# MUGA Scan and Cardiac Function Assessment

## Radionuclide Ventriculography (RVG) -- MUGA Scan

### Overview

The multi-gated acquisition (MUGA) scan, also known as radionuclide angiography or ventriculography, is an ECG-gated blood pool imaging study using Tc-99m-labeled red blood cells. It provides accurate and highly reproducible measurement of left ventricular ejection fraction (LVEF) and was historically considered the gold standard for serial LVEF monitoring. Its primary clinical application is monitoring for cardiotoxicity from chemotherapy agents, particularly anthracyclines and trastuzumab.

### Radiopharmaceutical: Tc-99m Labeled RBCs

Three methods exist for labeling red blood cells with Tc-99m. The in vitro method is preferred for MUGA scanning. Using the UltraTag RBC kit, blood is withdrawn, stannous pyrophosphate is added to reduce the technetium, and then Tc-99m pertechnetate is introduced. This method achieves a labeling efficiency exceeding 97% and produces the best image quality with the lowest free pertechnetate. The modified in vivo method involves intravenous injection of stannous pyrophosphate, followed by a 20-minute wait, then withdrawal of blood to which pertechnetate is added in vitro before reinjection, achieving a labeling efficiency of approximately 90 to 95%. The in vivo method, which is least preferred, involves injecting stannous pyrophosphate intravenously, waiting 20 minutes, then injecting pertechnetate intravenously. This achieves only 75 to 80% labeling efficiency, resulting in higher free pertechnetate concentrations and inferior image quality. The typical administered dose is 20 to 30 mCi (740 to 1110 MBq) of Tc-99m.

| Labeling Method | Technique | Labeling Efficiency | Image Quality |
|---|---|---|---|
| In vitro (UltraTag kit) | Blood withdrawn, stannous ion + Tc-99m added ex vivo | >97% | Best |
| Modified in vivo | IV stannous ion → withdraw blood → add Tc-99m ex vivo → reinject | 90–95% | Good |
| In vivo | IV stannous ion → IV Tc-99m pertechnetate | 75–80% | Poorest (free pertechnetate) |

## Acquisition Technique

### Planar MUGA

The left anterior oblique (LAO) view at approximately 45 degrees is the primary acquisition view, optimized by adjusting the angle until the interventricular septum is seen on edge, providing the best separation of the right ventricle from the left ventricle. This is the view used for LVEF calculation. The anterior view is obtained to assess the right ventricle, aorta, and pulmonary outflow. The left lateral view separates the left ventricle from the left atrium posteriorly.

### ECG Gating

The R-wave on the electrocardiogram triggers data acquisition, and the cardiac cycle is divided into 16 to 24 frames (bins), with each frame representing a time segment of the cardiac cycle, typically 30 to 50 milliseconds. Data is accumulated over hundreds of cardiac cycles and summed into each frame to achieve adequate count statistics. Beat rejection is applied to exclude irregular R-R intervals caused by arrhythmias and premature ventricular contractions (PVCs), which would otherwise degrade gating accuracy. Atrial fibrillation poses a particular challenge because the highly variable R-R intervals make gating difficult; a wider acceptance window or list-mode acquisition may be used. When frequent PVCs are present, ectopic beats are rejected, and sufficient normal beats must be acquired for statistical validity.

### Acquisition Parameters

Standard acquisition uses a 64 by 64 matrix (or 128 by 128 for higher resolution), with frame duration determined by the number of frames and heart rate. Total acquisition takes 5 to 10 minutes per view to accumulate sufficient counts. A zoom of 1.5 to 2.0 times is applied to center the heart in the field of view.

## LVEF Calculation

### Count-Based Method

LVEF is calculated from the change in counts within the left ventricular ROI between end-diastole and end-systole. The formula is LVEF = (EDC - ESC) / EDC, where EDC represents end-diastolic counts and ESC represents end-systolic counts. Background subtraction is applied using an ROI placed adjacent to the left ventricle. This count-based approach does not require geometric assumptions about left ventricular shape, which is a fundamental advantage over echocardiography. The method is highly reproducible, with a coefficient of variation of approximately 3 to 5%. Normal LVEF is 50% or greater, though the exact threshold varies slightly by laboratory.

### Advantages of Count-Based LVEF

The count-based method is independent of geometric assumptions, making it accurate even in distorted ventricles such as those seen after myocardial infarction with remodeling or ventricular aneurysms. It is highly reproducible for serial monitoring, less operator-dependent than echocardiography, and provides consistent measurements across studies.

