An MPI, or myocardial perfusion imaging scan, is a noninvasive heart test that shows how well blood flows through your heart muscle. A small amount of a radioactive tracer is injected into your vein, and a special camera tracks where that tracer goes, creating a detailed map of blood flow. Areas that receive less blood show up as dark spots on the images, revealing blockages or damage that might not be visible any other way. It’s one of the most widely used tools for diagnosing coronary artery disease.
How the Test Works
The basic idea behind an MPI is straightforward: healthy heart muscle with good blood supply absorbs more of the radioactive tracer than areas starved for blood. After the tracer is injected into a vein, it travels through your bloodstream and gets taken up by heart muscle cells in proportion to the blood flow they receive. A gamma camera then captures images showing where the tracer concentrated and where it didn’t.
The test typically involves two sets of images. One set is taken while your heart is under stress (either from exercise or a medication that mimics exercise), and the other is taken while you’re at rest. Comparing the two reveals critical information. If a region looks normal at rest but shows reduced blood flow during stress, that suggests a blockage is limiting flow when the heart works harder. This pattern, called reversible ischemia, often means the muscle is still alive but at risk. If a region shows poor blood flow in both sets of images, that typically indicates permanent scarring from a previous heart attack.
Exercise Stress vs. Chemical Stress
When possible, the stress portion of the test involves walking on a treadmill until your heart rate reaches a target level. Exercise stress gives doctors additional useful information, like how well you tolerate physical activity and how your heart rhythm responds to exertion.
If you can’t exercise due to joint problems, severe fatigue, or other limitations, a medication is used instead to increase blood flow to your heart and simulate the effects of exercise. Both approaches are effective for identifying blockages. However, patients who receive chemical stress tend to be older and sicker overall, with higher rates of diabetes, high blood pressure, and prior heart attacks. A large meta-analysis found that patients who couldn’t exercise had higher rates of future cardiac events regardless of their scan results, making the inability to exercise itself an important risk marker.
What an MPI Is Used For
Doctors order an MPI for several specific reasons. The most common is evaluating chest pain, whether it’s new or has been occurring over days or longer, to determine whether narrowed coronary arteries are the cause. The test is also used after a heart attack to assess how much muscle was damaged, and after procedures like bypass surgery, angioplasty, or stent placement to check whether blood flow has improved. In some cases, it’s used before major surgery to evaluate cardiac risk in patients with known risk factors.
One advantage MPI has over some alternatives, like stress echocardiography, is its ability to detect areas of viable but threatened heart muscle. Research comparing the two approaches suggests that while stress echocardiography can detect coronary artery disease at comparable rates, MPI identifies significantly more at-risk tissue. This makes it particularly useful for deciding which patients need more aggressive treatment.
Diagnostic Accuracy
For detecting blockages in the main coronary arteries, MPI has a sensitivity and specificity around 67% to 71%, meaning it correctly identifies most significant blockages but can miss some, particularly in smaller branch vessels. Its accuracy drops somewhat in patients who have already had bypass surgery. These numbers mean a normal MPI result is reassuring but not a guarantee, and an abnormal result typically leads to further testing, such as a coronary angiogram, to confirm the findings and plan treatment.
The Radioactive Tracers
Two main types of radioactive tracers are used. The older one, thallium-201, is a potassium mimic that gets actively pulled into heart cells by the same pumps that transport potassium. The newer and now preferred option is technetium-99m, which is attached to a carrier compound that concentrates in heart muscle. Technetium-99m produces sharper images because of its energy properties, and its shorter half-life means a higher dose can be given for better image quality without increasing your overall radiation exposure. Some facilities still use thallium-201, and it remains reliable for identifying blood flow problems, though the images tend to appear slightly more blurred.
Radiation Exposure
An MPI does involve radiation, and the dose varies depending on which tracer is used and the specific protocol. The target for most facilities is to keep the total effective dose at or below 9 millisieverts, roughly equivalent to three or four years of natural background radiation. In practice, the average across U.S. labs has been closer to 11 to 15 millisieverts. Thallium-201 protocols deliver roughly double the radiation of technetium-99m protocols, around 26 millisieverts compared to 12 millisieverts. This difference is one reason technetium-based protocols have become the standard at most centers.
How to Prepare
Preparation starts the day before. You’ll need to avoid caffeine for 24 hours before the test, and that includes coffee, tea, soda, chocolate, and caffeine-containing medications like some headache remedies. Alcohol and tobacco products should also be avoided for 24 hours. On the day of the test, don’t eat or drink anything except water for six hours beforehand. Bring a list of all your medications. If you take nitrate medications for chest pain, your doctor will likely ask you to skip them the day before and the day of the exam. Other medication adjustments depend on your specific situation.
What to Expect on Test Day
A standard rest-and-stress MPI takes about 3 to 5 hours from start to finish. Most of that time is spent waiting between the injection and the imaging, not on the treadmill or under the camera. The tracer needs time to circulate and settle into your heart muscle before the camera can capture useful images. You’ll receive one injection for the stress portion and another for the rest portion (or vice versa), with separate imaging sessions after each.
Some newer protocols can shorten the process considerably. If only stress images are needed, the entire study can be completed in about 90 minutes. Dual-tracer protocols that use both thallium and technetium together have been tested at under 30 minutes total, though these aren’t yet standard everywhere.
The imaging itself is painless. You lie still on a table while the gamma camera rotates around your chest. The exercise portion, if you’re doing treadmill stress, involves walking at increasing speed and incline for several minutes. If you’re receiving a chemical stress agent, you’ll stay seated or lying down while the medication is administered through your IV. Some people feel flushed, slightly short of breath, or experience a brief headache during chemical stress, but these effects pass quickly.