What Is Myocardial Perfusion Imaging and How Does It Work?

Myocardial perfusion imaging is one of the most widely used tests for checking whether the heart muscle gets enough blood.¹ It is often called a nuclear stress test, because it pairs a small dose of radioactive material with a camera and a heart workout. It shows how blood flow changes between rest and stress, and because it shows how well blood actually reaches the muscle, not just what the arteries look like, doctors use it to decide on treatment.

How the Tracer and Camera Map Blood Flow

A tracer is a radioactive substance injected into a vein. Common ones are the technetium-based sestamibi and tetrofosmin, or thallium. Shortly after injection, the amount of tracer the muscle holds tracks blood flow closely over a wide range.² Well-supplied muscle glows brightly, and poorly supplied muscle looks dim. At very high flows the tracer under-reads, so the map shows relative differences better than exact amounts.³

The injection is a normal IV, and you cannot feel the tracer. It is not a dye and is not expected to cause the reactions linked to contrast dye. Its radioactivity fades within hours to days, and it leaves your body through urine, so drinking fluids afterward helps clear it.

Most tests use SPECT (single-photon emission computed tomography), a gamma camera that rotates around the chest to make 3D slices. Some centers use PET (positron emission tomography), a different scanner that gives sharper images.⁴ PET can also measure blood flow in absolute units.⁵

The test needs two sets of pictures because a narrowed artery may carry enough blood at rest. Under stress, healthy arteries widen to deliver more blood, but a narrowed artery has little room to widen, so the muscle it feeds gets relatively less tracer. Putting the stress and rest pictures side by side shows which dim areas appear only with effort and which are there all the time.

What Happens on Test Day

Many centers do rest and stress on one day, using a smaller dose for the first scan and a larger one for the second.⁶ Others split it over two days, or start with stress and skip the rest scan if the stress images are normal.⁷ You walk on a treadmill with rising speed and incline until you reach your target or need to stop. During exercise, people often reach about 9 in 10 of their predicted maximum heart rate.⁸

You check in, have an IV line placed, and get ECG stickers on your chest so the heart can be monitored throughout. After the resting injection there is usually a wait, often half an hour or more, so the tracer can reach the heart muscle. For each scan you lie still on a table, often with your arms above your head, for roughly 15 to 30 minutes. The stress tracer is injected near peak effort, or at the peak of the drug’s effect if you have a medicine test. Then comes another wait and the second scan. The whole visit often takes 3 to 4 hours.

Wear comfortable shoes and clothes you can walk in. Bring a list of all your medicines and something to read during the waits. If eating is allowed after the stress phase, a snack is useful to have with you.

Tell the staff right away if you feel chest pain, dizziness or breathlessness. They watch your ECG and blood pressure the whole time. Stopping early is allowed and safe, and they can often still use the results.

Medicine Stress Tests When You Can’t Exercise

Regadenoson, adenosine and dipyridamole widen the heart arteries and boost blood flow directly.⁹ Narrowed vessels cannot widen as much, so uneven flow shows up on the scan even without the heart working harder.¹⁰ Dobutamine works differently: it makes the heart beat harder and faster, like exercise.¹¹ Newer drugs such as regadenoson are simpler and better tolerated.¹²

Within seconds of the injection you may feel flushing, warmth, a racing heart or pressure in the chest. The most common side effects of regadenoson are shortness of breath and headache, mostly mild or moderate.¹³ These effects usually last only minutes as the drug wears off. Staff can give a reversal medicine if symptoms are troublesome. Tell them beforehand if you have asthma or severe lung disease, because some of these drugs can aggravate breathing problems and the choice of drug may change.

A drug test is common for people with arthritis, lung disease or balance problems, and for those who can’t reach a high enough heart rate. Exercise is preferred when possible because it also shows how fit you are.