### Sources of Error

Poor labeling efficiency results in free pertechnetate accumulating in the stomach, thyroid, and bladder, degrading cardiac image quality. Atrial overlap with the left ventricle may overestimate background counts, a problem that can be identified using the lateral view to confirm adequate separation. Arrhythmias compromise gating fidelity, and incorrect background ROI placement and patient motion during acquisition introduce additional measurement error.

## Wall Motion Analysis

### Visual Assessment

Cine display of the gated frames reveals ventricular wall motion patterns. Normal wall motion shows symmetric inward motion of all walls during systole. Hypokinesis refers to reduced wall motion, akinesis to absent wall motion, and dyskinesis to paradoxical outward motion during systole, which is characteristic of ventricular aneurysm. Regional wall motion abnormalities correspond to specific coronary artery territories.

### Phase and Amplitude Analysis

Fourier analysis of the time-activity curve for each pixel generates phase and amplitude images. The phase image displays the timing of maximal contraction for each pixel. In normal hearts, the phase is uniform across the ventricle, reflecting synchronous contraction. Abnormal regions show phase delay, and the phase histogram appears as a narrow peak in synchronous contraction versus a wide or bimodal distribution in dyssynchronous contraction. The amplitude image shows the magnitude of count change for each pixel, with high amplitude indicating good contractility and low amplitude indicating hypokinesis or akinesis. Phase analysis can detect ventricular dyssynchrony, which has some relevance for cardiac resynchronization therapy candidate selection, though this role remains limited.

## Diastolic Function Assessment

### Time-Activity Curve Analysis

The left ventricular time-activity curve provides information about filling and emptying patterns. Peak filling rate (PFR) measures the maximum rate of LV filling during early diastole, expressed as end-diastolic volumes per second, with normal values exceeding 2.5 EDV/s. Time to peak filling (TTPF) measures the interval from end-systole to maximum filling rate, with normal values below 180 milliseconds. Diastolic dysfunction is characterized by reduced PFR and prolonged TTPF. However, this assessment is performed less commonly than it was historically, as echocardiography is now preferred for evaluating diastolic function.

## Clinical Applications

### Chemotherapy Cardiotoxicity Monitoring

Anthracyclines (doxorubicin, epirubicin) cause dose-dependent cumulative cardiotoxicity, with the risk increasing substantially above a cumulative doxorubicin dose of 400 to 550 mg/m2. The mechanism involves free radical damage, mitochondrial dysfunction, and myocyte death, resulting in irreversible cardiomyopathy if not detected early. Trastuzumab (Herceptin) also causes cardiotoxicity, but unlike anthracyclines, the effect is usually reversible and is not dose-dependent. The risk of cardiomyopathy is higher when trastuzumab is used sequentially or concurrently with anthracyclines. The monitoring protocol involves a baseline LVEF measurement before starting therapy, serial LVEF assessments during and after treatment (typically every 2 to 3 months during anthracycline therapy), and withholding or discontinuing chemotherapy if the LVEF drops below a threshold, typically below 50% or a decrease of more than 10% from baseline.

### Role of MUGA vs. Echocardiography

MUGA offers highly reproducible LVEF measurement that is operator-independent and free of geometric assumptions. Echocardiography provides the advantages of no radiation exposure, wider availability, and additional structural and hemodynamic information including diastolic function, valvular disease, and strain imaging. Cardiac MRI is the most accurate LVEF method but is expensive, time-consuming, and has limited availability. The trend in clinical practice is toward echocardiography (particularly with 3D echo and strain imaging) increasingly replacing MUGA for cardiotoxicity monitoring. Global longitudinal strain (GLS) on echocardiography can detect subclinical cardiotoxicity before LVEF declines, and ASCO and ASE guidelines now support echocardiography as first-line.

| Modality | Reproducibility | Radiation | Geometric Assumptions | Additional Information | Availability |
|---|---|---|---|---|---|
| MUGA | Excellent (CV 3–5%) | ~7–8 mSv per study | None (count-based) | Limited (EF, wall motion) | Moderate |
| 2D Echocardiography | Good | None | Required (Simpson method) | Extensive (valves, diastolic, strain) | High |
| 3D Echocardiography | Excellent | None | Minimal | Extensive | Moderate |
| Cardiac MRI | Best | None | None | Most comprehensive | Low |

### Controversy: Declining Use of MUGA

MUGA use has declined significantly over the past decade for several reasons. Each study delivers approximately 7 to 8 mSv of radiation exposure, and the cumulative dose from serial monitoring is a concern, particularly for young women with breast cancer who represent the largest monitored population. Three-dimensional echocardiography now provides comparable reproducibility to MUGA, and GLS on echocardiography offers earlier detection of cardiotoxicity than LVEF measurement by any method. Nevertheless, some centers still prefer MUGA for its objectivity and reproducibility when echocardiographic windows are poor.

