Intracranial compliance (ICC) determines the ability of the intracranial space to accommodate increase in volume (e.g.,
brain swelling) without a large increase in intracranial pressure (ICP). Therefore, measurement of ICC is potentially
important for diagnosis and guiding treatment of related neurological problems. Modeling based approach uses an
assumed lumped-parameter model of the craniospinal system (CSS) (e.g., RCL circuit), with either the arterial or the
net transcranial blood flow (arterial inflow minus venous outflow) as input and the cranio-spinal cerebrospinal fluid
(CSF) flow as output. The phase difference between the output and input is then often used as a measure of ICC
However, it is not clear whether there is a predetermined relationship between ICC and the phase difference between
these waveforms. A different approach for estimation of ICC has been recently proposed. This approach estimates ICC
from the ratio of the intracranial volume and pressure changes that occur naturally with each heartbeat. The current study
evaluates the sensitivity of the phase-based and the direct approach to changes in ICC. An RLC circuit model of the
cranio-spinal system is used to simulate the cranio-spinal CSF flow for 3 different ICC states using the transcranial
blood flows measured by MRI phase contrast from healthy human subjects. The effect of the increase in the ICC on the
magnitude and phase response is calculated from the system's transfer function. We observed that within the heart rate
frequency range, changes in ICC predominantly affected the amplitude of CSF pulsation and less so the phases. The
compliance is then obtained for the different ICC states using the direct approach. The measures of compliance
calculated using the direct approach demonstrated the highest sensitivity for changes in ICC. This work explains why
phase shift based measure of ICC is less sensitive than amplitude based measures such as the direct approach method.
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