Introduction: Computational hemodynamic simulations of cerebral aneurysms usually rely on idealized boundary conditions of blood flow velocity and blood pressure, since patient-specific measurements are unavailable or difficult to collect. However, the lack of such direct physiological measurements may substantially alter calculations of flow rate and wall shear stress.[1] The need for patient-specific boundary conditions in computational modeling is controversial.
Methods: We performed simulations using both endovascular-derived patient-specific and typical literature-derived inflow and outflow boundary conditions. Detailed three-dimensional anatomical models of the cerebral vasculature were developed from rotational angiography data, and blood flow velocity and pressure were measured in situ by a dual-sensor Doppler guidewire at multiple peri-aneurysmal locations in ten unruptured cerebral aneurysms.[2] These measurements were used to define inflow and outflow boundary conditions for computational hemodynamic models of the aneurysms. The additional in situ measurements which were not incorporated into the simulation were then used to assess the accuracy of the simulated flow velocities and pressure drops.
Results: Simulated velocities using patient-specific boundary conditions showed good agreement with the guidewire measurements at measurement locations, with no bias in the agreement (Figure 1A). Simulated velocities using the simplified, literature-derived values showed a systematic bias and over-predicted velocity by ˜30% (Figure 1B).
Conclusions: Computational hemodynamics based on commonly-accepted reference values systemically overestimates blood flow velocity and thus wall shear stress. Modeling of the cerebral vasculature using endovascularly-measured patient-specific boundary conditions is more accurate and precise, and could improve the understanding of aneurysmal hemodynamics.
Patient Care: By improving the accuracy of hemodynamic models using Doppler guidewire measurements, we hope to better understand the hemodynamic environment of brain aneurysms and the effects of aneurysm treatment.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the role of boundary conditions in the accuracy of computational fluid dynamics models, 2) Understand how the Doppler guidewire can provide patient-specific measurements for incorporation into these models and 3) Discuss how improved accuracy of computational models could help investigators understand aneurysmal hemodyanmics.
References: 1. Venugopal P, Valentino D, Schmitt H, Villablanca JP, Viñuela F, Duckwiler G. Sensitivity of patient-specific numerical simulation of cerebal aneurysm hemodynamics to inflow boundary conditions. J Neurosurg. 2007 Jun;106(6):1051-60.
2. Levitt MR, McGah PM, Aliseda A, Mourad PD, Nerva JD, Vaidya SS, Morton RP, Ghodke BV, Kim LJ. Cerebral aneurysms treated with flow-diverting stents: Computational models with intravascular blood flow measurements. AJNR American Journal of Neuroradiology. Epub 2013 July 18.