How is a multiple gated acquisition (MUGA) scan performed?
A MUGA scan involves the radioactive tagging of the patient’s red blood cells (RBCs) so that the blood pool remains radioactive throughout the study. Radiolabeling can be performed either in vitro or in vivo. In vitro labeling requires blood to be drawn from the patient first; the radiolabeling is performed in the laboratory, and then the blood is reinjected into the patient. Alternatively, the RBCs can be labeled in the patient by first injecting stannous pyrophosphate, which allows the RBCs to take up 99m Tc. When the patient’s RBCs have been labeled, scintigraphic images are usually acquired in the left anterior oblique and anterior projections for approximately 15 minutes while the ECG is monitored. The R-R interval is divided into 8 or 16 segments, during which image acquisition occurs. The result is that the radiotracer activity in the left ventricle can be measured throughout the cardiac cycle, and the end-diastolic (ED) and end-systolic (ES) radiotracer activity levels and regional wall motion can be assessed. The ejection fraction is calculated based on the following equation: Ejection fraction = (ED − ES)/(ED − BG). The background (BG) counts are also important because placement of the background region over an area with significant radiotracer activity (e.g., the thoracic aorta) would result in a falsely elevated calculated ejection fraction.
How is a MUGA scan used clinically?
A MUGA scan is primarily used to evaluate myocardial wall motion and to measure left ventricular ejection fraction. Both of these can now be assessed with echocardiography, which can also be used to evaluate the structure and function of the cardiac valves. For this reason, MUGA scans are used less frequently than echocardiography even though MUGA scan results are less operator dependent. The principal use is currently for the determination of ejection fraction in cancer patients before and after they undergo chemotherapy. Patients taking chemotherapeutic agents that can affect cardiac function, such as doxorubicin, require serial measures of cardiac function before and after therapy. A MUGA scan provides true quantitative information regarding ejection fraction because the actual counts of radiotracer activity within the left ventricular cavity can be assessed, compared to echocardiography, which determines ejection fraction by estimation. General guidelines suggest that if the ejection fraction declines to less than 40% or has a decrease of more than 10% to 15%, then the patient should not continue chemotherapy with the same agent.