MPI stands for myocardial perfusion imaging, a noninvasive heart test that measures how well blood flows through the muscle of your heart. It’s one of the most common tools used to detect coronary artery disease, and you may also hear it called a nuclear stress test. The test works by comparing images of your heart at rest and under stress to reveal areas that aren’t getting enough blood.
What MPI Shows and Why It’s Ordered
MPI gives your cardiologist a detailed map of blood flow through the heart muscle. Areas that receive normal blood flow light up evenly, while areas with reduced flow appear as darker spots. By comparing images taken at rest with images taken during physical stress, the test can distinguish between two important findings: areas of the heart that are temporarily starved for blood (which signals an active blockage that could be treated) and areas of permanent scarring from a prior heart attack.
The test is most commonly ordered to evaluate chest pain, shortness of breath, or other symptoms that could point to blocked coronary arteries. But it also plays a role in several other situations: assessing risk before major surgery, checking how well a previous bypass or stent is holding up, determining whether heart tissue is still alive and worth revascularizing, and evaluating people at high risk for heart disease even without symptoms. If a standard treadmill test produces unclear results or your EKG is difficult to interpret, MPI is often the next step.
How the Test Works
MPI relies on a small amount of a radioactive tracer injected into a vein in your arm. The tracer travels through your bloodstream and is absorbed by heart muscle cells in proportion to blood flow. Healthy, well-supplied areas of the heart absorb more tracer; areas with restricted blood flow absorb less. A specialized camera called a gamma camera then rotates around your chest, detecting the energy emitted by the tracer and building a 3D image of your heart.
The most commonly used tracers are technetium-based compounds, which emit gamma rays and have a half-life of about six hours, meaning the radioactivity fades relatively quickly. An older tracer, thallium, is still used in some protocols. For facilities equipped with PET scanners, rubidium-82 is an alternative that offers higher image quality.
What to Expect During the Test
The full appointment typically takes several hours, though you won’t be actively doing something the entire time. The test has two main phases: rest and stress.
During the rest phase, you receive a tracer injection and then lie still on a table while the gamma camera captures images of your heart. Each imaging session takes about 15 to 20 minutes. You’ll need to keep your arms above your head and stay as motionless as possible, since even small movements can blur the images.
For the stress phase, the preferred method is walking on a treadmill. The speed and incline increase gradually while your heart rate and blood pressure are monitored. Once you hit your target heart rate (based on your age), the tracer is injected again and you continue exercising for another one to two minutes to let it circulate. Then a second round of images is taken.
If you can’t exercise due to joint problems, lung disease, or other limitations, a medication is given through your IV to mimic the effect of exercise by widening the coronary arteries. This pharmacological stress test takes about 20 to 30 minutes. Afterward, you’re monitored for roughly 10 minutes to make sure your heart rate and blood pressure return to normal.
How to Prepare
If your test will use a pharmacological stress agent, you’ll be asked to avoid all caffeine for 12 to 24 hours beforehand. This includes coffee, tea, energy drinks, chocolate, and some medications. Caffeine directly interferes with how the stress medication works, and consuming it beforehand can compromise the accuracy of your results. Your ordering physician will provide specific instructions about which of your regular medications to hold or continue. Wear comfortable clothes and walking shoes if you’ll be exercising on the treadmill.
Understanding Your Results
Your results hinge on the comparison between rest and stress images. There are three main patterns a cardiologist looks for:
- Normal perfusion: Blood flow appears even across the heart in both sets of images. This is a reassuring result that makes significant coronary artery disease unlikely.
- Reversible defect: An area shows reduced blood flow during stress but looks normal at rest. This pattern indicates ischemia, meaning a coronary artery is narrowed enough to limit flow when the heart is working hard. It often points to a blockage that may benefit from treatment.
- Fixed defect: An area shows reduced blood flow in both the stress and rest images. This typically means scar tissue from a previous heart attack, where the muscle is no longer viable.
Defects are also graded by size (small, medium, or large), severity (mild, moderate, or severe), and location within specific regions of the heart. A small, mildly reversible defect carries very different implications than a large, severely fixed one. Your cardiologist uses this information alongside your symptoms, risk factors, and other test results to decide on next steps.
How Accurate Is MPI?
The accuracy depends on the imaging technology used. The most widely available version, called SPECT (single-photon emission computed tomography), correctly identifies coronary artery disease in about 74% of people who have it and correctly rules it out in about 79% of people who don’t. PET-based MPI performs better, detecting disease in roughly 84% of cases with an 87% rate of correctly clearing those without significant blockages. MRI-based perfusion imaging is the most accurate of the available options, with sensitivity around 89% and specificity near 87%.
These numbers mean MPI is a strong screening tool, but not a perfect one. A normal result significantly lowers the probability that you have dangerous blockages, while an abnormal result may lead to further testing, such as a coronary angiogram, to confirm the finding and plan treatment.
Other Meanings of MPI
In medical research, MPI can also refer to magnetic particle imaging, a newer technology introduced in 2005 that uses superparamagnetic iron oxide nanoparticles instead of radioactive tracers. It’s radiation-free and offers high sensitivity, with potential applications in cancer imaging, vascular imaging, and monitoring cell therapies. However, magnetic particle imaging is still largely in the research phase and not yet a routine clinical tool. If your doctor ordered an MPI, they’re almost certainly referring to myocardial perfusion imaging.