Simulation of Cerebral Infusion Tests Using a Poroelastic Model

In an infusion test the apparent rate of cerebrospinal fluid (CSF) production is temporarily increased through injection of fluid directly into the CSF system with the result that CSF pressure rises, in theory to a new plateau average, and the change in pressure level gives a measure of resistance to CSF outflow and the rate of approach to the plateau gives information about cerebral compliance. In the first part of this paper we give details of a two-fluid (blood and CSF) spherically symmetric poroelastic model that can simulate an infusion test which includes oscillations in blood pressure. This model has been applied to clinical data where the infusion rate and arterial blood pressure are input and an oscillatory CSF pressure is computed along with spatial parenchyma displacement, strain and local changes in CSF content. In the later part of this paper, the poroelastic model is simplified by spatial integration resulting in a one-compartment model that includes blood pressure oscillations but which, when they are ignored, reduces to a well known one-compartment model. When the arterial pressure pulsations are included, their interaction with a non-linear compliance results in solutions that have to be interpreted very carefully to predict parameter values.