Quantitative assessment and imaging protocols:
Standard volumetric assessment of LV function is done using SSFP imaging, which gives accurate measurements LV systolic and diastolic dimensions and volumes, stroke volume, and ejection fraction; these can be interpreted both as absolute and indexed and standardized based on body surface area (4,10,30,31). Having accurate, gold-standard measurements for these values is crucial; ACC/AHA guidelines ejection fraction and LV systolic dimension into the decision-making process for determining appropriateness and timing of mitral valve surgery. Left atrial and ventricular volumes are included in ASE recommendations in assessing MR. (4,28) The intra and inter-reader variability for calculating LV size and function is low for CMR, thus making it an excellent tool for long term patient follow-up(16,17).
There are many methods to assess the severity of mitral regurgitation via CMR. Most commonly, indirect methods are used to quantify mitral regurgitant volume utilizing 3D ventricular stroke volumes or phase-contrast velocity encoded mapping. (10) Other less semi-quantitative assessment include the use of signal void, regurgitant jet size and regurgitant orifice area measures using phase velocity encoded mapping. (7,10,32,33) Here we will discuss the most common method utilizing LVSV and forward flow to determine regurgitant volume. (Table II)
Left ventricular stroke volume (LVSV) can be quantified using volumetric data obtained from SSFP imaging. This “volumetric” stroke volume includes both forward stroke volume as well as regurgitant volume; in short, any volume of blood leaving the left ventricle during systole via any route.
The forward stroke volume can be obtained using flow data using phase velocity mapping the proximal aorta (or main pulmonary artery, assuming no shunt is present).
By subtracting out the forward stroke volume (from flow data), from the volumetric stroke volume, one can accurately derive the regurgitant volume. This calculation assumes no aortic regurgitation (AR) or cardiac shunt.
If there is significant AR, the equation can be modified to incorporate the AR volume: MR volume=LVSV by volumetric data- (AR by phase velocity mapping + Net Stroke volume through aorta by phase velocity mapping).
For reference, when no valve disease or shunts are present, the LVSV by volumetric data, right ventricular stroke volume (RVSV) by volumetric data, aortic flow, and pulmonary artery flow should all measure approximately the same.
In patients with mitral regurgitation, the LVSV includes not only forward flow but also mitral regurgitant volume, making it larger. The regurgitant volume can thus be obtained by taking the difference between that volumetric stroke volume and any of the other measures of forward stroke volume: RVSV (assuming no tricuspid or pulmonic regurgitation), aortic flow or pulmonary artery flow. (4,7,10,21) (Figure 4)
Garg et al suggest a comprehensive MR protocol should assess the mitral valve anatomy and function to define the cause of the MR, LV and right ventricular (RV) volumes and function, in addition to quantifying the MR. They observe that in those patients for whom high quality TTE images are able to be obtained which are adequate for the assessment of LV and RV function, an abbreviated quantification-based protocol can be used as opposed to the comprehensive protocol. Figure 5 demonstrates the protocol they suggest for imaging of MR via CMR.
The following values of regurgitant fraction and regurgitant volume to categorize MR have been proposed based on data from prognostic studies (Table III).(14,21,22)
CMR can also assess the impact of systolic variation on mitral regurgitation estimation. Uretsky et al determined a method to assess this systolic variation by dividing systole into 3 parts: early, mid, and late. The MR jets were categorized as holosystolic, early, or late based on the portions of systole the jet was visible. The aortic flow and left ventricular stroke volume (LVSV) acquired by CMR are then plotted against time. The instantaneous regurgitant rate is calculated for each third of systole as the difference between the LVSV and the aortic flow. They concluded that there is significant variation of the mitral regurgitant rate even among patients with holosystolic MR jets and concluded that quantitative ways of assessing MR as suggested above via CMR effectively take into consideration this important confounding factor. A subsequent multicenter study determined that systolic variation is one of the factors that accounts for the discordance between echocardiographic and CMR assessment of MR, as the use of the color Doppler jet at a single point in time is the basis of many echocardiographic methods, and hence cannot factor in systolic variation.(34,35)Qualitative assessment of mitral valve anatomy and MR: When assessing mitral valve anatomy, the anterior and posterior leaflets, the annulus, the subvalvular apparatus (papillary muscles), and LV contractility including any regional wall motion abnormalities, are carefully visually assessed and their pathophysiology is commented upon. Abnormal leaflet morphology includes thickening, calcification, redundancy, perforation, vegetations, other masses and clefts. These abnormalities should be described in detail (diffuse versus focal, the size and the leaflet location). Such observations have far reaching clinical implications in areas such as mitral regurgitation. CMR with its superior spatial resolution as compared to echocardiography has developed an in-vivo contemporary model of human anatomy and defines the cypress tree like pattern of the papillary muscle adding a new dimension to our understanding of mitral valve apparatus. This understanding can help guide therapy decisions, with restoration of local heart geometry and reconstruction of appropriate tethering length potentially helping surgeons cure MR. (36)
Abnormal subvalvular morphology can involve chordal rupture, thickening, fusion, very large vegetations and masses, which should similarly be described in detail by size and location. Abnormal annular morphology comprises dilatation and/or calcification (seen as signal loss).
The long-axis stack is best for making the visual assessment of the mitral valve leaflets. Mitral annular disjunction is defined by the displacement of atrial junction and the mitral valve leaflet at end systole in long-axis cines and is considered significant if the distance is ≥1.0mm. Leaflet motion can be described using Carpentier’s classification: type I (normal leaflet motion); type II (excessive leaflet motion); and type III (restricted leaflet motion), subcategorized as type IIIa (restricted during both systole and diastole) and type IIIb (restricted only during systole). (Figure 6) The etiology should be consistent with the overarching diagnosis. (4,7) Our lab has described how tenting area, annulus and posterior leaflet length are possible determinants of MR severity. Additionally, there is single center data from our lab demonstrating that even visual assessment of cardiac regurgitant lesions is reliable, accurate and reproducible when compared to formal quantitative analysis via CMR.(37,38)