A New Method for Hemodynamic Flow Assessment in Graft Anastomoses using MRI Imaging

Richard F. Neville, MD, Christopher J. Elkins, PhD, Marcus T. Alley, MD, Angela M. Yamauchi, PhD and Ryan B. Wicker, PhD  
Georgetown University, Washington, DC, Stanford University, Stanford, CA and  University of Texas El Paso, El Paso, TX

Objectives: Intimal hyperplasia (IH) remains a major cause of prosthetic graft failure in lower extremity bypass procedures. Hemodynamic forces at the distal anastomosis are thought to play an important role in IH formation, but the relationship between these flow forces and IH is not completely understood. We report upon a MRI technique (4D-MRV) capable of measuring pulsatile 3D flow across graft anastomoses revealing detailed quantitative flow measurements.
Methods: 4D-MRV is a non-invasive method for assessing flow while varying anatomic configurations in a user-defined 3D volume model. In this preliminary study, anastomotic models were created by suturing commercially available grafts (PTFE) to 6 mm ID silicone tubing (Tygon 3350). The silicone tubing was clamped between the proximal and distal anastomoses to simulate occlusion of a native artery. Pulsatile flow was used in a pattern consistent with a normal cardiac cycle. Flow rates (mean 0.16 L/min, peak ~0.8 L/min, and negative peak ~-0.2 L/min) and pressures were physiologic. Blood was simulated with a solution of 40% glycerol and a trace amount of gadolinium in distilled water. Measurements were made using sub-millimeter resolution in a volume surrounding the distal anastomosis of each model.
Results: Resultant 4D-MRV data produced streamlines and isosurfaces useful for visualizing the time-varying 3D geometry of each anastomosis while under 3D pulsatile flow. Images were produced that compared flow streamlines through anastomotic areas. Hemodynamic patterns were identified with regions of high and low shear stress evident in the streamline flow visualization. The 4D-MRV data was also analyzed to quantify shear rates and flow distribution across the length of each anastomosis. The flow differences in small diameter anastomotic geometries were effectively identified with marked differences demonstrated between standard and cuffed end-to-side anastomotic configurations.
Conclusions: The 4D-MRV technique produces 3D hemodynamic data with sufficient resolution to quantify geometry and flow velocity in graft anastomoses. This technique can be used to determine the effects of anastomotic geometry on true hemodynamic flow patterns. Based on this preliminary data, further studies are planned to evaluate postoperative bypass patients and investigate the relationship between IH and in vivo flow hemodynamics.