Recruitment of collateral pathways in the Circle of Willis during vasospasm. A small cohort patient-specific computational fluid dynamics study.*

Vasospasm – involuntary constriction of blood vessels – afflicts 50-90% of patients after subarachnoid hemorrhage in the brain. This reduces perfusion to the brain, with some patients exhibiting neurological deficits. In stroke patients, the collateral pathways in the Circle of Willis (CoW) direct flow to the site of the occlusion to try to maintain normal perfusion; however, the role of collateral pathways in vasospasm patients is an active area of research. Only 40% of the population exhibits a complete, balanced CoW, limiting the ability of collateral pathways to be recruited in stroke or vasospasm. Three different patients with distinct CoW variants are compared using the magnitudes and directions of collateral flow as well as loss metrics such as vascular resistance and dissipation. Patient-specific computational fluid dynamics simulations are created by applying transcranial Doppler ultrasound measurements of the velocity in the main CoW vessels on a segmentation of the cerebral vasculature from contrast tomography. The pre-vasospasm simulations are benchmarked against literature values of diameter, velocity, and flow rate to validate the model. Virtual angiograms are compared to clinical angiograms to confirm flow reversal. The influence of CoW morphology and patient characteristics are analyzed in vasospasm.