The MUGA scan

What it is, how it is used

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The MUGA scan (MUltiple Gated Acquisition scan) is a noninvasive test used to evaluate cardiac function. The MUGA scan produces a moving image of the beating heart, and from this image several important features can be determined about the health of the left and right ventricles (the heart’s major pumping chambers).

How Is the MUGA Scan Done?

A MUGA scan is performed by attaching Technetium 99 (a radioactive substance), to red blood cells then injecting the red blood cells into the patient’s bloodstream.

The patient is then placed under a special camera (a gamma camera), which is able to detect the low-level radiation being given off by the Technetium-labelled red cells. Since the red blood cells (including those that are radio-labelled) fill the cardiac chambers, the image produced by the gamma camera is essentially an outline of those chambers.

With sophisticated software techniques, the final product is a "movie" of the heart beating.

What Can Be Learned From the MUGA Scan?

Several important features of cardiac function can be measured from the MUGA scan. If a patient has had a heart attack, or any other disease that affects the heart muscle (such as heart failure), the MUGA scan can localize the portion of the heart muscle that has sustained damage, and can assess the degree of damage. But more importantly, the MUGA scan gives an accurate and reproducible means of measuring and monitoring the “ejection fraction” of the cardiac ventricles.

The left ventricular ejection fraction (LVEF) is an excellent, and the most commonly used, measure of overall cardiac function.

The ejection fraction is simply the proportion of blood that is expelled from the ventricle with each heart beat. So, for instance, if the left ventricle ejects 60% of its blood volume with each beat, the LVEF is 0.6.

(A normal LVEF is 0.5 or greater.)

When Is the MUGA Scan More Useful Than Other Heart Tests?

There are two general advantages of the MUGA scan over other techniques (such as the echocardiogram) for assessing cardiac function.

First, the LVEF obtained with a MUGA scan is highly accurate, probably more accurate than that obtained by any other technique.

Second, the LVEF measured by a MUGA scan ejection fraction is highly reproducible. That is, if the LVEF measurement is repeated several times in succession, nearly the same answer is always obtained. (With other tests, variations in the measured LVEF are much greater.)

These two features make the MUGA scan particularly useful for detecting subtle changes in a patient's cardiac function over time.

A common situation in which repeated MUGA scans are useful is in following a patient's cardiac function during the delivery of chemotherapy for cancer. Some chemotherapeutic agents (Adriamycin being the most notable) can be quite toxic to the heart muscle.

The MUGA scan is accurate and reproducible enough to detect subtle, early changes in cardiac function that might easily be missed by other techniques. By measuring the LVEF with periodic MUGA scans, oncologists can determine whether it is safe to continue with the therapy, or whether certain medications need to be stopped.

Limitations of the MUGA Scan

Because the MUGA scan requires the use of a radioactive substance, the patient is exposed to radiation. The amount of radiation associated with a MUGA scan is about 6.2 mSv, which is roughly twice the normal background radiation a person receives in one year, and about 10 times the radiation obtained with a mammogram.

The accuracy of the LVEF obtained with a MUGA scan tends to be diminished in patients with irregular heart rhythms.

Also, the MUGA scan usually does not give much information about the function of the heart valves, or whether there is ventricular hypertrophy. The echocardiogram, in contrast, is excellent for obtaining such information.

Sources:

Naik MM, Diamond GA, Pai T, et al. Correspondence of left ventricular ejection fraction determinations from two-dimensional echocardiography, radionuclide angiography and contrast cineangiography. J Am Coll Cardiol 1995; 25:937.

Wackers FJ, Berger HJ, Johnstone DE, et al. Multiple gated cardiac blood pool imaging for left ventricular ejection fraction: validation of the technique and assessment of variability. Am J Cardiol 1979; 43:1159.

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