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.