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Introduction While Transcranial Doppler (TCD) ultrasonography provides real-time cerebrovascular monitoring following aneurysmal subarachnoid hemorrhage (aSAH) for severe complications like cerebral vasospasm, existing methods lack the ability to translate raw flow measurements into quantitative, patient-specific hemodynamic parameters. We developed a 1-dimensional (1D) simulation pipeline capable of modeling cerebral blood flow in the Circle of Willis (CoW) from TCD and Computed Tomography Angiography (CTA) data, establishing an open-source patient-specific computational model from which changes in hemodynamic parameters can be quantitatively measured in patients with vasospasm.Materials and Methods A 3-dimensional (3D) model of the CoW is segmented from whole brain CTA images. Blood flow velocity waveforms are extracted from TCD data for inlets and outlets of the CoW vessels and processed to refine input velocity profiles. To reconcile measured data with conservation of mass, velocity profiles and CoW geometry are inputted refined via Bayesian analysis. 1D simulations are generated using CTA derived centerline network and TCD derived flow rates at inlets using open source 1D hemodynamic solver openBF. Resistance and Capacitance values of the distal vasculature at each outlet are derived via nonlinear optimization by minimizing the error between simulation and TCD derived flow at outlets. Changes in Resistance and Capacitance between baseline and vasospasm were calculated for 5 patients.Results Preliminary results (n = 5) demonstrate the accuracy of the method and the ability to calculate hemodynamic changes in vasculature that occur during vasospasm. 1D Simulation results matched state-of-the-art 3D simulation with an RMSE of only 6.4%. Results show statistically significant increases in blood vessel Resistance (RMSE=325%, p=0.027) and Capacitance (RMSE=27.5%, p=0.045) when comparing baseline and vasospasm conditions for each patient.Conclusions A 1-dimensional simulation of the Circle of Willis can accurately model changes in hemodynamic parameters from baseline to vasospasm conditions, demonstrating the utility of patient-specific computational modeling of CoW blood flow. This approach may offer an accessible computational method of monitoring patients for the development of vasospasm.Disclosures M. Fung: None.