Yuki Onishi

Accurate determination of patient‐specific boundary conditions in computational vascular hemodynamics using 3D cine phase‐contrast MRI

In the patient-specific vascular CFD, determination of the inlet and outlet boundary conditions (BCs) is an important issue for a valid diagnosis. The 3D cine phase-contrast MRI (4D Flow) velocimetry is promising for this issue; yet, its measured velocities contain relatively large error and are not admissible as the BCs without any correction. This paper proposes a novel correction method for determining the BCs accurately using the 4D Flow velocimetry. First, we reveal that the error of the velocity measured by the 4D Flow at each measurement voxel is large but is distributed symmetrically. Secondly, our method pays attention to the incompressibility of the blood and the fact that the volume flow rate (VFR) in each vessel is constant on any cross sections. We reveal that the average of the cross-sectional VFRs integrated from many measurement voxel in each vessel is accurate despite the large error. Finally, we propose the novel correction method, which applies a smoothing to the measured velocities on each inlet or outlet boundary with a low-pass filter and then corrects them with the VFR. The results of the several phantom studies are presented to validate the accuracy of our method. A demonstrative analysis for an actual aneurysm is also presented to show the feasibility and effectiveness of our method.

Accurate Viscoelastic Large Deformation Analysis Using F-Bar Aided Edge-Based Smoothed Finite Element Method for 4-Node Tetrahedral Meshes (F-BarES-FEM-T4)

A state-of-the-art tetrahedral smoothed finite element method, F-barES-FEM-T4, is demonstrated on viscoelastic large deformation problems. The stress relaxation of viscoelastic materials brings nea...

F-Bar Aided Edge-Based Smoothed Finite Element Method With 4-Node Tetrahedral Elements for Static Large Deformation Elastoplastic Problems

A new type of smoothed finite element method (S-FEM), F-barES-FEM-T4, is demonstrated in static large deformation elastoplastic cases. F-barES-FEM-T4 combines the edge-based S-FEM (ES-FEM) and the node-based S-FEM (NS-FEM) for 4-node tetrahedral (T4) elements with the aid of the F-bar method in order to resolve the major issues of Selective ES/NS-FEM-T4. As well as most of the other S-FEMs, F-barES-FEM-T4 inherits pure displacement-based formulation and thus has no increase in DOF. Moreover, the cyclic smoothing procedure introduced in F-barES-FEM-T4 is effective to adjust the smoothing level so that pressure checkerboarding (oscillation) is suppressed reasonably. Some examples of static large deformation analyses for elastoplastic materials proof the excellent performance of F-barES-FEM-T4 in contrast to the conventional hybrid T4 element formulation.

Hemodynamic vascular biomarkers for initiation of paraclinoid internal carotid artery aneurysms using patient-specific computational fluid dynamic simulation based on magnetic resonance imaging

PurposeWe performed computational fluid dynamics (CFD) for patients with and without paraclinoid internal carotid artery (ICA) aneurysms to evaluate the distribution of vascular biomarkers at the aneurysm initiation sites of the paraclinoid ICA.MethodsThis study included 35 patients who were followed up for aneurysms using 3D time of flight (TOF) magnetic resonance angiography (MRA) and 3D cine phase-contrast MR imaging. Fifteen affected ICAs were included in group A with the 15 unaffected contralateral ICAs in group B. Thirty-three out of 40 paraclinoid ICAs free of aneurysms and arteriosclerotic lesions were included in group C. We deleted the aneurysms in group A based on the 3D TOF MRA dataset. We performed CFD based on MR data set and obtained wall shear stress (WSS), its derivatives, and streamlines. We qualitatively evaluated their distributions at and near the intracranial aneurysm initiation site among three groups. We also calculated and compared the normalized highest (nh-) WSS and nh-spatial WSS gradient (SWSSG) around the paraclinoid ICA among three groups.ResultsHigh WSS and SWSSG distribution were observed at and near the aneurysm initiation site in group A. High WSS and SWSSG were also observed at similar locations in group B and group C. However, nh-WSS and nh-SWSSG were significantly higher in group A than in group C, and nh-SWSSG was significantly higher in group A than in group B.ConclusionOur findings indicated that nh-WSS and nh-SWSSG were good biomarkers for aneurysm initiation in the paraclinoid ICA.

