Fadi Glor

Reproducibility study of magnetic resonance image-based computational fluid dynamics prediction of carotid bifurcation flow

AbstractThe importance of shear stress in the initiation and progression of atherosclerosis has been recognized for some time. A novel way to quantify wall shear stress under physiologically realistic conditions is to combine magnetic resonance imaging (MRI) and computational fluid dynamics. The present study aims to investigate the reproducibility of the simulated flow by using this combined approach. The right carotid bifurcations of eight healthy subjects were scanned twice with MRI within a few weeks. Three-dimensional geometries of the vessels were reconstructed for each scan and each subject. Pulsatile flows through these models were calculated to assess errors associated with the predicted flow parameters. This was done by comparing various wall shear stress indices, including the time-averaged wall shear stress (WSS), oscillating shear index (OSI), WSS Gradients (WSSG) and WSS Angle Deviation (WSSAD). Qualitatively, all the wall shear parameters proved to be highly reproducible. Quantitatively, the reproducibility was over 90% for OSI and WSSAD, but less impressive (60%) for other parameters. Our results indicated that WSS and WSSG values were extremely sensitive to subtle variations in local geometry and mesh design, particularly in regions around the bifurcation apex where WSS values were high and least reproducible.

Image-based carotid flow reconstruction: a comparison between MRI and ultrasound.

Atherosclerosis is a major cause of morbidity and mortality. Its apparent link with wall shear stress (WSS) has led to considerable interest in the in vivo estimation of WSS. Determining WSS by combining medical images with computational fluid dynamics (CFD) simulations can be performed both with magnetic resonance imaging (MRI) and three-dimensional ultrasound (3DUS). This study compares predicted 3D flow patterns based on black blood MRI and 3DUS. Velocity fields in the carotid arteries of nine subjects have been reconstructed, and the haemodynamic wall parameters WSS, oscillatory shear index (OSI), WSS gradients (WSSG) and angle gradients (WSSAG) were compared between the two imaging techniques. There was a good qualitative agreement between results derived from MRI and 3DUS (average correlation strength above 0.60). The root mean square error between haemodynamic wall parameters was comparable to the range of the expected variability of each imaging technique (WSS: 0.411 N m(-2); OSI: 0.048; temporal WSSG: 150 N s(-1) m(-2); spatial WSSG: 2.29 N m(-3); WSSAG: 87.6 rad m(-1)). In conclusion, MRI and 3DUS are capable of providing haemodynamic parameters when combined with CFD, and the predictions are in most cases qualitatively and quantitatively similar. The relatively high cost of MRI and continuing improvement in ultrasound favour US to MRI for future haemodynamic studies of superficial arteries.

Carotid geometry reconstruction: a comparison between MRI and ultrasound

Image-based Computational Fluid Dynamics (CFD) has become a popular tool for the prediction of in vivo flow profiles and hemodynamic wall parameters. Currently, Magnetic Resonance Imaging (MRI) is most widely used for in vivo geometry acquisition. For superficial arteries such as the carotids and the femoral artery, three-dimensional (3-D) extravascular ultrasound (3-DUS) could be a cost-effective alternative to MRI. In this study, nine healthy subjects were scanned both with MRI and 3-DUS. The reconstructed carotid artery geometries for each subject were compared by evaluating cross-sectional areas, centerlines, and carotid nonplanarity. Lumen areas agreed very well between the two different acquisition techniques, whereas centerlines and nonplanarity parameters showed measurable disagreement, possibly due to the different neck and head positions adopted for 3-DUS versus MRI. With the current level of agreement achieved, 3-DUS has the potential to become an inexpensive and fast alternative to MRI for image-based CFD modeling of superficial arteries.

Operator dependence of 3-D ultrasound-based computational fluid dynamics for the carotid bifurcation

The association between vascular wall shear stress (WSS) and the local development of atherosclerotic plaque makes estimation of in vivo WSS of considerable interest. Three-dimensional ultrasound (3DUS) combined with computational fluid dynamics (CFD) provides a potentially valuable tool for acquiring subject-specific WSS, but the interoperator and intraoperator variability associated with WSS calculations using this method is not known. Here, the accuracy, reproducibility and operator dependence of 3DUS-based computational fluid dynamics were examined through a phantom and in vivo studies. A carotid phantom was scanned and reconstructed by two operators. In the in vivo study, four operators scanned a healthy subject a total of 11 times, and their scan data were processed by three individuals. The study showed that with some basic training, operators could acquire accurate carotid geometry for flow reconstructions. The variability of measured cross-sectional area and predicted shear stress was 8.17% and 0.193 N/m/sup 2/ respectively for the in vivo study. It was shown that the variability of the examined parameters was more dependent on the scan operators than the image processing operator. The range of variability of geometrical and flow parameters reported here can be used as a reference for future in vivo studies using the 3DUS-based CFD approach.

