HEMODYNAMICS OF HUMAN CAROTID-ARTERY BIFURCATIONS - COMPUTATIONAL STUDIES WITH MODELS RECONSTRUCTED FROM MAGNETIC-RESONANCE-IMAGING OF NORMAL SUBJECTS

Purpose: The precise role played by hemodynamics, particularly wall sh ear stress, in the development and progression of vascular disease rem ains unclear, in large part because of a lack of in vivo studies with humans. Although technical challenges remain for noninvasively imaging wall shear stresses in humans, vascular anatomy can be imaged with su fficiently high resolution to allo is reconstruction of three-dimensio nal models for computational hemodynamic studies. Ln this paper we pre sent an entirely noninvasive magnetic resonance imaging (MRI) protocol that provides carotid bifurcation geometry and flow rates from which the in vivo hemodynamics can be computed. Maps of average, oscillatory and gradients of wall shear stress are presented for two normal human subjects, and their data are compared with those computed for an idea lized carotid bifurcation model. Methods: An MRI protocol was develope d to acquire all necessary image data in scan times suitable for patie nt studies. Three-dimensional models of the carotid bifurcation lumen were reconstructed from serial black blood MR images of two normal vol unteers. Common and internal carotid artery flow rate waveforms were d etermined from MRI phase-contrast velocity imaging in the same subject s and were used to impose fully developed velocity boundary conditions for the computational model. Subject-specific time-resolved velocitie s and wall shear stresses were then computed with a finite element-bas ed Navier-Stokes equation solver Results: Models reconstructed from in vivo MRI of two subjects showed obvious differences in branch angle, bulb size and extent, and three-dimensional curvature. Maps of a varie ty of wall shear stress indices showed obvious qualitative differences in patterns between the in vivo models and between the in vivo models and the idealized model. Secondary, helical flow patterns, induced pr imarily by the asymmetric and curved in vivo geometries, were found to play a key role in determining the resulting wall shear stress patter ns. The use of in vivo flow rate waveforms was found to play a minor b ut noticeable role in some of the wall shear stress behavior observed. Conclusions: Conventional ''averaged'' carotid bifurcation models mas k interesting hemodynamic features observed in realistic models derive d from noninvasive imaging of normal human subjects. Observation of in tersubject variations in the in vivo wall shear stress patterns suppor ts the notion that more conclusive evidence regarding the role of hemo dynamics in vascular disease may be derived from such individual studi es. The techniques presented here, when combined with subject-specific MRT measurements of carotid artery plaque thickness and composition, provide the tools necessary for entirely noninvasive, prospective, in vivo human studies of hemodynamics and the relationship of hemodynamic s to vascular disease.