  The recently proposed glymphatic system suggests that bulk flow is important
for clearing waste from the brain, and as such may underlie the development of
e.g. Alzheimer's disease. The glymphatic hypothesis is still controversial and
several biomechanical modeling studies at the micro-level have at least
partially dismissed the system and its assumptions. In contrast, at the
macro-level, there are many experimental findings in support of bulk flow.
Here, we will investigate to what extent the CSF tracer distributions seen in
novel magnetic resonance imaging (MRI) investigations over hours and days are
suggestive of bulk flow or diffusion. In order to include the complex geometry
of the brain, the heterogeneous CSF flow around the brain, and the transport
over the time-scale of days, we employed the methods of partial differential
constrained optimization to identify the apparent diffusion coefficient (ADC)
that would correspond best to the MRI findings. We found that the computed ADC
in grey and white matter was respectively 23\% and 82\% larger than the ADC
estimated with DTI, which suggests that diffusion may not be the only mechanism
governing transport.
