Myocardial Perfusion Imaging (MPI) is a non-invasive diagnostic test that evaluates blood flow to the heart muscle. It helps healthcare professionals understand how well the heart receives blood, both at rest and during activity. This imaging technique is a valuable tool for assessing heart function.
Understanding Myocardial Perfusion Imaging
Myocardial Perfusion Imaging detects conditions like coronary artery disease (CAD), which involves narrowed arteries supplying the heart. It also assesses existing heart disease, determines heart attack risk, or evaluates heart treatment effectiveness.
The core principle behind MPI uses a small amount of a radioactive tracer. This tracer is injected into the bloodstream and travels to the heart muscle, where it is absorbed by healthy tissue. A special camera detects the energy emitted, creating images that show how blood is flowing through the heart. Areas with good blood flow appear differently from those with reduced or absent flow, providing a visual map of the heart’s perfusion.
Preparing for the Scan
Preparing for a Myocardial Perfusion Imaging scan involves several steps to ensure accurate results. Patients typically fast for four to six hours before the procedure, avoiding all food and drinks except plain water.
Avoid caffeine for at least 12 to 24 hours prior, as it can interfere with the stress portion of the test. This includes coffee, tea, sodas, chocolate, and certain over-the-counter medications. Patients should discuss all medications with their doctor, as some, such as beta-blockers or nitrates, might need to be temporarily stopped. Specific instructions for managing diabetes medications may also be provided. Wearing comfortable, loose-fitting clothing is recommended.
The Imaging Procedure
The Myocardial Perfusion Imaging procedure involves two main phases: a rest phase and a stress phase. During the rest phase, a small amount of radioactive tracer is injected into a vein. A specialized gamma camera then captures images of the heart, showing blood flow under normal, resting conditions.
Following the rest images, the stress phase begins. This can be induced through physical exercise, such as walking on a treadmill or riding a stationary bicycle, or through medication if the patient cannot exercise. If medication is used, drugs like adenosine or dobutamine are administered intravenously to simulate exercise effects. A second dose of the radioactive tracer is then injected at the peak of stress, and another set of images is taken. The entire procedure, including both phases, can take several hours, often between three to five hours.
Interpreting the Results
Interpreting the results of a Myocardial Perfusion Imaging scan involves comparing images from the rest and stress phases. Areas of the heart muscle that receive adequate blood flow during both rest and stress appear uniform and bright on the images. This indicates healthy blood supply.
If an area shows normal blood flow at rest but reduced flow during stress, it suggests a temporary lack of blood supply, known as ischemia. This often points to a partial blockage in a coronary artery that restricts blood flow when the heart works harder. Conversely, if an area shows reduced or absent tracer uptake during both rest and stress, it indicates a myocardial infarction, or heart attack, where heart muscle tissue has been permanently damaged. These findings help diagnose heart conditions, assess their severity, and guide treatment strategies.