Characterization of Vortex Flow in the Left Ventricle by Phase Contrast Magnetic Resonance Imaging

Introduction: Characterization of turbulence phenomena in fluid transport systems is useful in understanding energy conservation, dissipation, and exchange. Vortices, structures exhibiting circular or swirling motion, are central in defining flow fields and flow structure. Formation of vortices and changes in their structure are determined by structural geometry of the system as well as other global system properties. Previous work [1] suggests that the geometric structure of chambers in the heart optimizes vortex formation and vortex flow. Asymmetry in both chamber inlets and outlets and chamber geometry assist in optimizing energy conservation and dissipation during specific portions of the heart cycle. Studies using magnetic resonance imaging [2] and direct numerical calculation [1] show that vortex formation helps redirect blood flow between chambers in the heart and prevent blood flow collisions, thereby minimizing excessive energy dissipation in the heart. Abnormal asymmetry in chamber geometry due to cardiac disease or other pathophysiological conditions may adversely affect the efficiency of the heart; such asymmetry may exhibit itself as a change in vortex formation and flow that are noticed in vivo [3,4]. Therefore, the study of vortex formation and flow in the heart may be a useful tool for evaluating and diagnosing cardiac conditions. Low temporal and spatial resolutions limit the effectiveness of imaging techniques to evaluate blood flow fields in the heart. Previous studies [3, 4] have demonstrated the use of contrast echocardiography and echo particle image velocimetry to evaluate vortex formation; however, these methods require echo contrast injection and the particle tracking process reduces spatial resolution. Phase contrast magnetic resonance imaging (PC-MRI) directly measures the velocity field, eliminating the step of particle tracking and preserving spatial resolution. The goal of our study was to demonstrate feasibility of using PCMRI to quantitatively evaluate vortex formation and flow using the visualization methods developed for contrast echocardiography. Methods: Eight subjects underwent MRI on a 1.5T system (Avanto, Siemens, Malvern, PA). Of these eight subjects, five had normal LV function and three had either severe LV dysfunction or cardiomyopathy. A two-dimensional in-plane (x-velocity and y-velocity) velocity field was acquired in a three-chamber horizontal long-axis view of the left ventricle of each subject using a breath-hold segmented k-space gradient echo acquisition with parallel acceleration rate 2 (TR/TE 5.7/3.1 ms, temporal resolution 68 ms, voxel size 2.5mm x 2.5mm x 6mm). Omega Flow (Siemens Ultrasound, Mountain View, CA) was used to process velocity field data to obtain graphical visualization of LV vortex formation and flow as well as obtain parameters characterizing vortex formation (vortex depth (VD), vortex length (VL), vortex width (VW), sphericity index (SI), relative strength (RS), vortex relative strength (VRS), vortex pulsation correlation (VPC)).