Reconstruction and classification of distal anastomoses of peripheral by-pass grafts: from Magnetic Resonance Imaging to CFD

Abnormal haemodynamic conditions are implicated in the development of anastomotic myointimal hyperplasia (MIH), which constitutes the main cause of failure of peripheral by-pass grafts. However these conditions are difficult to determine in-vivo, prompting research using ex-vivo idealised models. To relate the understanding gained in idealised geometries to anatomically correct conditions we have investigated an approach to classify in-vivo distal graft anastomoses and their inter-patient variability.

In-vivo distal anastomotic geometries were acquired by magnetic resonance (MR) angiography from 13 patients who had undergone infrageniculate autologous venous bypass surgery. On average, the images were acquired 2 weeks post-operatively. Five patients also underwent repeat examinations 2 to 7 weeks later. For each geometry, the surface of the arterial lumen is reconstructed from MR images using a zero level set of an implicit function constructed from radial basis functions that can be shown to minimise curvature. The final surface representation is a tessellation obtained by the marching cubes algorithm and is the initial support for CFD mesh generation.

The three-dimensional binary image created from the interpolated surface is processed using a skeletonisation algorithm to obtain the centreline of each branch in the geometry. This allows the definition and the computation of the set of parameters that constitute the base of a geometric classification. In this study we have chosen to measure the branching angles between straight line approximations of the centrelines obtained by averaging over a characteristic length of the anastomosis.