What Your Results Mean

A reversible defect is a dim area on the stress images that improves or vanishes at rest.¹⁴ That muscle is alive but short of blood during effort, usually because of a narrowed artery. A fixed defect looks the same at rest and under stress.¹⁵ A severe one most likely means scar from a past heart attack, while a milder one may be weakened but still living muscle.¹⁶

Reports also list the ejection fraction, the share of blood the heart pumps out with each beat (pumping strength), along with wall movement and how large and severe each defect is.¹⁷ The heart is usually divided into 17 segments to locate problems.¹⁸ Doctors care about weakened-but-alive muscle because reopening its artery can restore function and outlook, which doesn’t happen with dead muscle.¹⁹

“Normal perfusion” means no dim areas were found, so blood appeared to reach the muscle evenly at rest and under stress. A “small, mild reversible defect” suggests a limited area of reduced flow during stress. “Large or severe ischemia” means a big or markedly underserved area of muscle, and it often leads to further tests. A pumping strength of about 55% or more is usually considered normal, though labs vary slightly.

After an abnormal result, medicines and lifestyle changes may be enough for small areas. Larger areas may lead to a catheter angiogram, an invasive look at the arteries. The choice depends on your symptoms as well as the images, so ask what size and location your defect is.

How Accurate the Test Is

Standard SPECT catches about 88% of people who have a significant blockage.²⁰ Among people without one, it correctly clears only about 61%, so false alarms are fairly common.²¹ PET clears about 81% of people without blockage, making false alarms less common.²²

Body tissue such as breasts or belly fat can block the signal and create fake dim areas. Correction software improves accuracy.²³ With correction, more scans can be read as normal without missing people at risk.²⁴ Weight alone is not a reason to skip the test.

The scan compares regions to the best-supplied part of the heart.²⁵ So if all three main arteries are equally narrowed, flow drops evenly and the scan can look falsely normal.²⁶ This is rare.²⁷ PET’s absolute flow measurement can help detect such widespread disease.²⁸

A dim area on the scan can lead to an angiogram that shows clean arteries. Some dim areas come from tissue blocking the signal rather than reduced blood flow, so it is reasonable to ask whether that could explain yours. If the answer is unclear, another type of scan may help settle it.

Radiation and Safety

The dose is more than a chest X-ray but similar to some CT scans. Newer cameras and software have cut the dose by at least half for full rest-and-stress tests.²⁹ If the stress scan is normal and the rest scan is skipped, the dose can fall by up to 89%.³⁰ PET tracers are very short-lived, which lowers exposure too.³¹

Tell staff beforehand if you are pregnant, might be pregnant, or are breastfeeding, since the plan may need to change. You may be advised to limit close contact with infants or pregnant women for a short time, following local advice. In normal contact you are not dangerous to other people after the test. Ask whether a stress-first protocol is offered, since it can reduce your dose if the first scan is normal.

The added cancer risk from one test’s dose is small, while missing a blocked artery carries bigger risks. Repeated tests add up, so ask whether another is needed.

How to Prepare

Caffeine blocks the drugs that widen arteries, so centers usually ask you to avoid coffee, tea, cola and chocolate for 12 to 24 hours. In one study, coffee within 4 hours did not change regadenoson’s effect on blood flow.³² Follow your center’s instruction anyway, since rules differ and the test may be rescheduled. Decaf still has some caffeine, and some painkillers and energy drinks contain it. If you had any by mistake, tell the staff.

Ask whether to pause beta blockers or nitrates before the test, because they can blunt the heart’s response to stress. Do not stop any medicine on your own. Fasting for a few hours is often required, so check the exact timing. If you have diabetes, ask how to adjust insulin or other medicines around fasting.

How It Compares With Other Heart Tests

Cardiac MRI stress tests catch about 89% of blockages and clear about 76% of people without them.³³ A CT coronary angiogram, which images arteries and plaque, predicted which arteries truly limited flow better than perfusion imaging in one large trial.³⁴ Perfusion imaging, though, has decades of evidence for predicting future heart risk.³⁵ Computer programs trained on scans can match or beat specialists and speed up reading, though these are still emerging.³⁶

A CT angiogram needs contrast dye and works best with a slow, steady heartbeat. Cardiac MRI can be difficult with some implants or if you are claustrophobic. Perfusion imaging suits people with known heart disease or stents, because it shows how blood flow behaves rather than only what the arteries look like. Your doctor’s choice often also depends on which equipment and expertise are available locally.