Imaging the Heart: An Important Part of the Stress Test
Imaging adds additional information to an exercise stress test and is an essential part of a pharmacologic stress test. Imaging also makes your stress test more accurate. The two types of imaging commonly used are nuclear and echocardiographic (ultrasound).
Nuclear Imaging: The Nuclear Stress Test
The stress can be either exercise, such as with a treadmill, or pharmacologic with one of several agents. It's called nuclear because of the radioactive material used to image your heart. This radioactive material, or isotope, is injected into your blood stream at the peak of exercise, when you have symptoms suggesting coronary disease, or after sufficient pharmacologic material is infused into your body. The radioactive isotope is absorbed by healthy heart muscle.
The two commonly used isotopes are thallium and technetium. The technetium isotope is further broken down into sestamibi (MIBI) and myoview. The test may thus be called a stress MIBI, adenosine MIBI, or dipyridamole myoview.
A nuclear camera detects the radiation from the isotope and creates an image of the heart with the help of computer software. The amount of radioactivity corresponds to the amount of blood flow to heart muscle. If your coronary arteries have significant blockage or you have damaged muscle from a prior heart attack the affected area of the heart will have less or no radioactivity while healthy areas will light up.
Many nuclear cameras can also evaluate the squeezing strength of the heart (ejection fraction). This is an important indicator of heart damage and can help determine prognosis after a heart attack.
Most nuclear stress tests require two sets of images. You'll be placed under the camera both at rest and after exercise.
A major advantage of the nuclear stress test is its ability to obtain quality images. Very obese people may have poor quality images with the ultrasound machine (see below). The nuclear camera can provide better images and important information about the heart in this setting.
Disadvantages of the nuclear stress test include the small amount of radiation injected into your body. You'll be asked to avoid close contact with small children and pregnant women for 24 hours after the test as a precaution. Also, the test takes several hours and requires an intravenous (IV) catheter.
Echocardiographic (Ultrasound) Imaging: The Stress Echo
As with the nuclear stress test, the stress can be either exercise or pharmacologic. Imaging is done at rest and after exercise or the pharmacologic stress is complete.
The stress echo does not use radiation. Only a small amount of a jelly-like substance is placed on your chest to allow for better contact with the ultrasound probe. Additionally, an IV catheter is usually not required for an exercise stress echo. It is required for a pharmacologic stress echo. Finally, the stress echo offers added benefits. It can accurately measure the heart, evaluate for abnormalities of heart valves, and measure the pressure inside the heart.
Sources:
ACC/AHA/ASNC 2003 Guidelines for the Clinical Use of Cardiac Radionuclide Imaging.
American College of Cardiology Appropriateness Criteria for a Nuclear Stress test.
ACC/AHA 2002 Guideline Update for Stress Testing.