Experimental verification of brain tissue incompressibility using digital image correlation.

For decades, incompressibility has been a major assumption in the mechanical study of brain tissue. This assumption is based on the hydrated nature of the biological tissues and the incompressibility of fluids. In this paper, an experimental validation of this assumption using digital image correlation is presented. Unconfined compression tests, relaxation tests and cyclic tests were performed on cylindrical samples of swine brains at loading rates suitable for neurosurgical applications. Digital image correlation was used to evaluate the evolution of the volume ratio throughout the tests. The preparation of the samples is described and it is demonstrated that it causes no statistically significant change of their mechanical properties. The results indicate that the brain tissue incompressibility assumption is verified.

Modelling the arterial wall by finite elements.

The mechanical behaviour of the arterial wall was determined theoretically utilizing some parameters of blood flow measured in vivo. Continuous experimental measurements of pressure and diameter were recorded in anesthetized dogs on the thoracic ascending and midabdominal aorta. The pressure was measured by using a catheter, and the diameter firstly, at the same site, by a plethysmograph with mercury gauge and secondly, by a sonomicrometer with ferroelectric ceramic transducers. The unstressed radius and thickness were measured at the end of each experiment in situ. Considering that the viscous component is not important relatively to the nonlinear component of the elasticity and utilizing several equations for Young modulus calculation (thick and thin wall circular cylindrical tube formulas and Bergel's equation) the following values were obtained for this parameter: 0.6 MPa-2 MPa in midabdominal aorta and 2 MPa-6.5 MPa in thoracic ascending aorta. The behaviour of the aorta wall was modelled considering an elastic law and using the finite element program "Lagamine" working in large deformations. The discretized equilibrium equations are non-linear and a unique axi-symmetric, iso-parametric element of 1 cm in length with 8 knots was used for this bi-dimensional problem. The theoretical estimation of radius vessel, utilizing a constant 5 MPa Young modulus and also a variable one, are in good agreement with the experimental results, showing that this finite element model can be applied to study mechanical properties of the arteries in physiological and pathological conditions.