MUGA

 

• Multigated acquisition (MUGA)

• Equilibrium radionuclide angiocardiography (ERNA)

• Radionuclide ventriculography (RVG)

• Procedure in which patient’s RBCs are radiolabeled and gated cardiac scintigraphy is obtained.

• Data are collected from several hundred cardiac cycles to generate an image set of the beating heart that is presented as a single, composite cardiac cycle.

  
  

  Parameters obtained from MUGA include:

  • Global ventricular systolic function

  • Regional wall motion

  • Ventricular volumes

  • Responses of above parameters to stress

  • Systolic and diastolic function indices

  • Stroke volume ratios

    Clinical settings:

    • Evaluation of cardiac function in patients undergoing chemotherapy

    • Determination of accurate LVEF in patients with intractable heart failure

    • Determination of regional LVEF

    • Identification of diastolic dysfunction in patients of heart failure with preserved LVEF

    • Demonstration of dyssynchrony prior to cardiac resynchronization

    • Known or suspected CAD

    • Assessment of ventricular function in patients with valvular heart disease.

      Patient preparation

      • REST STUDY

      • No special preparation required

      • Fasting state is preferred

      
      

      • STRESS STUDY

      • Fasting of 3-4 hours

      • Medications that may alter the heart rate response can be withheld.

      
      

      • ECG is taken before the study to confirm a regular rhythm (< 10% PVC)

      
      

    Radiopharmaceuticals

    • Adults: 15-30 mCi of autologous RBCs labeled with Tc-99m using in vivo, modified in vivo or in vitro techniques

    • Children: 0.2-0.4 mCi/kg (minimum dose of 2-4 mCi)

    
    

    • Tc-99m radiolabeled human serum albumin (HSA) is an alternative – poor image quality, longer acquisition periods and labeling is stable for a shorter time period

    Procedure

    • Patient is injected with 20 mCi of 99mTc-RBC intravenously

    • Radiolabeled RBCs achieve a state of equilibrium after 15-20 mins and patient is imaged with a LEAP parallel hole collimator

    • ECG gating is carried out using R wave as a trigger (10-15% R-R window)

    • Cardiac cycle is divided into 16-32 frames with frame duration of 50 msec or less

    • Supine imaging in 3 standard views are taken – best septal view (LAO at 35˚-45˚ or at the angle that allows best separation of the right and left ventricles), anterior and left lateral views

    • Acquire for a total of 3-7 million counts (250k counts/frame).

      • If performing stress study, images may be acquired at multiple levels of exercise (2-3 minutes at each level once a stable heart rate is attained, usually beginning after 1 minute at the new level).

      
      

  • Processing

    
    
    
    

    • Visual assessment of left ventricular systolic function should be performed before calculating LVEF

    • ROI should be drawn encompassing the left ventricle

    • Background ROI should be drawn along the lateral side of left ventricle edge, somewhat crescent shaped

    • Generate time-activity curve

    Interpretation

    • Cardiac morphology, orientation, sizes of cardiac chambers, thickness of pericardial silhouette and ventricular wall is evaluated subjectively

    • Global left ventricular function should be assessed qualitatively and compared with calculated EF

    • Normal LVEF – 50-80% at rest and 56-86% at stress

    • Impaired LV function – mild: 40-49%, moderate: 30-39% and severe: < 30%

    • Normal RVEF – 45-70%

    • Abnormalities in contraction are described in terms of mild, moderate or severe hypokinesia, akinesia or dyskinesia

    • Regional EF can be calculated (Regional values can be abnormal even when global EF is normal as seen in a dyskinetic segment of myocardium)

  • ED frame – frame with highest counts in LV ROI

  • ES frame – frame with lowest counts

  • Global EF = net ES counts/net ED counts × 100

  • Volume curve is generated by plotting net counts of each LV ROI as a function of time

  • Shows how blood volume changes through the cardiac cycle

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      Ventricle can be divided into 5-8 segments to evaluate regional EF and wall motion

    • Septal side would normally have slightly lower EF than lateral wall or apex

     

    • Peak of R wave corresponds to end diastole

    • End systole corresponds to end of T wave

      Other parametric images

      
      
      
      

      • Stroke Volume image – Obtained by subtracting end systole (ES) image from end diastole (ED) image

      • Paradox image – Obtained by subtracting ED image from ES image, this represents the atrial stroke volume

      • Phase display - consists of an image and a graph, it shows the contractile pattern of myocardium at a given point of time, helps in determination of synchronicity of the LV myocardium

      • Amplitude image – color scaled picture that shows the magnitude or force of contraction of myocardium

      • Peak filling rate (PFR) and time to peak filling rate (t-PFR) are two parameters obtained from the LV TAC, which help in diagnosing diastolic dysfunction

        Phase analysis

        
        
        
        

• Type of parametric imaging that produces a display of heart contraction
• Each pixel in the series of frames of cardiac cycle is transformed into a temporal phase wave
• Pixels in the region of heart will produce a sine wave
• Heart contraction begins in the atria and spreads through the ventricles
• For each pixel, the phase wave will differ depending on its location in the heart
• All pixels that are in the same phase of contraction are displayed with the same color
• Phase histogram shows the number of pixels that are contracting together with the same phase angle
• Phase angle is an arbitrarily chosen point on the sine wave, such as peak or ED point
• Dynamic phase analysis display is very useful for assessing conduction abnormalities and dyskinetic wall motion which are frequently associated with ventricular aneurysms
• Peak filling rate and time to peak filling rate are diastolic parameters
• Peak emptying rate and time to peak emptying rate are systolic parameters
• Rates are obtained by measuring the slope of the curve and expressed as end diastolic volume per second (EDV/s).
• Source of Errors:
• Certain medications (eg: hydralazine) and diseases like chronic renal failure will decrease RBC labeling efficiency and reduce target-to-background ratio
• EF may be inaccurately calculated by inadequate separation of LV from other cardiac structures, especially left atrium
• Poor ECG signal or one in which complexes other than QRS complex are dominant may result in spurious gating, care should be taken to ensure that QRS complex is the triggering signal
• Significant heart rate variability may compromise the determination of diastolic filling indices, so for diastolic information, a narrow acceptance window of +/- 10-15% around the average heart rate R-R interval should be chosen.(HR variations do not affect the systolic portion of the curve and therefore, do not affect calculation of EF)
• Inclusion of non-ventricular activity or exclusion of ventricular activity from ventricular ROIs may cause underestimation or overestimation of EF.

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