MUGA scans (Deborah had one today) measure the left ventricle ejection
fraction (LVEF) and are a good indicator of overall heart health.

The LVEF measures the amount of blood being pumped out of the heart
and is measured in percent - a healthy adult will have an LVEF of 50%
or greater.

A MUGA scan provides information that echocardiography cannot.  From
about.com -

http://tinyurl.com/akubd

"The advantages of the MUGA scan over other techniques (such as the
echocardiogram) for measuring the LVEF are twofold. First, the MUGA
ejection fraction is highly accurate, probably more accurate than that
obtained by any other technique. Second, The MUGA ejection fraction is
highly reproducible. That is, if the LVEF measurement is repeated
several times, nearly the same answer is always obtained. (With other
tests, variations in the measured LVEF are much greater.) These
advantages - along with its noninvasive nature - make the MUGA scan
ideal for detecting subtle changes in a patient's cardiac function
over time.

A common clinical 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. By
measuring the MUGA ejection fraction periodically during chemotherapy,
oncologists can determine, on an ongoing basis, whether it is safe to
continue with the therapy, or whether certain medications need to be
stopped. The MUGA scan is accurate and reproducible enough to detect
subtle, early changes in cardiac function that might easily be missed
by other techniques. It is a highly effective, noninvasive means of
monitoring one of the worst side effects of chemotherapy, and allowing
that therapy to be delivered more safely and effectively than would
otherwise be possible."

More stuff -

http://tinyurl.com/9ncau

"The MUGA scan measures the heart's function and the flow of blood
through it. The strongest chamber in the heart is the left ventricle,
which serves as the main pump of blood through the body. The left
ventricular is assessed by measuring the amount of blood pumped with
each heartbeat (the ejection fraction), ventricle filling, and the
blood flow into the pumping chamber. A normal ejection fraction is 50%
or more. The heart's ejection fraction is one of the most important
measures of its performance. The right ventricle's ability to pump
blood to the lungs is also assessed, and any abnormalities in the
heart wall are identified. The MUGA scan is the most accurate,
non-invasive test available to assess the heart's ventricles."

Still more stuff...

http://tinyurl.com/bmzp8

"Multiple Gated Acquisition - MUGA for short - measures how well your
heart is functioning as a pump, much like an echo does, but more
accurately. The MUGA measures how much blood your heart pumps, or
"ejects" with each contraction (EF) and how quickly that blood is
ejected. EF is actually figured with a mathematical formula : EF =
(end diastolic volume - end systolic volume) divided by (end diastolic
volume - background). The nuclear camera (gamma camera) can separate
left heart function from right heart function by using a certain view,
called left anterior oblique.

Because this test is very accurate for measuring EF, it is preferred
by some doctors. However, MUGA equipment is both more scarce and more
expensive than echo equipment. It requires an injection of a
radioactive material and is also more uncomfortable for the patient.
MUGA does not "see" valve disease as well as an echo. If you have lung
disease, this is the test of choice to measure EF because echo does
not work well for people with lung disease.
    Remember that with this test, images that are taken during
irregular beats are rejected. That's why patients with multiple odd
ventricular beats like PVCs require longer scan times to gather enough
"good data." MUGA is not suited for patients with a really irregular
heart rhythm. Echo has the same problem but not as badly."