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

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. 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. 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). 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.