Myocardial Perfusion Imaging (MPI), often called a nuclear stress test, is a medical imaging technique that provides detailed information about blood flow to the heart muscle, the myocardium. By using small amounts of radioactive material, MPI helps physicians assess the function and health of the heart, particularly in the context of coronary artery disease. The test is a common method for evaluating symptoms like chest pain and shortness of breath.
Defining Myocardial Perfusion Imaging
Myocardial Perfusion Imaging is a type of molecular imaging. The procedure involves injecting a radiopharmaceutical, a radioactive tracer such as Technetium-99m, into the bloodstream. This tracer travels through the circulatory system and is absorbed by healthy heart tissue in proportion to the blood flow it receives. A specialized device, like a Single Photon Emission Computed Tomography (SPECT) camera, detects the gamma rays emitted by the tracer within the myocardium.
The resulting images create a map of the heart’s blood supply, showing which areas are receiving adequate blood flow and which are not. Areas with good blood flow will “light up” on the scan due to tracer uptake. Conversely, areas with reduced or absent flow will appear as “cold spots” or defects, allowing medical professionals to visualize the functional impact of blockages in the coronary arteries.
Clinical Applications and Indications
Physicians primarily order an MPI scan for the diagnosis and evaluation of Coronary Artery Disease (CAD). Since CAD involves the narrowing of vessels supplying the heart muscle, MPI determines if this narrowing significantly reduces blood flow. The test is particularly useful for assessing unexplained chest pain or other symptoms suggesting a cardiac origin.
MPI provides data that helps stratify a patient’s risk of a future cardiac event. For individuals with known CAD, the scan assesses the severity and extent of blockages. Furthermore, it monitors the effectiveness of revascularization procedures, such as angioplasty or bypass surgery. The images can also identify areas of the heart muscle damaged by a previous heart attack.
How the Imaging Procedure Works
The MPI procedure involves two separate phases: a “Rest” study and a “Stress” study. During the Rest study, the radioactive tracer is injected while the patient is lying down, and images are acquired shortly after. This initial scan establishes the baseline blood flow to the heart muscle. The patient lies still on a table while the SPECT camera captures three-dimensional images.
The Stress study increases the heart’s workload to reveal blood flow deficiencies not apparent at rest. Stress is commonly achieved through exercise, such as walking on a treadmill or using a stationary bicycle. If a patient cannot exercise due to physical limitations, a pharmacological stress agent is administered intravenously to chemically simulate exertion. These medications, such as adenosine or dipyridamole, cause the coronary arteries to dilate, mimicking the increased blood flow demand of exercise.
At the peak of either exercise or pharmacological stress, a second dose of the radiotracer is injected. This tracer circulates to the heart muscle, reflecting blood flow under maximum demand. Immediately afterward, the patient is moved back to the scanner for a second set of images.
The SPECT or sometimes PET scanner uses specialized detectors to capture the radiation emitted by the tracer. A computer processes these signals to reconstruct detailed, cross-sectional, three-dimensional views of the left ventricle. The difference in tracer uptake between the rest and stress images provides the functional data physicians use to assess the heart’s health. The entire process, including wait times between injections and imaging, can take a few hours to complete.
Interpreting the Scan Results
Interpreting an MPI scan involves comparing tracer uptake in the heart muscle during the stress phase with uptake during the rest phase. A normal scan shows a uniform distribution of the radioactive tracer in both sets of images. This equal uptake indicates that blood flow to all areas of the myocardium is adequate, even when the heart is working hard.
A reversible defect strongly indicates myocardial ischemia, a temporary lack of blood flow. In this pattern, the image shows reduced tracer uptake during the stress study but normal uptake during the rest study. This suggests a coronary artery blockage is present and restricts blood flow only when the heart’s demand for oxygen increases.
A fixed defect, also known as an irreversible defect, appears as a persistent reduction in tracer uptake in the same area on both the stress and rest images. This pattern usually signifies myocardial infarction or scar tissue, meaning the heart muscle has been permanently damaged from a prior heart attack. The tissue is no longer viable and cannot absorb the tracer regardless of the blood flow condition. By analyzing the size, location, and reversibility of defects, the physician can determine the severity of the disease and plan the appropriate management strategy.