Cardiovascular magnetic resonance (CMR) offers complementary information to echocardiography for the patient with mitral stenosis. The fundamentals of valve assessment are similar, but due to the limited availability, susceptibility to arrhythmias (especially atrial fibrillation), lower resolution and less validation compared to echocardiography, CMR is essentially considered in the following circumstances:
Evaluation of mitral stenosis by CMR is based on cine images, which allow for both morphological and functional assessment of the mitral valve. Long-axis views, in particular the left ventricular outflow tract or 3-chamber view, can provide an excellent overview relative to the aetiology and severity of mitral stenosis, as presented below.
Mitral stenosis is typically quantified by direct planimetry. A stack of cines aligned perpendicularly to the mitral valve orifice ensures complete coverage of the mitral valve. Thin slices (e.g. 5mm) are desirable to obtain better spatial resolution and more accurate measurements of the mitral valve orifice. Cine images may be contiguous (i.e. no gaps between them), but ideally should be overlapping each other for better coverage of the mitral valve orifice (e.g. 5mm slice thickness, 3mm slice interval). After selection of the most appropriate cine (at the tip of the valve) and still-frame image (at maximal opening) for planimetry, a line is drawn over the inner edges of the leaflets, resulting in an area that is used to determine the size of the mitral valve orifice.
Planimetry of the mitral valve. Top left, left ventricular outflow tract view. This still frame image is in early diastole at the widest separation of the valve tips. From this image, a stack of images is laid out perpendicular to the mitral valve orifice, as represented by the solid lines. Top right, mitral valve stack still-frame images in early diastole. In this case, the slice thickness and interval are 5mm (i.e. no gaps between slices), but the slice interval can be reduced for overlapping images and more accurate selection of the valve tips for planimetry. The image highlighted in yellow is defined at the tips as the most distal one where the valve contours are still seen. Bottom right, planimetry is performed by drawing a continuous line over the inner edges of the leaflets. In this case, the final value (0.9 cm2) shows good agreement with the estimated area by echo Doppler imaging (1.0 cm2).
Flow measurements are possible, following the same principles by Doppler echocardiography. Hence, peak mitral early diastolic (E) and late diastolic (A) velocities can be calculated, alongside peak and mean gradients, as well as the pressure half-time, using the formula calibrated for echocardiography (mitral valve area = 220/pressure half-time). However, flow measurements by CMR are not routinely used to assess mitral stenosis since they are time consuming and sparsely validated.
Flow assessment of mitral stenosis. Top panels, magnitude image of the 3-chamber view in early diastole with corresponding velocity map. These in-plane images allow for proper alignment with the stenotic jet at peak velocity (yellow line), which is usually immediately after the valve tips. Bottom panels, resulting magnitude image and through-plane velocity map across the stenotic jet at peak velocity. In these examples, bright signal basically corresponds to the diastolic blood flow across the mitral valve, whilst dark signal essentially corresponds to the systolic blood flow across the left ventricular outflow tract. As signal intensity of each individual pixel is directly proportional to the velocity in each particular time frame, peak velocity curves can be generated across the mitral valve and along the cardiac cycle, as displayed in the graph on the right. Peak E and A velocities can be estimated from this graph, as well as the mitral valve area by pressure half-time, as depicted by the gray double arrow.
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