A myocardial perfusion scan, also known as a nuclear stress test, is a diagnostic imaging test that shows how well blood flows through the heart muscle. It uses a small amount of a radioactive tracer to create images. These images help healthcare providers visualize areas of adequate and inadequate blood flow. This non-invasive procedure assesses heart health.
Purpose of the Scan
The primary purpose of a myocardial perfusion scan is to evaluate blood flow to the heart muscle (myocardium). This test helps detect areas of reduced blood supply (ischemia) or heart muscle damage (infarction). It helps diagnose coronary artery disease (CAD), which involves narrowing of the heart’s arteries.
Beyond initial diagnosis, the scan assesses the extent and severity of known CAD. It can determine if chest pain originates from insufficient blood flow. Scans also guide treatment decisions, such as whether revascularization procedures like bypass surgery or angioplasty would be beneficial. They can also check for scar tissue from a previous heart attack or other heart conditions.
How the Scan is Performed
A myocardial perfusion scan involves several steps, typically spanning a few hours. The process begins with injecting a small amount of radioactive tracer into a vein, usually in the arm or hand. Common tracers include technetium-99m (Tc-99m) sestamibi or tetrofosmin for SPECT scans, and rubidium-82 (Rb-82) or nitrogen-13 (N-13) ammonia for PET scans. The tracer travels through the bloodstream and is absorbed by healthy heart muscle.
After the initial tracer injection, images are taken while the patient is at rest. The patient then undergoes a stress test, which can be either exercise-induced or pharmacologic. For an exercise stress test, the patient walks on a treadmill, with intensity gradually increasing while heart rate and blood pressure are monitored. If exercise is not possible, a pharmacologic stress agent (e.g., adenosine, dipyridamole, or dobutamine) is administered intravenously to simulate exercise effects on the heart.
Following the stress phase, another dose of tracer is injected, and a second set of images is acquired. A special camera, either a Single Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET) scanner, detects the energy emitted by the tracer. The scanner rotates around the patient, who lies still on a table, to capture detailed pictures of the heart. Comparing rest and stress images helps identify differences in blood flow.
Understanding Your Results
The images from a myocardial perfusion scan reveal patterns of blood flow within the heart muscle. Areas with normal blood flow show an even distribution of the radioactive tracer. Conversely, regions with reduced or absent tracer uptake indicate blood supply issues. These “cold spots” or “defects” signify either reduced blood flow (ischemia) or damaged heart muscle (infarction).
Healthcare professionals analyze these images to distinguish between reversible defects and fixed defects. A reversible defect shows decreased tracer uptake during stress that normalizes at rest, indicating myocardial ischemia (insufficient blood flow only when the heart works harder). A fixed defect, however, shows reduced tracer uptake in both stress and rest images, often indicating scar tissue from a prior heart attack or non-viable myocardium. The size and severity of these defects are also assessed, helping to determine the extent of heart involvement and potential next steps for care.
Safety and Considerations
Myocardial perfusion scans are generally considered safe. The procedure involves exposure to a small amount of radiation from the radioactive tracer. Patients can help flush the remaining tracer by drinking plenty of fluids after the test. Allergic reactions to the tracer are rare.
If a pharmacologic stress agent is used, patients might experience temporary side effects such as flushing, headache, dizziness, nausea, chest pain, or shortness of breath. Certain medications, like those containing caffeine or theophylline, may need to be avoided for 12 to 48 hours before the scan, as they can interfere with pharmacologic stress agents. Fasting for a few hours before the test is typically required. Patients should also inform their healthcare provider about any medications they are taking, allergies, or if they are pregnant or breastfeeding, as these factors can influence the procedure.