CMR is the future of imaging in MR: comparison of CMR with TTE and its role in prognostication and procedural planning
A factor that significantly impacts the decision of intervening upon mitral valve in the setting of MR is its impact on left ventricular deterioration and the lack of reliable and reproducible markers to assess the same. Many clinicians opt for the strategy of watchful waiting especially in those patients who have no symptoms, or in whom other pathophysiology may be contributing to symptoms in addition to mitral regurgitation. (49) However, as noted in the ACC/AHA guidelines, there is concern that “mitral regurgitation begets mitral regurgitation”. The notion is that the initial level of MR causes LV dilatation, which perpetuates a cycle of ever-increasing LV volumes and MR by causing stress on the mitral apparatus, which in turn leads to more severe MR and further LV dilation. This perpetual volume overload leads to irreversible LV dysfunction and thereby poor long term prognosis.(9) Patients with severe MR who develop an EF ≤60% or LVESD ≥40 have already developed LV systolic dysfunction.(50-53) There is data to show that for LV function and size to normalize after mitral valve repair, the left ventricular ejection fraction (LVEF) should be >64% and LVESD <37mm.(50) LVEF and LVESD have been used as surrogates for determining left ventricular decline- however there is concern that in using them alone the window of opportunity for LV recovery may have been crossed by the time the mitral valve is intervened upon.(54) Myocardial fibrosis may prove to be a helpful guide here.(55) CMR has two techniques to detect left ventricular fibrosis which include late gadolinium enhancement (LGE) and parametric T1 mapping. LGE can detect myocardial fibrosis as noted above (Figure 7a, Figure 7b, Figure 7c). T1 mapping is better at detecting diffuse fibrosis than LGE. When T1 is acquired pre- and post-contrast, the myocardial extra cellular volume can be calculated, which is a surrogate for extracellular matrix and diffuse fibrosis. (56)
Interestingly, in a subgroup of patients undergoing mitral valve surgery, MRI‐based severity grading had superior prognostic value over echocardiography in predicting the degree of postsurgical LV remodeling. Also, recent large‐scale studies found MRI‐derived regurgitant volume to be a better predictor of referral for surgery and all‐cause mortality than echocardiographic parameters. (21,22)  Myerson et al, demonstrated that quantifying MR with CMR showed a strong association with the future need for surgery over the subsequent 5 years. They studied the impact of regurgitant volumes and left ventricular size on surgery free survival. The study shows high discriminative power with cut off limits of 55ml of regurgitant volume and 40% of regurgitation fraction in predicting survival. While the study was limited by small sample size, it provided important data on the impact of regurgitant volume of MR as measured by CMR as a variable that can be used to determine need for surgery. (21) (Figure 8)
Uretsky et al, in a prospective multicenter trial demonstrated that agreement between MRI and echocardiographic estimates of MR severity was modest in the overall cohort, and there was a poorer correlation in the subset of patients sent for mitral valve surgery. There was a strong correlation between post-surgical LV remodeling and MR severity as assessed by MRI, and no correlation between post-surgical LV remodeling and MR severity as assessed by echocardiography. (14)
In the seminal paper on the role of CMR in MR, Urestky et al describe significant discordance in quantification between CMR and echocardiography using the ASE integrated method for assessment of MR. There was low to moderate concordance between the two modalities, with a r value of 36%-70%. Quantification of severe MR by either modalities had a r value of 20-66% with TTE more frequently diagnosing severe MR.(10) These limits of agreement, when placed in a fuller socio-economic context, suggest a sobering commentary on the state-of-the-art contemporary echocardiographic MR assessment.
Penicka et al in a prospective observational study demonstrated that CMR derived assessment of primary MR can better identify patients with severe MR and adverse outcomes than echo derived integrative approach. The CMR derived regurgitant volume showed the largest area under the curve to predict mortality or its combination with the development of indication for mitral valve surgery.
CMR can also play a valuable role in the assessment of cardiac reverse remodeling and the impact of that on MR post procedures such as pulmonary vein isolation for atrial fibrillation and transcatheter aortic valve replacement for aortic stenosis. For example, in those with durable maintenance of normal sinus rhythm, cardiac reverse remodeling demonstrated by 3D CMR occurs and is matched by marked improvements in MR and mitral apparatus, likely contributing to continued maintenance of sinus rhythm.(57) Meta-analysis of post TAVR patients has found that there is strong association between moderate to severe MR and 1 year mortality after TAVR.(58) A study from our lab also showed that post aortic valve replacement in 24 severe AS patients who were followed by CMR for 4 years, there was stabilization or reduction of MR in 80% of the patients which correlated to changes in LV mass and LV EDVI and LVEF post AVR which was in conjunction with improvement in clinical sequelae. (59)
Additionally, there have been recent advances in mitral percutaneous repair techniques for high surgical risk patients with use of transcatheter mitral valve systems including Mitra Clip (Abbott) and Pascal (Edwards Lifesciences) currently used mostly for primary MR. The quantification of MR post MitraClip can be challenging due to limited visualization, acoustic shadowing and multiple complex eccentric jets secondary to a double orifice mitral valve. In two small studies post-MitraClip, CMR had excellent reproducibility and lower interobserver variability in comparison with TTE. Cine-CMR is useful for the assessment of prosthetic mitral valve-associated MR, which manifests concordant quantitative and qualitative changes in size and density of inter-voxel dephasing. It provides an accurate non-invasive means of screening for TEE-evidenced severe MR (60) In one of the these studies, clinical benefit and LV remodeling had good correlation with CMR in post-procedural follow up.(61,62) This signifies an emerging role of CMR in clinical follow up of this subgroup of patients.
The use of MitraClip for functional MR is bound to expand with the recent CO-APT and MITRA-FR trials. There were key differences in outcomes of these trials related to the presence of LV remodeling and the presence of disproportionate versus proportionate MR. CMR is positioned to play a unique role for identifying the right patient at the right time for mitral valve interventions given its strengths as we described above. In a recent study by Cavalcante et al quantification of functional mitral regurgitation along with myocardial infarction size using CMR, CMR was a powerful predictor of adverse outcomes.(26) This further underscores the importance of the interplay between LV remodeling, myocardial infarct size (MIS), and volume overload in functional MR where CMR has a distinct advantage over TTE and will play an important role in determining candidacy in the growing field of transcatheter MV therapies in the future.
In summary, CMR has proven to be the more reliable imaging modality in the qualitative assessment of MR as compared to echocardiography. It also has important advantages in terms being able to better define the etiology as well as more accurately prognosticate the impact of procedural interventions on MR. With tools such as LGE enhancement as well as T1 mapping, it accurately detects scar and the impact of it not only on MR but also on underlying cardiomyopathy. As noted extensively in our review, it has repeatedly proven to accurate predict the appropriate time as well as impact of surgical intervention of MR on the left ventricle and long-term outcomes. The role of CMR in MR has important implications as the cardiovascular community embarks on more sophisticated surgical and non-surgical approaches for which accurate determination of MR is naturally, critical. CMR overcomes most of the current limitations of TTE including inter-observer variability, over-estimation of MR and inability to predict the impact of intervention of post-surgical outcomes. Hence, we conclude that CMR is the future of the imaging assessment of mitral regurgitation.