Nuclear Myocardial Perfusion Imaging

A Myocardial Perfusion Imaging (MPI) test, also known as a Myocardial Perfusion Scan or Nuclear Stress Test, is a non-invasive heart imaging test that shows how well blood flows through to your heart muscle and how well your heart muscle pumps blood to the rest of the body.

It helps your doctor discern which area of the heart muscle is not getting enough blood flow. The scans are taken when your heart is under exercise stress and when it is at rest to detect any irregularities.

An MPI involves a tiny amount of radioactive substance called a tracer. This radioactive tracer is injected into a vein in your arm, flows through the bloodstream, and will be absorbed by healthy heart muscles.

When viewed through a gamma camera, the areas where the tracer has been absorbed fully will look different from areas that have not. Areas that do not absorb the tracer properly may be areas of the heart that are damaged or have poor blood flow. These areas are called “cold spots” or “defects” and suggest significant blockages in the coronary arteries supplying these areas.

There are 2 techniques for Myocardial Perfusion Imaging (MPI), and the main difference is the type of radioactive tracers used:

• Single Photon Emission Computed Tomography (SPECT)
• Positron Emission Tomography (PET)

Your cardiologist may recommend a myocardial perfusion imaging test if you experience chest discomfort or shortness of breath, either during normal activities or during exercise. An abnormal MPI test result may indicate the need for additional diagnostic tests, like a coronary angiogram. A normal MPI test result helps in ruling out significant issues related to your heart.

An MPI test can also help your doctor:

• Find out if there are narrowings or blockages in your coronary (heart) arteries accounting for your chest discomfort
• Find out how much heart muscle is damaged following a heart attack
• Decide whether you would benefit from angioplasty or bypass surgery
• Determine the effectiveness of the treatment you had to improve blood flow (coronary angioplasty or bypass surgery)
• How well your heart can handle physical activity

The test is very accurate. Studies report the accuracy of an MPI test to be between 83% – 94% in diagnosing coronary artery diseases (CAD).

MPI tests are generally very safe. The amount of radiation is very low and is not known to cause any side effects or cause cancer. An MPI test is not recommended if you are likely to be pregnant or breastfeeding. Please discuss your concerns with your doctor about medical radiation, the risks versus benefits of this test, and whether any alternatives are available.

Fasting is not required for the test, however, you should:

• Avoid food and drinks that contain caffeine, such as coffee, tea or chocolate, at least 12 hours prior.
• Avoid medications for high blood pressure or other heart conditions such as beta-blockers and nitrates at least 48 hours before the test.
• Avoid smoking at least 6 hours before the test.
• Refrain from heavy meals prior to the test.

It is important that you inform your doctor of the following:

• Any allergies or sensitivity to medications, contrast dye or iodine
• Current and previous medical conditions
• Medications you are taking
• Any supplements or vitamins you are taking
• If you are likely to be pregnant or are breastfeeding

Exercise Stress Myocardial Perfusion Imaging Test

An exercise stress myocardial perfusion imaging test helps assess the efficacy of blood flow to the heart muscles during exercise and gives the doctor an indication of how your heart pumps during exercise.

You will be put through two phases, the stress phase and the rest phase, either carried out within the same day or divided into two days.

Stress phase

1. Before the exercise test, basic measurements will be taken first, such as your height, weight, blood pressure and baseline ECG. An intravenous (small plastic) cannula will be inserted into the vein in your arm.
2. You will then be exercising on the treadmill machine. You will start off with slow walking, subsequently, the treadmill speed and incline will increase gradually every three minutes.
3. Once your heart rate reaches a predefined target, you will stop the treadmill exercise and a small amount of tracer is injected through the intravenous cannula into the vein in your arm.
4. You will lie still on a table for 15 minutes while the gamma camera takes pictures of your heart.

Rest phase

1. You will be asked to rest for 2-3 hours, or come back the next day for the rest scan. You will be allowed to leave the scan room and return at the appointed time.
2. You will be injected with the tracer again and another set of pictures will be taken by the gamma camera.

Vasodilator Stress Myocardial Perfusion Imaging test

Some people may not be able to perform treadmill exercise for various reasons. In such instances, your doctor will recommend replacing exercise with medications to either dilate the arteries or increase the contraction of the heart muscles.

You will be put through two phases, the stress phase and the rest phase, either carried out within the same day or divided into two days.

Stress phase

1. Before the vasodilator test, basic measurements will be taken first, such as your height, weight, blood pressure and baseline ECG. An intravenous (small plastic) cannula will be inserted into the vein in your arm.
2. You will be given a vasodilator medication infusion for 4-6 minutes through the intravenous cannula into the vein in your arm.
3. After the infusion, a small amount of tracer is injected through the intravenous cannula into the vein in your arm.
4. You will lie still on a table for 15 minutes while the gamma camera takes pictures of your heart.

Rest phase

1. You will be asked to rest for 2-3 hours, or come back the next day for the rest scan. You will be allowed to leave the scan room and return at the appointed time.
2. You will be injected with the tracer again and another set of pictures will be taken by the gamma camera.

After the procedure

• Once the procedure is complete, the intravenous cannula will be removed. The site will be checked for any signs of redness or swelling. If you notice a reaction when you return home, please inform your doctor.
• You can usually go back to your normal activities right away.
• Drink a lot of fluids for the next 2 days to flush out the small amount of tracer from your body.

Since a radioactive tracer is used during the test, you should stay away from infants and pregnant women for at least 24 hours after the test. If you are breastfeeding, you can only resume after 24 hours following the test.

You will be scheduled to return on another day to discuss the results of your test with your doctor. However, if the MPI test picks up a severe abnormality that requires prompt medical attention, your doctor may contact you earlier.

A myocardial perfusion scan is a non-invasive and accurate test to examine your blood flow to the heart and heart performance. It is one of the many cardiac tests available and is sometimes used in combination with other tests.

Your doctor will best advise you whether an MPI test is suitable for your condition and whether further tests and treatment are necessary following an MPI test.