Generation of Patient Specific Finite Element Head Models

Abstract: Traumatic brain injury (TBI) is a great burden for the society worldwide and the statisticsindicates a relative constant total annual rate of TBI. It seems that the present preventativestrategies are not sufficient. To be able to develop head safety measures against accidents ine.g. sports or automobile environment, one needs to understand the mechanism behindtraumatic brain injuries. Through the years, different test subjects have been used, such ascadavers, animals and crash dummies, but there are ethical issues in animal and human testingusing accelerations at injury-level and crash dummies are not completely human-like. In aFinite Element (FE) head model, the complex shape of the intracranial components can bemodeled and mechanical entities, such as pressure, stresses and strains, can be quantified atany theoretical point. It is suggested that the size of the head, the skull-brain boundarycondition, the heterogeneity, and the tethering and suspension system can alter the mechanicalresponse of the brain. It can be seen that the shape of the skull, the composition of gray andwhite matter, the distribution of sulci, the volume of cerebrospinal fluid and geometry of othersoft tissues varies greatly between individuals. All this, suggests the development of patientspecific FE head models.A method to generate patient specific FE head model was contrived based on the geometryfrom Magnetic Resonance Imaging (MRI) scans. The geometry was extracted usingexpectation maximization classification and the mesh of the FE head model was constructedby directly converting the pixel into hexahedral elements. The generated FE model had goodelement quality, the geometrical details were more than 90 % accurate and it correlated wellwith experimental data of relative brain-skull motion. The method was thought to beautomatic but some hypothetically important anatomical structures were not possible to beextracted from medical images. This leads to investigations on the biomechanical influence ofthe cerebral vasculature, the falx and tentorium complex. It was found that biomechanicalinfluence of the cerebral vasculature was minimal, due to the convoluting geometry and thenon-linear elastic material properties of the blood vessels. It suggests that futurebiomechanical FE head model does not necessarily have to include these blood vessels. Theinclusion of falx and tentorium in an FE head model has on the other hand a substantialbiomechanical influence by affecting its surrounding tissue. Therefore, in the investigation ofthe biomechanical influence of the sulci, the falx and tentorium were manually added to theanatomically detailed 3D FE head model. The biomechanical influence of the sulci haspreviously not been studied in 3D and the results indicated an obvious reduction of the strainin the model with sulci compared to the model without sulci in all simulations, and mostinteresting was the consistent reduction of strain in the corpus callosum. The development ofgyri not only produces a larger area for synapses but also forms the sulci to protect the brainfrom external forces.Based on the results, a patient specific FE head model for traumatic brain injury predictionshould at least include the skull, cerebrospinal fluid, falx, tentorium and pia mater, in additionto the brain. With these anatomically detailed 3D models, the injury biomechanics can bebetter understood. Hopefully, the burden of TBI to the society can be alleviated with betterprotective systems and improved understanding of the patients’ condition and hence, theirmedical treatments