Bi-plane video-based determination of strains in arterial bifurcations

Abstract

Subarachnoid hemorrhage following the rupture of a cerebral aneurysm is one of the most devastating neurological catastrophes affecting adults. It has been well documented that cerebral aneurysms frequently form at arterial branching sites; more specifically, on either side of the apex of a bifurcation. For decades it has been assumed that an inherent medial deficiency, termed "medial defect," at the apex was one source for initial development. Based upon recent histological studies of the apical region in cerebral arteries, Finlay et al (1998) hypothesized that a stress concentration may exist in the region adjacent to the apex. Along the apex they found a narrow and highly-oriented collagen band, which refutes previous claims of apical weakness; on the other hand, the existence of such a structure may lead to a sharp material discontinuity in the region, promoting the formation of cerebral aneurysms. To test the validity of their claim, a device was constructed capable of orienting a bifurcation in its native configuration, applying static loads, and optically measuring marker displacements. Bovine left coronary arteries were excised, and excess adventitia was removed from the apical region. Seventy-five to one hundred black microspheres, approximately 50æm in diameter, were glued to the surface of the branching region. The marker group was spatially reconstructed using bi-plane video imaging. Subsequently, a mesh of marker triplets was created and a 2-D homogeneous approximation of Green's strain (E) was found for each domain at four pressures: 40, 60, 80, and 120mmHg (the reference pressure was 20mmHg). A color assignment, based upon the relative intensity of the first invariant of E (I[E]), was given to each domain within the mesh and plotted accordingly. Our results revealed no apparent trends with regard to high strain gradients near the apical region. Difficulties with system resolution introduced pixel selection error, leading to a high degree of uncertainty for strain calculations. For example, errors as large as one third the value of I[E] for smaller domains could be substantiated. Yet, strains were typically within one standard error in most domains and we concluded that the approximated strain field in the coronary bifurcation was nearly homogeneous to within experimental error.