Impact of measurement location on direct mitral regurgitation quantification using four-dimensional flow cardiovascular magnetic resonance

Background: Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) shows promise for quantifying mitral regurgitation (MR) by allowing for direct regurgitant volume (RVol) measurement using a plane precisely placed at the MR jet. However, the ideal location of a measurement plane remains unclear. This study aims to systematically examine how varying measurement locations affect RVol quantification and determine the optimal location using the momentum conservation principle of a free jet. Methods: Patients diagnosed with MR by transthoracic echocardiography (TTE) and scheduled for CMR were prospectively recruited. Regurgitant jet flow volume (RVol_jet) and regurgitant jet flow momentum (RMom_jet) were quantified using 4D flow CMR at seven locations along the jet axis, x. The reference plane (mid-plane, x = 0 mm) was positioned at the peak velocity of the jet at each cardiac phase, and three additional planes were positioned on either side of the jet, each 2.5 mm apart. RVol_jet was compared to RVol_TTE, measured by the proximal isovelocity surface area method, and RVol_indirect, measured by subtracting aortic forward flow volume from the left ventricle stroke volume derived from two-dimensional phase contrast at the aortic valve and a stack of short-axis cine CMR techniques. Results: RVol_jet and RMom_jet were quantified in 45 patients (age 63±13, male 26). In patients with RVol_jet at x = 0 mm ≥ 10 mL (n = 25), RVol_jet consistently increased as the plane moved downstream. RVol_jet measured furthest upstream (x = −7.5 mm) was significantly lower (39±11%, p<0.001) and RVol_jet measured furthest downstream (x = 7.5 mm) was significantly higher (16±19%, p<0.001) than RVol_jet at x = 0 mm. RMom_jet similarly increased from x = −7.5 to 0 mm (57±12%, p<0.001) but stabilized from x = 0–7.5 mm (−2±17%). From x = −7.5 to 7.5 mm, RVol_jet was in consistent moderate agreement with RVol_indirect (n = 41, bias = −2±24 to 8±32 mL, intraclass correlation coefficient = 0.55–0.63, p<0.001). Conclusion: The location of a measurement plane significantly influences RVol quantification using the direct 4D flow CMR approach. Based on the converging profile of RMom_jet, we propose the peak velocity of the jet as the optimal position.