Modeling And Application Of Axonal Fiber In Finite Element Head Model

Traumatic brain injuries（TBIs）are the most common major injuries in road traffic accidents（RATs）,more than 30%of the victims were died from serious TBIs among all the deaths caused by RATs,which seriously endangers the people’s security.Hence,the biomechanics of TBIs should be clearly understood,which supplies scientific foundation to develop the personal protective equipments,vehicle safety technology and improve the vehicle safety regulations.Related researches have important academic value and practical significance.The objective of this study was to investigate a multi-scale coupling modeling method to incorporate axonal fibers into a finite element（FE）brain model and develop a fiber-coupled FE head model.The fiber-coupled FE head model was validated through analyzing intracranial dynamic responses of the model under mechanical impacts referring to experimental results;and the correlation mechanism of diffuse brain injuries（DBIs）between the mechanical impacts and intracranial dynamic response was investigated.Firstly,the FE fiber model was developed based on whole-brain tractography data,a fiber-coupled FE head model was developed by incorporating axonal fibers into existing FE brain model.Axonal tractography data of 71 regions of interest（ROIs）were obtained using DSI-Studio software based on an averaged template of human head magnetic resonance images（MRI）,while axonal fiber diameters of 71 ROIs were calculated.Based on LS-DYNA explicit FE method,a FE fiber model was developed by independent programming.A global scaling method was used to align the fiber model with the mesh-refined THUMS-head baseline model,which the fiber model was in the same anatomical space as the volumetric baseline model brain.Embedded FE method was used to constraint the fibers and brain solid elements.Secondly,an automatic material determination pipeline was developed to obtain the material parameters of brain matrix and fiber through optimization algorithm.FE simulations of brain tissue dynamic experiments,with a maximum strain of 0.5 and strain rates of 30 s^-1,were developed referring to experimental data.A method combined by FE simulation and optimization algorithm was applied to the construction of automatic material determination pipeline with mode FRONTIER,LS-DYNA and MATLAB.Correlation ratings were calculated between the engineering stress-strain curves predicted by simulations and averaged curves of experiments,which CORrelative and Analysis（CORA）were assigned as objective function;and the multi-objective genetic algorithm was applied to the optimization iterative solution of the material parameters.Thirdly,based on the existing experimental data,the validation of fiber-coupled FE head model was performed for predicting intracranial dynamic responses under mechanical impacts.FE reconstructions of six experiments were developed using the fiber-coupled FE head model.CORA was used to calculate the correlation ratings between the skull-brain relative displacements predicted by simulations and experimental curves.Meanwhile,the deformation processes of brain tissue under mechanical impact were analyzed,and verified the effectiveness of the fiber-coupled FE head model for predicting intracranial dynamic responses.Finally,simulation matrix was constructed for DBIs under mechanical impact and investigated the correlation mechanism between head impacts and intracranial dynamic responses.Based on related data of human brain injuries caused by mechanical impacts,the characteristics of mechanical impacts were analyzed.60 simulations were calculated using the fiber-coupled FE head model after the design of simulation matrix.Quantitatively compared the strain-related dynamic responses（Cumulative Strain Damage Measure-CSDM and axonal strain damage distribution）predicted by simulations under different mechanical impacts,and investigated the influence of head impacts on DBIs.In conclusion,the automatic material determination pipeline in this study could effectively determine material parameters of brain tissue,and the fiber-coupled FE head model was an effective tool for investigating the biomechanics of DBIs;The correlation mechanism between head impacts and intracranial dynamic responses were discussed:the strain responses of brain tissue were mainly caused by the rotating load;head impact direction,the shape of load waveform,the durations of rotational acceleration and rotational deceleration had an obviously effect on strain responses.