Editorial for “4D Flow Cardiac MR in Primary Mitral Regurgitation”

Mitral valve regurgitation is one of the most prevalent valvular diseases of the developed world.1 Moderate to severe mitral regurgitation is associated with a high risk of mortality and morbidity, largely due to adverse cardiac remodeling leading to pulmonary hypertension and heart failure. Thus, patients must be stratified appropriately using both imaging and clinical assessment to be considered for medical or operative treatment.1 Current American Heart Association guidelines recommend transthoracic echocardiography (TTE) as first line imaging for diagnostic testing and follow up in patients with mitral regurgitation.2 Assessment of mitral regurgitation severity with TTE relies on evaluation of the valvular anatomy, as well as hemodynamic parameters such as regurgitant fraction or regurgitant volume. Patients are thereby categorized into different disease severity levels based on imaging parameters and clinical symptoms. Mitral valve intervention is typically recommended in patients with severe primary mitral regurgitation with reasonably low perioperative mortality risk.2, 3 Traditional surgical interventions involve placement of the patient on cardiopulmonary bypass with subsequent repair or replacement of the mitral valve. When feasible, repair has shown decreased operative mortality and improved survival compared with replacement.4 However, TTE is well known to be limited by operator dependence and body habitus in the assessment of complex regurgitant jets.5 Cardiac magnetic resonance imaging (cardiac MRI) using functional/anatomical imaging and 2D phase-contrast protocols, has gained increasing traction in recent years for the evaluation of mitral regurgitation, and has demonstrated possible superiority in identifying patients with mitral regurgitation who may benefit from surgical intervention. While cardiac MRI-derived mitral regurgitation severity has shown only moderate correlation with TTE-derived estimates, MRI has been shown to be a better predictor of adverse outcomes in patients with severe mitral regurgitation, and demonstrated strong correlation between postsurgical left ventricular (LV) remodeling and mitral regurgitation severity as assessed by cardiac MRI, with no correlation when assessed by TTE.6, 7 More recently, 4D flow cardiac MRI methods, which enable robust, retrospective analysis of 3D hemodynamics, have further demonstrated excellent reproducibility, enabling both indirect and direct methods of quantifying regurgitation.8, 9 In this issue of JMRI, Gorecka et al construct a single-center prospective, observational cohort study to compare the utility of TTE, 2D phase-contrast MRI, and 4D flow in evaluating post-treatment remodeling.10 They evaluate these modalities in patients with at least moderate primary mitral regurgitation who underwent either “watchful waiting” and guideline directed medical therapy, mitral valve surgery, or mitral valve repair. The authors use LV end-diastolic volume index, derived from the LV cine short-axis images, as a marker of remodeling. They find that the post-surgical change in LV end-diastolic volume index is negatively correlated with the 4D flow- and 2D phase contrast-derived change in presurgical regurgitant volume, while there is no correlation with the TTE-derived values. They also find that the reduction in LV end-diastolic volume index was not significantly different between mitral valve replacement and repair groups. These results are overall aligned with previous studies and support a possible prognostic superiority of cardiac MRI over TTE. While there is not yet robust data in the literature demonstrating prognostic superiority of 4D flow over 2D phase-contrast in the setting of mitral regurgitation, this study does show a modest advantage over 2D phase-contrast in predicting post-surgical change in LV end-diastolic volume index of greater than 32 mL/m2, although it is unclear if this advantage outweighs the notably longer scan times of 4D flow imaging. An additional potential benefit of 4D flow is the ability to directly measure regurgitant jets without needing to scan in multiple oblique planes. Direct assessments of mitral regurgitation by 4D flow have demonstrated accurate and consistent quantification, especially in cases of severe regurgitation.8, 9 Despite this, the authors calculate regurgitant volume using only the indirect method. Recent clinical guidelines also include transcatheter edge-to-edge repair in patients with mitral regurgitation and significant concurrent comorbidities, which will likely need to be addressed in future studies.2 While treatment guidelines using 2D-phase-contrast or 4D flow MRI remain to be established, this study is a step in the right direction.