Recovery of flow patterns in an abdominal aortic aneurysm phantom from phase contrast MRI

Magnetic Resonance Angiography (MRA) has become an important tool for the clinical evaluation of vascular disease. Flow measurement with Phase-Contrast (PC) MR imaging provides a powerful method for evaluation of blood velocity information inside vessels. However, image artifacts from complex flow patterns including slow flow, recirculation zone and pulsatile flow can adversely affect accuracy of results. Here, the authors introduce a new numerical formulation for improving the accuracy of PC velocity fields and corresponding streamlines, based on a physical constrain from fluid dynamics. The formulation which makes use of a stream function, automatically enforces continuity constraint of incompressible flow, and reconstructs the flow streamlines from PC images. The authors applied the algorithm to complex MRI flow velocities obtained in a flow phantom of an axisymmetric abdominal aortic aneurysm (AAA). The algorithm significantly improved streamline results especially inside the recirculation zone, where artifacts are more pronounced. A velocity reconstruction method in primitive variable form is also presented and results are compared to the stream function method. In order to validate flow characteristics derived from PC MR images, the authors used the FLUENT computational fluid dynamics (CFD) software package, to simulate flow patterns within the same geometry as their phantom. There was a good agreement between the numerical simulations and recovered PC streamline results. Processed stream function, in both streamline and primitive variable methods, were more realistic and provided more precise flow patterns than unprocessed PC data. These results may have significant implications for determining the in-vivo flow characteristics in various growth stages of AAAs from PC MRI