Wall shear stress calculations based on 3D cine phase contrast MRI and computational fluid dynamics: a comparison study in healthy carotid arteries Merih Cibis a† , Wouter V. Potters b† , Frank J. H. Gijsen a , Henk Marquering b,c , Ed vanBavel c , Antonius F. W. van der Steen a,d , Aart J. Nederveen b and Jolanda J. Wentzel a * Wall shear stress (WSS) is involved in many pathophysiological processes related to cardiovascular diseases, and knowledge of WSS may provide vital information on disease progression. WSS is generally quantified with computational fluid dynamics (CFD), but can also be calculated using phase contrast MRI (PC-MRI) measurements. In this study, our objectives were to calculate WSS on the entire luminal surface of human carotid arteries using PC-MRI velocities (WSS MRI ) and to compare it with WSS based on CFD (WSS CFD ). Six healthy volunteers were scanned with a 3 T MRI scanner. WSS CFD was calculated using a generalized flow waveform with a mean flow equal to the mean measured flow. WSS MRI was calculated by estimating the velocity gradient along the inward normal of each mesh node on the luminal surface. Furthermore, WSS was calculated for a down-sampled CFD ve- locity field mimicking the MRI resolution (WSS CFDlowres ). To ensure minimum temporal variation, WSS was analyzed only at diastole. The patterns of WSS CFD and WSS MRI were compared by quantifying the overlap between low, medium and high WSS tertiles. Finally, WSS directions were compared by calculating the angles between the WSS CFD and WSS MRI vectors. WSS MRI magnitude was found to be lower than WSS CFD (0.62 ± 0.18 Pa versus 0.88 ± 0.30 Pa, p < 0.01) but closer to WSS CFDlowres (0.56 ± 0.18 Pa, p < 0.01). WSS MRI patterns matched well with those of WSS CFD. The overlap area was 68.7 ± 4.4% in low and 69.0 ± 8.9% in high WSS tertiles. The angles between WSS MRI and WSS CFD vectors were small in the high WSS tertiles (20.3 ± 8.2°), but larger in the low WSS tertiles (65.6 ± 17.4°). In conclusion, although WSS MRI magnitude was lower than WSS CFD , the spatial WSS patterns at diastole, which are more relevant to the vascular biology, were similar. PC-MRI-based WSS has potential to be used in the clinic to indicate regions of low and high WSS and the direction of WSS, especially in regions of high WSS. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: shear stress; carotid arteries; phase contrast MRI; CFD INTRODUCTION Flow induced wall shear stress (WSS) is an important biomechanical parameter, widely accepted to influence the endothelial function in the vasculature and thereby involved in many pathophysiological processes related to cardiovascular diseases (1). The majority of studies have shown that, in the presence of risk factors, atheroscle- rotic plaques mainly form in the regions of low and oscillatory WSS (2–7). Other studies have also suggested that high WSS has a pathogenic effect on the initiation of aneurysm formation, while low WSS facilitates aneurysm growth (8). These findings suggest that knowledge of WSS may provide vital information about the initiation and progression of vascular diseases. Nevertheless, WSS assessment has not been integrated into clinical practice. This is mainly due to the difficulty of determining WSS in vivo. WSS can be calculated by multiplying the blood viscosity by the wall shear rate (WSR) the lat- ter being the gradient of blood flow velocity in the normal direction of the vessel wall. The most commonly used method for determin- ing 3D blood flow velocities and WSS is computational fluid dynamics (CFD). CFD has the advantage of solving velocities at a high spatial and temporal resolution. However, the disadvantage of CFD is that it requires non-clinical expertise and extensive * Correspondence to: J. J. Wentzel, Department of Biomedical Engineering, Erasmus MC Rotterdam, The Netherlands. E-mail: j.wentzel@erasmusmc.nl a. M. Cibis F. J. H. Gijsen A. F. W. van der Steen J. J. Wentzel Department of Biomedical Engineering, Erasmus MC Rotterdam, The Netherlands b W. V. Potters H. Marquering A. J. Nederveen Department of Radiology, Academic Medical Center, Amsterdam, The Netherlands c. H. Marquering, E. vanBavel Department of Biomedical Engineering and Physics, Academic Medical Center, Amsterdam, The Netherlands d. A. F. W. van der Steen Department of Imaging Science and Technology, Delft University of Technology, Delft, The Netherlands † Contributed equally. Abbreviations used: CCA, common carotid artery; CFD, computational fluid dynamics; ECA, external carotid artery; ICA, internal carotid artery; PC-MRI, phase contrast MRI; SNR, signal-to-noise ratio; TFE, transient field echo; WSSCFD, WSS based on CFD; WSSCFDlowres, WSS based on down-sampled CFD velocity field; WSSMRI, WSS based on PC-MRI; WSR, wall shear rate. Research article Received: 2 January 2014, Revised: 28 March 2014, Accepted: 28 March 2014, Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/nbm.3126 NMR Biomed. 2014 Copyright © 2014 John Wiley & Sons, Ltd.