What Is Myocardial Perfusion Imaging?
Myocardial perfusion imaging, commonly called an MPI scan, is a nuclear medicine test that shows how well blood reaches the muscle of your heart. “Myocardial” refers to the heart muscle, and “perfusion” means blood flow. The scan creates a map of blood supply to different regions of the heart, revealing whether any areas are receiving less blood than they should.
An MPI scan is one of the primary tools cardiologists use to evaluate suspected coronary artery disease (CAD), the condition where the arteries supplying the heart become narrowed or blocked by plaque buildup. It is also used to assess the severity of known CAD and to guide treatment decisions.
Why Two Phases: Stress and Rest
The scan is performed in two phases because the heart behaves differently at rest versus under stress.
At rest, even a moderately narrowed artery may deliver enough blood to keep the heart muscle adequately supplied. The problem shows up when the heart is working harder and demands more blood flow. A narrowed artery that is “good enough” at rest may not be able to keep up during exertion.
This is why the MPI scan includes both a stress phase and a rest phase. By comparing images from both conditions, the nuclear medicine physician can identify areas of the heart that receive adequate blood flow at rest but become underperfused during stress, a pattern known as ischaemia.
Exercise Stress
If you are physically able, the stress phase is performed using a treadmill. You walk on the treadmill while the speed and incline are gradually increased until your heart reaches a target rate (usually 85% of your age-predicted maximum heart rate). At peak exercise, the radioactive tracer is injected, and you continue walking for another minute to ensure the tracer distributes throughout the heart muscle at maximum stress.
Pharmacological Stress
If you cannot exercise adequately due to joint problems, poor fitness, or other medical conditions, a medication is used instead to simulate the effect of exercise on your heart. Common pharmacological stress agents include adenosine, dipyridamole, and regadenoson. These drugs dilate the coronary arteries and increase blood flow, mimicking what exercise would do. The tracer is then injected during the pharmacological stress.
How the Scan Works
The tracer used in an MPI scan is typically Tc-99m sestamibi or Tc-99m tetrofosmin, both of which are taken up by heart muscle cells in proportion to blood flow. Areas receiving good blood flow take up more tracer and appear brighter on the images. Areas with reduced blood flow take up less tracer and appear dimmer.
Here is the typical sequence:
Day of the scan: Depending on the protocol used at your centre, both stress and rest phases may be done on the same day (separated by a few hours) or on two separate days.
Stress phase: After reaching target stress (either by exercise or medication), the tracer is injected. You then wait 30 to 60 minutes for the tracer to distribute and settle in the heart muscle. Images are acquired using a gamma camera or SPECT (single-photon emission computed tomography) scanner, which takes about 15 to 20 minutes.
Rest phase: A second injection of tracer is given while you are resting. After another waiting period, a second set of images is acquired. These rest images show baseline blood flow without any stress on the heart.
Total time: If both phases are done the same day, expect to spend three to four hours at the imaging centre, including waiting times. The actual time spent under the camera is relatively short.
What the Results Show
The nuclear medicine physician and cardiologist review the stress and rest images side by side, looking for specific patterns:
Normal Perfusion
All areas of the heart show uniform tracer uptake during both stress and rest. This is a reassuring result that suggests the coronary arteries are delivering adequate blood flow even under stress.
Reversible Defect (Ischaemia)
An area of the heart shows reduced tracer uptake during stress but normal uptake at rest. This means the blood supply is compromised during exertion but recovers when the heart is not working as hard. This pattern indicates ischaemia and suggests a significant narrowing in the coronary artery supplying that region. Depending on the extent and location, this may warrant further investigation with coronary angiography or changes to medication.
Fixed Defect (Scar)
An area shows reduced tracer uptake during both stress and rest. This typically indicates that the heart muscle in that region has been permanently damaged, usually from a previous heart attack. Scarred muscle cannot be revived by opening the artery, so the treatment implications are different from ischaemia.
Partially Reversible Defect
A combination of scar and ischaemia in the same region. Some muscle is viable but underperfused, while some is permanently damaged.
How It Guides Treatment Decisions
The MPI scan results directly influence what happens next in your cardiac care:
- A normal scan in a patient with chest pain symptoms is reassuring and may mean that no invasive procedures are needed
- Mild ischaemia limited to a small area may be managed with medications (such as aspirin, statins, beta-blockers) and lifestyle changes
- Moderate to severe ischaemia, particularly involving a large portion of the heart muscle, may prompt a referral for coronary angiography to evaluate whether stenting or bypass surgery is appropriate
- A fixed defect from a previous heart attack is important to document for ongoing management and risk assessment
The scan also provides information about left ventricular ejection fraction (LVEF), a measure of how well the heart is pumping, which is a key indicator of overall cardiac function.
Preparation for the Scan
Your doctor and the nuclear medicine team will give you specific preparation instructions. Common guidelines include:
- Avoid caffeine (coffee, tea, chocolate, energy drinks, some medications) for at least 24 hours before the scan if pharmacological stress is planned, as caffeine interferes with adenosine and related stress agents
- Certain heart medications (beta-blockers, calcium channel blockers, nitrates) may need to be paused before the scan to allow an accurate stress response. Your doctor will advise which medications to hold and for how long
- Wear comfortable clothing and walking shoes if a treadmill stress test is planned
- Eat a light meal a few hours before the scan, or follow the fasting instructions provided by your centre
Is the Scan Safe?
The radiation exposure from an MPI scan is within accepted medical limits and is considered safe for the diagnostic information it provides. The tracers leave the body within hours to days. The pharmacological stress agents can cause temporary symptoms such as flushing, headache, or mild chest tightness, but these resolve quickly and are closely monitored by the medical team.
If you have been experiencing chest pain, breathlessness on exertion, or if your doctor suspects coronary artery disease, an MPI scan can provide the functional information needed to guide the next steps in your cardiac care. Speak with your cardiologist about whether this test is appropriate for your situation.