145 Aberration Retrieval Method using spot images with astigmatism offset

In this study, aberration retrieval method from spot images with astigmatism offset has been developed. The amplitude of the generalized pupil function and the wavefront aberration which is the phase of the generalized pupil function are expanded with the lower Zernike polynomials, respectively. The present aberration retrieval problem is reduced to the nonlinear least square problem minimizing the difference between the observed images on the imaging device and the corresponding model images which can be calculated by assuming these Zernike coefficients. In the mathematical model for calculating the captured spot images, the mathematical model considering the characteristics of the imaging device are applied to the current model. In order to demonstrate the effectiveness of this method, the numerical verification simulation and the aberration retrieval form practically captured images are performed.

Obtaining Corrosion Rates by Bayesian Estimation: Numerical Simulation Coupled with Data

Protecting structures from corrosion is one of the most important challenges in engineering. Cathodic protection using sacrificial anodes or impressing current from electrodes is applied to many marine structures. Prediction of the corrosion rates of structures and the design of cathodic protection systems have been traditionally based on past experience with a limited number of empirical formulae. Recently, application of numerical methods such as the boundary element method (BEM) or finite element method (FEM) to corrosion problems has been studied intensively,1-8 and these methods have become powerful tools in the study of corrosion problems.23-24 With the progress in numerical simulations, “Inverse Problems” have received a great deal of attention. The “Inverse Problem” is a research methodology pertaining to identifying unknown information from external or indirect observation utilizing a model of the system, as shown in Fig. 1. The background of inverse problems is the modeling and simulation of natural phenomena. When observations are taken of these phenomena, the observation data are used to infer knowledge about physical states. One of the most spectacular successes in the field of inverse problems was the invention of an inversion algorithm for computed tomography by Cormack (1963) and its experimental demonstration by Hounsfield (1973). The two shared the Nobel Prize in Physiology/Medicine in 1979. Applications of inverse problems arise in many fields of engineering as well. In the field of corrosion engineering, there are many issues that benefit from an inverse analysis approach. In this article, the use of numerical simulation for evaluating corrosion rates is explained with the boundary element method as an example. Then, an inverse analysis method for identifying corrosion rates or cathodic protection currents from the (easily measured) potential distribution around marine structures is introduced. This method is based on the Bayesian estimation, with the measured data and numerical simulation focused on the potential distributions around a seaside structure.

Development and Nobel Application of Numerical Corrosion Simulation

Numerical simulation is gathering many expectations in the field of corrosion engineering with the development of computer. In this review the two example topics are introduced. The first topic is “Numerical method for time-dependent localized corrosion analysis with moving boundaries”. The numerical method for time-dependent localized corrosion analysis is presented. Electromigration, mass diffusion, chemical reactions, and moving boundaries are considered in the numerical simulation of localized corrosion of engineering alloys in an underwater environment. The method combines the finite volume method (FVM)and the voxel method. In the second topic, the forecasting system of the current from sacrificial anodes using data assimilation was introduced. Some numerical verification example analysis are shown to demonstrate the effectiveness of the methods.

Efficient Elasto-Dynamic FEM for Objects with Uniform Cross-Section.

An efficient dynamic finite element method (FEM) for an elastic body with uniform cross-section was developed. In this method, the finite element mesh is generated in such a way that the body to be analyzed is at first sliced into layers with the same thickness and then each layer is discretized in the same way. The computational time and memory can be saved by utilizing the regularity in the mass, viscosity and stiffness matrices and making them repetitive block tridiagonal matrices, which results from the above mesh. A few example problems were solved with this method to demonstrate its effectiveness. This method is also applicable to many other problems, such as thermal and electrical problems.