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)