MUGA scan
Overview
A MUGA scan (Multiple Gated Acquisition Scan) is a nuclear medicine test to evaluate the function of the heart ventricles. It provides a movie-like image of the beating heart, and allows the doctor to determine the health of the heart’s major pumping chambers. The advantages of MUGA is that it is more accurate than an echocardiogram and it is non-invasive.
Purpose
MUGA is typically ordered for the following patients:
- With known or suspected coronary artery disease, to diagnose the disease and predict outcomes
- With lesions in their heart valves
- Who have recently had a heart attack, to assess damage to heart tissue and predict the likelihood of future cardiac events
- With congestive heart failure
- Who have undergone percutaneous transluminal coronary angioplasty, coronary artery bypass graft surgery, or medical therapy, to assess the efficacy of the treatment
- With low cardiac output after open-heart surgery
- Who are undergoing chemotherapy or immunotherapy (herceptin)
Procedure
At a high level, the MUGA test involves the introduction of a radioactive marker into the bloodstream of the patient. The patient is subsequently scanned to determine the circulation dynamics of the marker, and hence the blood.
The introduction of the radioactive marker can either take place in vivo or in vitro In the in vivo method, stannous (Tin) ions are injected into the patient's bloodstream. A subsequent intravenous injection of the radioactive substance, Technetium-99m-pertechnetate, labels the red blood cells in vivo. With an administered activity of about 800 MBq, the effective radiation dose is about 8 mSv. In the in vitro method, some the patient's blood is drawn and the stannous ions (in the form of stannous chloride) are injected into the drawn blood. The Technetium is subsequently added to the mixture as in the in vivo method. In both cases, the stannous chloride dilutes the Technetium and prevents it from leaking out of the red blood cells during the procedure.
The patient is placed under a gamma camera, which detects the low-level 140keV gamma radiation being given off by technetium-99m. As the gamma camera images are acquired, the patient's heart beat is used to 'gate' the acquisition. The final result is a series of images of the heart (usually sixteen), one at each stage of the cardiac cycle.
Depending on the the objectives of the test, the doctor may decide to perform either a resting or a stress MUGA. During the resting MUGA, the patient lies stationary, whereas during a stress MUGA, the patient is asked to exercise during the scan. The stress MUGA measures the heart performance during exercise and is usually performed to assess the impact of a suspected coronary artery disease. In some rare cases, a nitroglycerin MUGA may be performed, where nitroglycerin (a vasodilator) is administered prior to the scan.
The resulting images show the blood pool in the chambers of the heart and the images can be analyzed on a computer to calculate the ejection fraction of the heart together with other useful clinical parameters. This scan gives an accurate and reproducible means of measuring and monitoring the ejection fraction of the left ventricle, which is one of the most important metrics in assessing heart performance.
Results
Normal results
Normal ventricular function is characterized by an even distribution of Technetium. Elevated ejection fractions may be seen on a stress MUGA.
Abnormal results
An uneven distribution of technetium in the heart indicates that the patient has coronary artery disease, a cardiomyopathy, or blood shunting within the heart. Abnormalities in a resting MUGA usually indicate a heart attack, while those that occur during exercise usually indicate ischemia. In a stress MUGA, patients with coronary artery disease may exhibit a decrease in ejection fraction. For a patient that has had a heart attack, or is suspected of having another disease that affects the heart muscle, this scan can help pinpoint the position in the heart that has sustained damage as well as assess the degree of damage. MUGA scans are also used to evaluate heart function prior to and while receiving certain chemotherapies (e.g. Adriamycin) or immunotherapy (specifically, herceptin) that have a known effect on heart function.