Comparison of the effects of antihypertensive treatment with angiotensin II blockade and beta-blockade on carotid wall structure and haemodynamics: protocol and baseline demographics

Several systemic factors have been shown to contribute to the acceleration of large vessel atheroma. Correction of these factors leads to a reduction in the progression of plaque formation and associated arterial wall thickness. Atheroma remains, however, a focal disease, developing at characteristic sites within the arterial tree. These sites are typically at areas of vessel branching or marked curvature, and correspond to regions of high tensile stress and low sheer stress, leading to the hypothesis that local haemodynamic factors and vessel wall mechanics potentiate the focal development of atheroma. Current assessment of vascular haemodynamics suffers from an inability to handle complex flow, and does not allow accurate determination of locally varying flow, and shear stress patterns. The application of computational fluid dynamic (CFD) flow simulation techniques to ultrasound and local pressure data, however, allows a comprehensive, non-invasive appraisal of haemodynamic flow parameters to be performed. The Candesartan cilexetil and Atenolol Carotid Haemodynamic Endpoint Trial (CACHET) study compares the effects of two antihypertensive regimens, one β-blocker-based, the other angiotensin receptor blocker based, on carotid intima-media thickness. The collection of ultrasound and pressure data on each subject provides a unique opportunity to apply these data to the CFD model to study the effects of these antihypertensive regimens on local fluid dynamics. This will lead to a greater understanding of the relationship of these factors to atheroma formation and regression.

Carotid Artery Hemodynamics: Observing Patient-specific Changes with Amlodipine and Lisinopril by Using MR Imaging Computation Fluid Dynamics

PURPOSE To assess whether using magnetic resonance (MR) imaging combined with computational fluid dynamics (CFD) could reveal changes in common carotid artery (CCA) flow and wall shear stress (WSS) that might contribute to differences in CCA remodeling between amlodipine, a calcium channel blocker, and lisinopril, an angiotensin-converting enzyme inhibitor, despite similar reductions in blood pressure (BP). MATERIALS AND METHODS Institutional review board approval was obtained, and participants gave informed consent. Nine subjects with hypertension were recruited into a double-blind placebo-controlled randomized three-way crossover study to compare the hemodynamic effects of 7 days of treatment with placebo, amlodipine, or lisinopril. After each treatment period, patients underwent CCA ultrasonography, BP measurement, and MR imaging with CFD. Analyses were performed by using repeated-measures analysis of variance, followed by the Tukey test or the Wilcoxon matched-pairs test. RESULTS Amlodipine and lisinopril lowered BP similarly, but CCA flow rate was significantly higher (P < .01) and distal vascular resistance was lower (P = .016) after amlodipine treatment than after lisinopril treatment. WSS on the inner wall of the CCA was significantly lower after lisinopril treatment than after amlodipine treatment (P = .03). The change in WSS in the CCA correlated with the change in vascular resistance (r = -0.85, P < .001). CONCLUSION Amlodipine causes increased blood flow and increased time-averaged WSS in the CCA compared with lisinopril, despite similar reductions in BP. Differences in the subacute hemodynamic effects of amlodipine and lisinopril could contribute to the differences in CCA remodeling seen in long-term studies. SUPPLEMENTAL MATERIAL http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.10100788/-/DC1.

3D ultrasound-based CFD for carotid flow prediction: a reproducibility study

Image-based CID has been a prime technique for studying arteriosclerosis, plaque formation, aneurysm rupture and bypass design in the past decade. The imaging techniques used for vessel geometry acquisition are usually X-ray (CT-scan), intravascular ultrasound or most frequently MRI. It has been shown in previous studies that 3D extravascular ultrasound (3D US) can provide a cost-effective alternative for imaging superficial arteries like the carotid bifurcation or femoral arteries. In this study, the carotid bifurcation of 9 healthy subjects have been scanned twice within two to six weeks. CFD models for each subject and each scan were built using the corresponding anatomical data acquired in vivo. Overall reproducibility was satisfactory. Two main sources of error were identified. (1) Blurred border between vessel lumen and endothelium, causing the operator to overestimate the lumen area; and (2) altered neck angles. It is expected that with further improvement in ultrasound image quality and with standardisation of the imaging protocol, 3D US has a huge potential to become a viable alternative to MRI both for clinical and research uses.