Myocardial perfusion imaging (MPI) is a type of nuclear heart scan that shows how well blood flows through your heart muscle. A small amount of radioactive tracer is injected into your bloodstream, and a specialized camera captures where the tracer travels, creating a detailed map of blood flow. Areas that receive healthy blood flow light up brightly, while areas with reduced flow appear darker, revealing potential blockages in your coronary arteries. The test is one of the most established tools for diagnosing coronary artery disease without surgery.
How the Tracer Works
The radioactive tracers used in MPI are designed to concentrate in heart muscle cells in proportion to blood flow. When a region of your heart has good blood supply, cells in that area absorb more tracer and appear bright on the scan. When a coronary artery is partially or fully blocked, the heart muscle it feeds receives less tracer, creating a visible “perfusion defect” on the images.
Different tracers work in slightly different ways. One older tracer, thallium-201, mimics potassium and gets actively pulled into heart cells by the same pumps that move potassium. It has the highest initial uptake of any tracer used for this purpose, with about 85% of it absorbed by heart muscle on its first pass through the bloodstream. More commonly today, a technetium-based tracer is used. Technetium on its own doesn’t concentrate in heart tissue, so it’s attached to a carrier compound that directs it there. These technetium tracers have somewhat lower initial uptake (54% to 65%), but they produce clearer images because of their energy properties and are more practical for modern cameras.
Why Your Doctor Might Order One
MPI is most useful when there’s an intermediate chance that coronary artery disease is causing your symptoms, based on your age, sex, risk factors, and symptom pattern. It fills a gap: if your risk is very low, the test probably isn’t necessary, and if your risk is very high, you may need a more direct procedure like a catheterization. The sweet spot is the large group of people in between.
Common reasons for ordering MPI include:
- Chest pain with an unclear cause. If you arrive at the emergency department with chest pain but your ECG and blood tests come back normal, a resting MPI can help determine whether your heart muscle is at risk.
- Abnormal baseline ECG. Certain heart conditions, like left bundle branch block or a pacemaker rhythm, make a standard treadmill stress test unreliable. MPI provides an alternative way to assess blood flow.
- Risk stratification in known heart disease. If you’ve already been diagnosed with coronary artery disease, MPI can show how severe the blockages are and which parts of your heart are affected.
- Heart failure evaluation. In patients with weakened heart function, MPI can determine whether coronary artery disease is a contributing factor, even when chest pain isn’t present.
- High-risk patients without symptoms. People with diabetes or multiple cardiovascular risk factors may benefit from screening even before symptoms develop.
SPECT vs. PET Scanning
Two types of cameras can capture the tracer images: SPECT and PET. SPECT (single-photon emission computed tomography) is the workhorse of nuclear cardiology and is available at most hospitals. PET (positron emission tomography) produces sharper images with better spatial resolution, which translates to more accurate results, particularly in certain patient groups.
In large analyses, PET consistently outperforms SPECT in detecting significant coronary blockages. PET sensitivity runs around 67% to 76% depending on heart size, compared to 43% to 61% for SPECT. The gap widens in people with smaller hearts, where SPECT’s lower resolution leads to more false results. PET also delivers less radiation in many protocols and can measure absolute blood flow through the heart muscle, something SPECT cannot easily do. That said, PET scanners are less widely available and more expensive, so SPECT remains the default at many centers.
The Stress Component
MPI requires comparing your heart’s blood flow at rest to blood flow under stress. In a healthy heart, blood flow increases dramatically during exertion. In a heart with a significant blockage, the narrowed artery can’t keep up, and the difference between healthy and compromised areas becomes visible.
If you’re able to exercise, you’ll walk on a treadmill with increasing speed and incline until you reach a target heart rate. The tracer is injected at peak exercise. If you can’t exercise adequately, a pharmacological stress agent is used instead. These medications work by either dilating your coronary arteries (mimicking the increased blood flow of exercise) or by increasing your heart rate directly. The inability to exercise is itself a strong predictor of cardiac events, which is one reason your care team notes which method was used.
Pharmacological stress agents can cause temporary side effects. Chest pain occurs in roughly 20% of patients receiving certain agents, though this is often a drug effect rather than a sign of a blockage. Other common side effects include headache, flushing, dizziness, nausea, and a drop in blood pressure. These effects are short-lived, typically resolving within minutes.
What to Expect on Test Day
Plan for the appointment to take about 2 to 4 hours. The process involves two rounds of imaging, one at rest and one after stress, with waiting periods in between.
You’ll receive an IV line, and the tracer will be injected. After the first injection, you’ll wait roughly 45 minutes for the tracer to concentrate in your heart muscle, then lie still under the camera for 15 to 20 minutes while images are captured. Next comes the stress portion (treadmill or medication), followed by another injection, another 45-minute wait, and a second round of imaging. Some protocols reverse this order or use a stress-only approach if the stress images look normal, which can shorten the visit and reduce radiation exposure.
Preparation matters. You’ll need to avoid all caffeine for 24 hours before the test, including coffee, tea, energy drinks, chocolate, and even decaffeinated versions, which contain trace amounts that can interfere with results. Your doctor may also ask you to hold certain medications, particularly beta-blockers, nitrates, or diabetes drugs, so confirm this ahead of time.
Reading the Results
The key comparison is between your stress images and your rest images. Three patterns tell the story:
- Normal perfusion. Both the stress and rest images show even tracer uptake throughout the heart. This is a reassuring result that makes significant coronary blockages unlikely.
- Reversible defect. An area appears dark on the stress images but fills in on the rest images. This pattern indicates ischemia, meaning part of your heart isn’t getting enough blood during exertion but recovers at rest. It points to a significant blockage that may need treatment.
- Fixed defect. An area appears dark on both stress and rest images. A severe fixed defect typically represents scar tissue from a prior heart attack. However, a mild or moderate fixed defect can sometimes indicate “hibernating” muscle, tissue that is alive but not functioning well due to chronically reduced blood flow, and may recover if blood flow is restored.
Overall, MPI has strong diagnostic accuracy. PET-based perfusion imaging detects significant coronary artery disease with roughly 89% sensitivity and 84% specificity per patient. The test performs somewhat differently based on sex: women tend to have higher specificity (94%) and men higher sensitivity (90%), likely reflecting differences in heart size and disease patterns.
Radiation Exposure
Because MPI uses radioactive tracers, it does involve radiation exposure. For standard SPECT protocols, a rest-stress study delivers about 9 to 11 millisieverts (mSv), roughly equivalent to 3 to 4 years of natural background radiation. A stress-only SPECT protocol cuts that to about 7 to 8 mSv. PET protocols using certain tracers can go much lower, around 2 to 3 mSv in some cases.
For context, a CT scan of the chest delivers about 7 mSv, so MPI falls in a similar range. Nuclear cardiology organizations have pushed to keep average exposure below 9 mSv per study, and newer cameras and protocols continue to drive doses down. The diagnostic benefit of the test generally outweighs this exposure in patients with a meaningful clinical question about their heart.