Contouring techniques in cardiac magnetic resonance assessment of right ventricular volumes in repaired tetralogy of fallot

Abstract

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 < 0.001 for all comparisons). Stroke volume from contours by thresholding correlated most strongly with velocity mapping (r = 0.87, p < 0.001). Interobserver agreement was strongest for smooth (end-diastolic volume ICC = 1.0 (confidence interval (CI):0.999-1.0, p < 0.001); end-systolic volume ICC = 0.999 (CI:0.994-1.0, p < 0.001) and weakest for manual contours ((end-diastolic volume ICC = 0.89 (CI:0.33-0.99, p = 0.01); end-diastolic volume ICC = 0.88 (CI:0.30-0.99, p = 0.01). Intra-observer agreement was uniformly high (ICC≥0.996, p < 0.001 for all). In 4/24 cases, contouring technique altered threshold triggers for pulmonary valve intervention.

Conclusions: Differences in contouring technique for measuring right ventricular volumes in patients with RTOF can affect thresholds triggering referral for pulmonary valve intervention. Standardisation of right ventricular measurement is needed in congenital cardiac MRI practice.

Keywords: Cardiac magnetic resonance; Computer assisted; Congenital heart disease; Image processing; Pulmonary valve; Right ventricle.

Fig.1

End-diastolic frame of short-axis cine (steady state free precession) at mid-right ventricle level demonstrating a) smooth, b) semi-automated detailed thresholding, and c) detailed manual contouring methods. In a), trabeculae were counted as part of the blood pool, whilst for b) and c), trabeculae were counted as part of the myocardium.