Flow in the Circle of Willis during vasospasm: A poorly understood manifold for complex intracranial hemodynamics through patient-specific simulations.

Vasospasm is an acute constriction of blood vessels following subarachnoid hemorrhage affecting 50-90% of patients. Perfusion to the brain is severely diminished, frequently leading to decreased patient responsiveness. While the role of flow paths in the Circle of Willis during embolic stroke is well-documented, changes during vasospasm remain unquantified. This study aims to quantify the differences in blood flow between pre-vasospasm and vasospasm conditions. Patient-specific computational fluid dynamics simulations are conducted by segmenting computed tomography scans and applying Womersley profile boundary conditions from transcranial Doppler ultrasound patient data. Bayesian analysis is applied to minimize uncertainty in model parameters – vessel diameters and mean velocities – and identify an optimal set that satisfies mass conservation. The final diameters and flow rates are compared to the distribution of values in the literature. Virtual angiograms using the diffusive-convective motion of a passive tracer are compared to clinical angiograms. Circle of Willis flows change significantly during vasospasm, including flow reversal in major intracranial vessels.