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Introduction: Right ventricular size and function on cardiac MRI provides thresholds for referral for pulmonary valve intervention in repaired Tetralogy of Fallot (RTOF). However, different contouring techniques are available to calculate right ventricular volumes and ejection fraction. It is not known whether these contouring techniques impact on threshold triggers for intervention. Methods: Right ventricular volumes on cardiac MRI for 24 consecutive subjects with RTOF were measured by two independent, experienced observers using three contouring techniques: smooth, detailed manual and detailed with semi-automated thresholding. Results: End-diastolic and end-systolic volumes were significantly different between contouring techniques: largest for smooth contours, intermediate for thresholding and smallest for manual (EDV: smooth 233 ml, SD 105 ml; thresholding 215 ml, SD 95 ml; manual 206 ml, SD 94 ml; ESV: smooth 122 ml, SD 71 ml; thresholding 113 ml, SD 67 ml; manual 103 ml, SD 64 ml; p