In order to study the blood flow patterns in the left atrium, as well as the pulmonary venous inflow, Magnetic Resonance Imaging (MRI) was used to visualize the three-dimensional velocity fields in the transversal plane Left Ventricular Outflow Tract (LVOT) view and sagittal plane LVOT view in humans. A dedicated visualization and analysis software named ‘cardiac MARIAN’™ was developed on a Silicon Graphics Onyx computer. Temporal velocity profiles were measured and vector animations of the flow fields produced throughout the cardiac cycle. This provided a dynamic visualization of the complex flow fields in the left atrium.
At the beginning of systole, flow coming from the pulmonary veins is observed in the transversal and sagittal plane LVOT views. In the transversal view, blood coming from the left and right pulmonary veins flows along the atrial walls. The two streams join as they reach the closed mitral valve to form a vortex in the atrial chamber. In the sagittal view, blood flows from the right upper pulmonary vein into the atrium away from the mitral valve. This flow impinges on the atrial septum and is deflected towards the closed mitral valve. In both views, a vortex is created at peak systole in the left atrium and coincides with the measured peak Swave in the pulmonary venous flow profile. During diastole, the flow no longer forms a vortex in the left atrium, but instead flows directly from the pulmonary veins into the ventricle. During this phase, the atrium acts as a passive conduit from the pulmonary veins to the left ventricle. At the end of the E-wave, reverse flow is also present in the pulmonary veins.
The flow fields obtained from MRI provide information on the complete flow pattern in the left atrium, and are complementary to that obtained by Doppler echocardiography. MRI is unique in its ability to measure the three-dimensional velocity fields and anatomy simultaneously. Therefore, MRI can be used to provide a detailed analysis of cardiac function.