A myocardial perfusion scan uses a small amount of a radioactive chemical to create images. These images show how well blood flows to the muscle of the heart.
Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
What is a myocardial perfusion scan?
A myocardial perfusion scan uses a small amount of a radioactive chemical to see how well blood flows to the muscles of the heart (the myocardium). Some doctors call this a 'thallium' or 'MIBI' scan. Often this scan is performed after gentle exercise to see how the heart muscle responds under stress.
What is the myocardium?
The heart is mainly made of special muscle called the myocardium. The muscle pumps blood into arteries (blood vessels) which take the blood to every part of the body.
Like any other muscle, the myocardium needs a good blood supply. When the blood supply to the heart is reduced it may 'complain' with pain; this pain is called angina. The heart (coronary) arteries supply the heart with blood. The usual cause of angina is narrowing of one or more of your coronary arteries. The blood supply may be enough when you are resting. However, your heart muscle needs more blood and oxygen when it works harder. For example, when you walk fast or climb stairs, your heart rate increases to deliver the extra blood. If the extra blood that your heart needs during exertion cannot get past the narrowed arteries, the heart 'complains' with angina pain. The diagram below illustrates how angina occurs.
What is a myocardial perfusion scan used for?
Myocardial perfusion scans can be used to try to find the cause of unexplained chest pain, or chest pain brought on by exercise. This test may also be done to:
- Show blood flow patterns to the heart walls.
- See whether the heart (coronary) arteries are blocked and by how much.
- Determine the extent of injury to the heart following a heart attack (myocardial infarction).
How does a myocardial perfusion scan work?
A myocardial perfusion scan uses a special chemical called a radionuclide. A radionuclide (sometimes called a radioisotope or isotope) is a chemical which sends out (emits) a type of radioactivity called gamma rays. In a myocardial perfusion scan a tiny amount of radionuclide is put into the body, usually by an injection into a vein.
There are different types of radionuclides. Different ones tend to collect or concentrate in different organs or tissues. So, the radionuclide used depends on which part of the body is to be scanned. A radionuclide that concentrates in heart muscle is used for a myocardial perfusion scan. (Other scans that use different radionuclides include a bone scan and a thyroid scan and are dealt with elsewhere. See separate leaflets called Bone Scan and Thyroid Scans and Uptake Tests for more details.)
The radionuclide travels through the bloodstream and into the heart muscle. As the radionuclide moves through the heart muscle, areas that have good blood flow take up (absorb) the radionuclide well. Areas that do not absorb radionuclide very well may have a poor blood supply due to narrowed heart (coronary) arteries, or may have been damaged by a heart attack. So, heart muscle tissue with a good blood flow will emit more gamma rays than areas with a poor blood flow or damaged tissue.
Gamma rays are similar to X-rays and are detected by a device called a gamma camera. The gamma rays which are emitted from inside the body are detected by the gamma camera, are converted into an electrical signal, and sent to a computer. The computer builds a picture by converting the differing intensities of radioactivity emitted into different colours or shades of grey.
For example, areas of the target organ or tissue (in this case, the heart) which emit lots of gamma rays may be shown as red spots ('hot spots') on the picture on the computer monitor. Areas which emit low levels of gamma rays may be shown as blue ('cold spots'). Various other colours may be used for 'in between' levels of gamma rays emitted. This creates a picture that shows which parts of the heart muscle have good blood flow and which parts do not.
What happens during a myocardial perfusion scan?
The myocardial perfusion scans can be carried out in a number of ways. The test can be carried out while you are resting, after gentle exercise, or on two separate occasions to combine resting and exercising.
The exact order in which the test is carried out varies between different hospitals.
In an exercise test you may be given an injection of a medication that makes your heart beat faster and stronger. Some people find that this gives them a tingling feeling in the chest or a sensation that their heart is beating strongly (palpitations). These sensations usually pass quickly after the test is over. When the heart is beating at a certain rate the radionuclide chemical is injected, usually into the hand.
If you are able to, you may be asked to do some pedalling on an exercise bicycle during your test. The amount of activity you are asked to do will depend on your individual condition.
You will usually be monitored during the test with a heart trace called an electrocardiogram (ECG). This means that several sticky pads, called electrodes, will be placed on your chest. The electrodes are connected to a machine which shows how your heart responds.
Sometimes you will have a set of images taken soon after the radionuclide chemical is given. Some hospitals ask you to eat something before the pictures are taken. This can help to make the images clearer. When it is time to do the scanning, you lie on a couch while the gamma camera detects the gamma rays coming from your body. The computer turns the information into a picture. You need to lie as still as possible whilst each picture is taken (so it is not blurred). Actual scanning time for each heart scan varies from 16-30 minutes, depending on the type of scanner used.
Depending on the reason for your test you may need to have a second scan. This may take place on the same day, 24 hours later or a few days later. You will usually only have the exercise test once. So, for the second test you will just receive an injection of the radionuclide chemical and then have the images taken.
A resting test is carried out in the same way except that there is no injection of a medication to make your heart beat faster, nor any exercise.
What should I do to prepare for a myocardial perfusion scan?
Your local hospital should give you specific information to help you prepare for these tests. As these tests involve a small amount of radiation, pregnant women should not have them. Let your doctor know if you are, or think you could be, pregnant. You should also let your doctor know if you are breast-feeding.
Generally there is not much preparation needed before this test. However, you may be asked not to eat or drink anything that contains caffeine before the test. In some cases your doctor may advise you not to take your medication for a few days before the scan. You may also be asked to bring a list of medication along with you on the day of the test. Your local hospital will advise if this applies to you.
What can I expect after a myocardial perfusion scan?
Myocardial perfusion scans do not generally cause any after-effects. Through the natural process of radioactive decay, the small amount of radioactive chemical in your body will lose its radioactivity over time. It may also pass out of your body through your urine or stool (faeces) during the first few hours or days following the test. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly.
If you have contact with children or pregnant women you should let your doctor know. Although the levels of radiation used in the scan are small, your doctor may advise special precautions. Your local hospital should give you more advice on this.
Are there any side-effects or complications from a myocardial perfusion scan?
Most people have a myocardial perfusion scan without any problems. It is possible, although rare, that the exercise or the medication that makes your heart beat faster could cause an abnormal heart rhythm (arrhythmia) or heart attack (myocardial infarction). The medication that makes your heart beat faster may, occasionally, make some people 'wheezy'. The risk of this happening is higher if you have asthma or other lung conditions.
The term 'radioactivity' may sound alarming. But, the levels of radioactive chemicals used in radionuclide scans are very small and considered to be safe, and they leave the body quickly. The dose of radiation that your body receives is very small. In many cases, the level of radiation involved is not much different to a series of a few normal X-rays. However:
- As with any other types of radiation (such as X-ray), there is a small risk that the gamma rays may affect an unborn child. So, tell your doctor if you are pregnant or if you may be pregnant.
- Rarely, some people have an allergic reaction to the injected chemical.
- Theoretically, it is possible to receive an overdose when the chemical is injected. This is very rare.
Dr Rachel Hoad-Robson
Dr Gurvinder Rull
Dr Helen Huins