Adenosine Cardiac Perfusion Imaging
What is Cardiac Perfusion Imaging?
Cardiac perfusion imaging is a medical test that uses radioactive substance, known as a tracer, to assess the flow of blood to the heart muscle.
Generally, cardiac perfusion imaging is done after an exercise (stress) test. For patients who are unable to exercise adequately, it may be done after injection of a drug that mimics the effect of exercise on the heart. This is known as pharmacologic stress test.
The drug most commonly used for pharmacologic stress testing is adenosine (Adenocard®). Another drug is dipyridamole (Persantine®).
(Other terms often used to describe cardiac perfusion imaging include: myocardial perfusion scan, cardiac nuclear imaging and radionuclide stress test. Based on the specific tracer that is used, it may also be called Thallium, Cardiolite or Myoview scan.
What Does it Show?
Cardiac perfusion imaging helps doctors diagnose coronary heart disease, which is caused by narrowed or blocked coronary arteries (the vessels that supply blood to the heart muscle).
During the test, adenosine or dipyridamole is injected into a vein. The drug causes the coronary arteries to dilate (widen), which increases blood flow to the heart muscle. Arteries that are diseased cannot dilate as much as healthy arteries. As a result, blood flows mostly to area supplied by healthy arteries.
Once the dilating has been given, a small amount of tracer is injected into the vein. The tracer collects in those parts of the heart muscle that have good blood flow.
The tracer gives off a small amount of radiation that is detected with a scanning camera. A computer processes the information and produces images that show how radioactivity is distributed in the heart.
If an area of the heart receives less blood than the rest of the heart (because of narrowed or blocked artery), it will pick up less radioactivity and will show up as a lighter area, called a "defect."
Additional tracer is injected while you are at rest,and another set of images is taken. By comparing the stress and rest images, doctors can identify areas of the heart muscle with reduced blood flow as well as area that are scarred from a previous heart attack.
Preparing For the Test...
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Generally, you will be instructed not to eat or drink for at least 4 hours before the test. If you have diabetes and take medication for it, you will need special instructions.
- This test may not be right for patients who have a history of Asthma, emphysema or chronic lung disease. If you have any of these, be sure to let the doctor or nurse know.
- Do not take any medications that contain theophylline (such as Theo-Dur®, Theo-24®, Theolair®, or Uniphyl®) for 48 hours before the test. These medications can affect the test. If you do not know which medications to avoid, ask your doctor or nurse.
- Do not drink coffee, tea, cocoa, colas or soft drinks containing caffeine (such as Tab or Mountain Dew) for 24 hours before the test. Avoid chocolate. Avoid medicines containing caffeine (such as Anacin or Excedrin).
- Wear comfortable clothing and shoes that are suitable for walking, as you may be asked to do low-level exercise.
- The procedure will be explained to you and you will be asked to sing a consent form. Feel free to ask any questions you may have.
What Happens During the Test?
Cardiac perfusion imaging is generally performed at a hospital, test center or clinic. It may be done on a single day or on two separate days.
Usually, two sets of images are taken. Once set is taken after the dilating drug has been given (the stress portion), another set is taken after a period of rest. Depending on the department's routine, either the stress or the rest part may be done first.
- Stress Portion
Several electrodes (small sticky patches) will be placed on your chest to obtain an ECG and monitor your heart beat. An intravenous (IV) line will be inserted into a vein in your arm.
The dilating drug (adenosine or dipyridamole) is then injected slowly through the IV into the vein. As the drug is given, you may feel flushed or experience chest pressure, headache, nausea, anxiety, dizziness or shortness of breath. Be sure to let the doctor, nurse or technologist know how you feel. If the side effects are sever or make you too uncomfortable, other drugs can sometimes be given to stop the effects.
In some cases you may be asked to perform low-level exercise after the dilating drug has been injected. This helps to improve the quality of the test and lessen side effects from the drug.
Once the dilating drug has been given, or soon after, the radioactive tracer is injected into the vein.
Next, you lie flat on a special table under a large scanning camera. During imaging, the camera moves slowly in an arc over the front of your chest, taking pictures of your heart from the different angles. Remain still while the pictures are being taken. The imaging part of the test takes about 20 to 30 minutes.
- Rest Portion
A tracer will be injected one other time, and another
set of pictures will be taken while you are at rest. These images
are then compared to the images taken during the exercise part of the
test.
How Long Does it Take?
Cardiac perfusion imaging can take from 2 to 5 hours. If you are an outpatient, you may be allowed to leave the test area between the two parts of the test. Or you may be asked to return the next day for more imaging.
Is the Test Safe?
The radiation exposure during perfusion imaging is small and the doses used are safe. However, if you are pregnant, suspect you may be or are a nursing mother, be sure to let your doctor know.
The injection of dilating drugs is also safe. A small amount of risk does exist, however, because the heart is stressed. Possible rare complications include severe shortness of breath, abnormal heart rhythms and heart attack. Trained personnel are there to handle any emergency.