## First-Pass Radionuclide Angiography

### Technique

First-pass radionuclide angiography involves a bolus injection of Tc-99m pertechnetate or another Tc-99m agent with rapid sequential imaging (25 to 50 frames per second) during the first transit of tracer through the heart. This technique allows separate assessment of right and left ventricular function and can detect intracardiac shunts. It is performed less commonly than equilibrium MUGA.

### Shunt Detection

The time-activity curve over the lungs during first pass is analyzed for shunt detection. Normal transit produces a single peak with gradual washout. A left-to-right shunt produces a recirculation peak (early reappearance of tracer), and the Qp/Qs ratio can be calculated from the gamma variate fit of the curve.

<image>A diagram showing the planar MUGA acquisition setup in the LAO view. Illustrate the patient positioning with the camera at the left anterior oblique angle optimized to separate the LV from the RV (interventricular septum seen on edge). Show the ECG gating with R-wave triggers dividing the cardiac cycle into 16 frames. Display the resulting end-diastolic and end-systolic frames with ROIs drawn around the LV and background region, demonstrating the count-based LVEF calculation formula: LVEF = (EDC - ESC) / EDC after background subtraction.</image>

<image>A clinical example of serial MUGA scans for chemotherapy cardiotoxicity monitoring. Show three time points: baseline (LVEF 62%), mid-treatment (LVEF 55%), and late treatment (LVEF 44%). For each, display the LAO view end-diastolic and end-systolic frames with calculated LVEF. Include the time-activity curve showing progressive reduction in amplitude. Mark the clinical decision point where LVEF dropped below 50% triggering chemotherapy modification.</image>

<image>A phase and amplitude analysis display from a MUGA scan. Show a normal patient with uniform phase image (all LV pixels contracting synchronously, narrow phase histogram) and uniform amplitude image (good contractility throughout). Compare with an abnormal patient with prior anterior MI showing delayed phase in the anterior wall (bright area on phase map, shifted peak on histogram) and reduced amplitude in the same region (dark area on amplitude map). Label the clinical significance of each pattern.</image>

## Clinical Pearls

MUGA provides the most reproducible LVEF measurement for serial monitoring due to its count-based method that requires no geometric assumptions. This was its primary advantage over 2D echocardiography.

The in vitro RBC labeling method is preferred for MUGA. Poor labeling with the in vivo method causes free pertechnetate to accumulate in non-cardiac structures, degrading image quality.

The LAO view is the key acquisition angle and should be optimized until the interventricular septum is seen on edge, providing the best separation of the right and left ventricles.

MUGA is declining in use as 3D echocardiography and strain imaging provide comparable reproducibility and earlier detection of cardiotoxicity without radiation exposure.

A cumulative doxorubicin dose exceeding 400 to 550 mg/m2 significantly increases cardiotoxicity risk, and serial LVEF monitoring is mandatory during anthracycline therapy.

Each MUGA scan delivers approximately 7 to 8 mSv. For young women undergoing serial monitoring, such as breast cancer patients, the cumulative radiation dose is a legitimate concern favoring echocardiography.

Arrhythmias, especially atrial fibrillation and frequent PVCs, degrade MUGA quality by disrupting ECG gating. Adequate beat rejection and sufficient acquisition time are essential for reliable results.

## References

- Schwartz RG, et al. Congestive heart failure and left ventricular dysfunction complicating doxorubicin therapy. *Am J Med*. 1987;82(6):1109-1118.
- Plana JC, et al. Expert consensus for multimodality imaging evaluation of adult patients during and after cancer therapy. *J Am Soc Echocardiogr*. 2014;27(9):911-939.
- Corbett JR, et al. Equilibrium radionuclide angiocardiography. *J Nucl Cardiol*. 2006;13(6):e56-e79.
- Armenian SH, et al. Prevention and monitoring of cardiac dysfunction in survivors of adult cancers: AHA Scientific Statement. *Circulation*. 2017;136(11):e51-e69.