The MUGA scan

What it is, how it is used

heart
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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). The MUGA scan is particularly good at giving a reading of the overall pumping ability of the heart.

How Is the MUGA Scan Done?

If you are having a MUGA scan, a tiny amount of Technetium 99 (a radioactive substance) is injected into your bloodstream. The Technetium 99 attaches to your red blood cells, and circulates within your bloodstream. You will then be 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 radio-labelled red blood cells fill the cardiac chambers, the image produced by the gamma camera is essentially a movie of the beating heart. This dynamic image can be digitally analyzed to determine a lot of useful information about your heart health. 

What Can Be Learned From the MUGA Scan?

Several important features of cardiac function can be measured from the MUGA scan. If a person 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 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 very accurate, and is regarded as being more accurate than LVEF measurements obtained by other kinds of cardiac tests. 

Second, the LVEF measured by a MUGA scan ejection fraction is highly reproducible. This means that if subsequent MUGA scans measure a difference in the LVEF, that difference is likely to reflect an actual change in the condition of the heart (and is not an artifact of an inaccurate measurement). With other tests, the variability in repeated LVEF measurements tends to be greater.

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

A common example of when repeated MUGA scans are useful would be in assessing an individual’s cardiac function during a course 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

As with any diagnostic test used in medicine, there are some drawbacks to the MUGA scan.

Because the MUGA scan requires the use of a radioactive substance, the person having this scan is exposed to a small amount of 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.

Further, the accuracy of the LVEF obtained with a MUGA scan tends to be diminished in patients with irregular heart rhythms, especially atrial fibrillation.

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 information like this. So, for many people with suspected heart problems, the MUGA scan must be used in combination with other cardiac tests to obtain as full a picture as possible of the person’s overall cardiac condition.

Sources:

Hendel RC, Berman DS, Di Carli MF, et al. ACCF/ASNC/ACR/AHA/ASE/SCCT/SCMR/SNM 2009 Appropriate Use Criteria for Cardiac Radionuclide Imaging: a Report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force, the American Society of Nuclear Cardiology, the American College of Radiology, the American Heart Association, the American Society of Echocardiography, the Society of Cardiovascular Computed Tomography, the Society for Cardiovascular Magnetic Resonance, and the Society of Nuclear Medicine. Circulation 2009; 119:e561.

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