Evaluation of Diastolic Dysfunction Using Cardiac Magnetic Resonance Imaging
• pulmonary vein flow – S/D ratio (ratio of the peaks of the pulmonary vein S and D waves) and A-wave amplitude and duration; and
• morphological evaluation – indexed left atrial (LA) volume and indexed LV mass.
Technique
Mitral Inflow
The CMR phase-contrast imaging technique is utilised for evaluation of mitral valve flow and velocity quantification. In phase-contrast CMR, protons moving through a magnetic field gradient produce a phase shift proportional to their velocity and direction. However, stationary protons produce no phase shift. Protons moving in the same direction as a magnetic field gradient, known as forward flow, create a positive phase shift in the MR echo signal that is proportional to their velocity. Protons flowing in the negative direction relative to the magnetic field gradient produce a proportional negative phase shift in the MR echo signal. Protons with zero velocity produce zero phase shift in the MR echo signal. The difference in phase shift between moving and stationary protons is measured. The calculated velocity is proportional to the measured phase difference between moving and stationary protons. Velocity equals the measured phase difference multiplied by the chosen velocity-encoding (VENC) factor (usually 100–200cm/second), divided by 180. The software calculates this during the flow analysis post-processing. First a cine-phase contrast ECG-gated CMR sequence is performed. The slice is carefully selected using multiplanar localisation to traverse the tips of the mitral valve leaflets and is placed perpendicular to the LV inflow (see Figure 3A). This generates short-axis cine-phase contrast images. A graphical contour of the mitral valve orifice is then drawn and automatically propagated (with manual override) to all time-frames of the cine loop in order to calculate the velocity, peak velocity and flow plots over time (see Figure 3B). Subsequently, the E-wave peak, A- wave peak, DT and E/A ratio are calculated (see Figure 4A).
Pulmonary Vein Flow
The cine-phase contrast technique described above is also employed to evaluate the pulmonary vein flow. Any pulmonary vein can be used for flow evaluation. In the experience of the authors, the right inferior pulmonary vein offers the most consistent visualisation and longest horizontal course, making it optimal for sampling. The VENC factor is usually 50–100cm/second. Again, automated contours with manual override are utilised to generate flow curves over time (see Figures 3C and 3D). The S/D ratio, A-wave amplitude and A-wave duration are
calculated (see Figure 4B).
Morphological Evaluation
Left Atrial Volume
The bi-plane area length method is the most frequently used technique for evaluation of the LA volume. LA area and length are measured in two- and four-chamber views using planimetry (see Figures 5A and 5B). The volume is given by the following formula:
LA volume = (0.85 x A1 x A2)/L
where L = LA length (the shorter of the LA lengths in two-chamber and four-chamber views), A1 = LA area in two-chamber view and A2 = LA area in four-chamber view (see Figures 5A and 5B). The resultant LA volume divided by the body surface area (BSA) yields the indexed LA volume.
EUROPEAN CARDIOLOGY
Figure 3: Cardiac Magnetic Resonance Evaluation of Mitral Valve and Pulmonary Vein Flow
A
B
C
D
A and B: The position of the slice across the mitral valve plane in all three of the left ventricular (LV) long-axis views, as shown by the yellow lines (A, top left), ensure imaging of the true short axis of the mitral orifice. On the resultant cine-phase contrast images (B, bottom left), contours are drawn across the valve orifice to generate a mitral inflow curve. Similarly, contours across a cross-section of the pulmonary vein cine-phase contrast sequence (C, top right) yield a pulmonary wave curve on cardiac magnetic resonance (CMR) imaging (D, bottom right).
Figure 4: Measurement of Mitral Valve and Pulmonary Vein Flow Parameters
E-wave peak
Systolic peak
A-wave peak
DT
A-wave amplitude
AB
Diastolic peak
A-wave duration
CD
A: Mitral inflow trace obtained from the cine-phase contrast sequence shows various inflow parameters such as E-wave peak, deceleration time and A-wave peak. B: Pulmonary vein flow curve shows the pulmonary vein systolic wave (PVs), diastolic wave (PVd) and A-wave (PVa) and calculation of A-wave amplitude and duration. C: Phase-contrast short-axis image through the mitral valve showing mitral valve contour in a case of diastolic dysfunction. D: Mitral inflow curve from the same case shows abnormal, highly elevated, restrictive pattern of the E–A ratio (>2:1).
Left Ventricular Mass
For calculation of LV mass, epicardial and endocardial contours are drawn on the cine short-axis views in end-diastole from base to apex. These are multiplied by the slice thickness to obtain LV volume, which is multiplied by the myocardial density (1.05) to obtain the LV mass. In practice, the myocardial contours are drawn manually and the LV mass is automatically calculated by the computer software (see Figures 5C and 5D). The resultant LV mass divided by the BSA yields the indexed LV mass.
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