Abstract Body (Do not enter title and authors here): Introduction/Background
Quantification of mitral regurgitation (MR) by CMR is performed indirectly by subtracting the aortic forward flow (measured by phase velocity mapping) from the left ventricular stroke volume (LVSV, measured by 3D EDV-ESV). However, 3D volumetric analysis of the LV can be performed by either including papillary muscles in the blood pool “Contour Method” or as part of myocardium “Blood Volume Method”. While both methods are considered acceptable to the Society for Cardiovascular Magnetic Resonance 2020 Guideline, using either method has a significant impact on the calculated LVSV, which in turn affects the calculated mitral regurgitant fraction (MRfr). Moreover, no universal cutoff for MR grading by CMR has been established yet.

Research Questions/Hypothesis
We sought to validate which method (Blood Volume or Contour) should be used to quantify MR by comparing the calculated MR grade by CMR (assessed by MRfr) to MR grade by echocardiography (none, trace, mild, moderate, or severe).

Methods/Approach
This is a single-center retrospective analysis. CMR studies were analyzed using Medis Suite (Version 4.0.50.2) Software. Each patient was analyzed twice; initially using Contour Method then Blood Volume Method.

Results/Data
A total of 100 patients with various grades of MR were included. 83% of patients had a transthoracic echocardiogram (TTE) while 17% had a transesophageal echocardiogram (TEE). 94% of patients had their echocardiogram performed within 6 months of CMR, and 80% performed within 3 months of CMR.

We found that the Contour Method tends to over-estimate the severity of MR, especially in patients with small ventricles (Figure 1) and in patients with milder grades of MR (less than moderate). This is because Contour Method yields a higher SV than Blood Volume Method. When compared to echocardiographic grades of MR, Blood Volume Method has a much better performance in quantifying MR severity (Kappa 0.89) than Contour Method (Kappa 0.44, Figure 2). Based on Blood Volume method, we suggest cutoffs of MR grading by CMR as listed in Table 1.

Conclusions
In this study, we present evidence that papillary muscles and trabeculations should be excluded from the blood pool in routine volumetric analysis for accurate assessment of LV stroke volume. This practice is not firmly established by current guidelines. We also present evidence-based reference ranges of mitral regurgitation grades according to the mitral regurgitant fraction calculated by CMR.