Compliance Effect on Amplitude and Phase of Cranio-Spinal CSF Flow Measured by MRI

Intracranial compliance (ICC) determines the ability of the intracranial space to accommodate increase in volume without a large increase in pressure. Indirect and direct approaches have been proposed for magnetic resonance imaging (MRI)-based noninvasive estimation of ICC from measured cerebral blood and cerebral spinal fluid (CSF) flow dynamics to and from the cranium during the cardiac cycle. The indirect measures are based on phase lag between the CSF flow and its driving force, either arterial inflow or net transcranial blood flow. The direct approach estimates ICC from the ratio of the volume and pressure changes during the cardiac cycle. This study compares the sensitivity of indirect and direct measures of ICC to changes in ICC. In vivo volumetric blood and CSF flows measured by MRI phase contrast from healthy volunteers and from patients with elevated ICP were used for the comparison. We utilized an RLC circuit model of the cranio-spinal system to simulate the effect of a change in ICC on the CSF flow waveform. These simulations quantify the affect of changes in intracranial compliance on phase and amplitude and further demonstrate that amplitude based measures such as the one used in the direct approach are a more sensitive and reliable estimate of ICC than the phase based indirect approach.