, higher dose of the tracer is injected, and a second set of images is acquired, capturing blood flow distribution under maximum cardiac demand.
Interpreting the Scan Results
The interpretation of the scan centers on comparing the resting and stress images to identify differences in tracer uptake. The radioactive tracer is absorbed by healthy, well-perfused heart tissue, appearing bright on the scan. Areas with reduced blood flow show less uptake and appear darker, or as “defects.” Physicians look for three primary patterns of tracer distribution to make a diagnosis.
Normal Scan
A normal scan is characterized by uniform and consistent tracer uptake across all segments of the heart in both the rest and stress images. This indicates healthy, unrestricted blood flow, suggesting the coronary arteries supply sufficient blood even when the heart is working hard.
Fixed Defect
A fixed defect is observed when an area of poor tracer uptake is present in the same location and with the same severity in both the rest and stress images. This pattern signifies scar tissue, often the result of a prior heart attack where muscle cells have died and can no longer absorb the tracer.
Reversible Defect
A reversible defect is the most significant finding for diagnosing active CAD, as it indicates myocardial ischemia. In this pattern, an area shows reduced tracer uptake during the stress image but normal uptake in the corresponding rest image. This confirms a flow-limiting blockage that restricts blood supply only when the heart’s demand increases.
Patient Preparation and Safety Overview
Patients must follow specific preparation instructions to ensure accurate scan results. Patients are typically required to fast for a minimum of four hours before the test to minimize interference from nearby organs that also absorb the tracer. A critical instruction is the avoidance of all caffeinated products, including decaffeinated coffee, tea, chocolate, and certain medications, for at least 12 to 24 hours prior to the scan, especially if a pharmacological stress agent is used.
Patients are often instructed to wear comfortable clothes and athletic shoes if an exercise stress test is planned. Specific cardiac medications, such as beta-blockers or calcium channel blockers, may need to be temporarily stopped before the test, as they can interfere with the heart’s response to stress.
The overall safety profile of the MPS is high, with minimal radiation exposure comparable to a standard diagnostic X-ray. The primary risks are associated with the stress component, which can include minor side effects like temporary flushing, shortness of breath, or chest discomfort, particularly with pharmacological agents. Patients are closely monitored throughout the procedure to manage any adverse reactions. Following the scan, patients are encouraged to drink plenty of fluids to help eliminate the remaining radioactive tracer.