MUGA Scan
The MUGA scan (multigated acquisition, or equilibrium radionuclide ventriculography) measures left and right ventricular ejection fraction and wall motion from the blood pool.
The patient's red cells are labelled with technetium-99m, and the gamma camera records counts across hundreds of heartbeats, divided into 16 to 32 frames of the cardiac cycle by the electrocardiogram R wave.
Red cell labelling uses an in vivo method (stannous pyrophosphate, followed after 20 minutes by 740 to 1100 MBq of pertechnetate) or a modified in vivo or in vitro method with a kit, which gives a higher labelling efficiency.
Planar images are taken in the anterior, best septal separation (left anterior oblique with a caudal tilt) and left lateral or left posterior oblique views, each for 5 to 10 minutes. Gated blood pool SPECT is used in some centres.
The count based ejection fraction does not depend on ventricular shape. It is highly reproducible, with a variation of about 2 to 3 ejection fraction points, so it is well suited to serial comparisons.
Peak ejection and filling rates, time to peak filling, regional wall motion, and phase and amplitude images (for the timing of contraction) are also obtained. The effective dose is about 6 to 8 mSv.
Cardiotoxicity from anthracyclines and trastuzumab is the commonest use. The risk of heart failure with anthracyclines rises with the cumulative dose, reaching about 5% at 400 mg/m2 of body surface and more with chest radiation, older age, hypertension and prior heart disease.
Normal resting left ventricular ejection fraction is 50 to 70% (lower limit about 50% for most centres), and normal right ventricular ejection fraction is about 40% or more.
Indications.
- Baseline and serial left ventricular ejection fraction before and during chemotherapy with anthracyclines, trastuzumab and other cardiotoxic drugs.
- Long term surveillance in survivors of childhood and adult cancer treated with cardiotoxic drugs or chest radiation.
- Heart failure and cardiomyopathy when echocardiography is not diagnostic.
- Timing of surgery in chronic aortic and mitral regurgitation.
- Right ventricular function in congenital heart disease, pulmonary hypertension and right ventricular cardiomyopathy.
- Assessment of ventricular function after infarction, and of transplant function.
- Mechanical dyssynchrony before resynchronisation therapy (phase analysis).
- Discordant ejection fraction between echocardiography and other tests.
- Exercise study of ejection fraction response when ischaemia or reserve is the question.
- Perioperative assessment of ventricular function before high risk surgery.
Contraindications and precautions.
- Pregnancy: avoid unless essential.
- Breast feeding: interrupt for about 12 hours.
- Allergy to the tracer or kit is rare.
- Atrial fibrillation or frequent ectopic beats degrade gating. Beat rejection, or list mode acquisition, is used and the ejection fraction may be underestimated.
- Drugs that reduce the efficiency of in vivo labelling include heparin, doxorubicin, hydralazine, methyldopa, digoxin, quinidine and prazosin. The in vitro method avoids this.
- Recent iodinated contrast and recent nuclear studies increase background.
- Patients unable to lie flat for 20 to 30 minutes need a modified position or a different test.
- For exercise MUGA, the contraindications to exercise testing apply.
Preparation and technique.
No fasting is needed. Electrocardiogram electrodes are placed with a clear R wave. Labelling is checked by looking at the heart, aorta and blood pool, with minimal activity in the stomach, thyroid and bladder, which would show free pertechnetate.
Beats outside an acceptance window around the average R-R interval are rejected, so that the curve is built from regular beats. Each view is acquired until at least 5 million counts are obtained. The left anterior oblique view is chosen to separate the two ventricles with the septum on edge.
Background regions are placed carefully outside the left ventricle (lateral and inferior to the end systolic edge), and the left ventricular time activity curve is built. Ejection fraction is calculated as end diastolic minus end systolic counts (background corrected) divided by end diastolic counts.
Wall motion is reviewed on a cine display and on phase and amplitude images.
