Optical characterization of acceleration-induced strain fields in inhomogeneous brain slices
Introduction
Annually 1.4 million people sustain a traumatic brain injury (TBI) in the United States, of which 20% is caused by vehicle traffic accidents [1]. Although vehicles are already equipped with belts and airbags, even more sophisticated preventive measures are needed to further reduce this number of injuries. The development of these measures can be based on injury predictions with numerical head models, by simulating crash situations.
Many numerical head models have been developed [2], [3], [4], [5], [6], [7], differing in the constitutive models used and the level of detail in the modelled geometries of the brain and the skull. Constitutive models describe the mechanical behaviour of tissue, which is nonlinear and visco-elastic in the case of brain tissue [8]. Moreover, brain tissue may be anisotropic and show inter-regional variations. The quality of numerical head model simulations depends partly on the ability of the constitutive model to describe this complex mechanical behaviour, and partly on the modelled geometry. Therefore, the constitutive model and the head model need to be validated in order to give reliable and representative injury predictions. However, only limited experimental data exist because of the inaccessibility of the cranium.
Pudenz and Shelden [9] measured the deformation in a macaque brain through a cranial window. Although this was one of the first successful strain measurements of the brain during acceleration, only the deformation of the surface could be observed. Furthermore, in the past two decades the use of living animals is being restricted due to legislations.
To validate model predictions, Brands et al. [3] used both open and closed cylindrical cups filled with silicon gel, which were subjected to transient rotational acceleration. In both setups, the gel response was measured using optical markers and a high-speed video camera. Ivarsson et al. [10], [11] studied the natural protection of the brain, also using gel and high-speed tracing of markers. More specifically, lateral ventricle substitutes were included in this physical model to investigate if these structures give strain relief during head impact. Margulies et al. [12] and Meaney et al. [13] recorded the motion of grid patterns painted on gel inside animal and human skulls during angular acceleration. The overall deformation pattern as a result of rotation was compared to the pathological portrait of diffuse brain injury, as determined from animal studies and autopsy reports. Although gel-based setups can provide insight in the global mechanical behaviour, they are unable to mimick the local brain structures like grey and white matter boundaries and the folding structure of the cortex.
Hardy and colleagues used neutral density accelerometers [14] and targets [15] to measure brain motion in human cadavers via high-speed X-ray imaging during angular acceleration. The spatial resolution of these measurements was too low to estimate local strain fields.
Bayly et al. [16] used MRI to measure the deformation of brain tissue induced by mild acceleration in human volunteers. Strains of 0.02–0.05 were typical during the occipital deceleration, and compression in anterior regions and stretching in posterior regions were observed. Moreover, the motion of the brain appeared to be constrained by structures at the frontal base of the skull. A drawback of volunteer tests is that they can only be performed at a level well below the injury threshold. The same method was used to obtain strain fields in the brain of a perinatal rat [17], [18]. In these experiments the brain was not accelerated, but the flexible skull was indented by 2 mm and during 21 ms. Lagrangian strains of more than 0.20 at strain rates exceeding 40 s−1 were observed.
The main objective of this study was to develop an experiment in which a realistic crash situation was mimicked and that can be used to validate numerical head models. Therefore, high-resolution strain-fields were measured in inhomogeneous, planar sections of fresh porcine brain tissue with a complex and detailed geometry, during translational acceleration normally occurring in crash situations. The planar brain samples contained both grey and white matter as well as the complex folding structure of the cortex. It was hypothesized that these inhomogeneities influence the acceleration-induced strain pattern. Since fresh porcine tissue was used, the experiment was conducted within 6 h post-mortem in order to prevent any time-related changes of the mechanical behaviour of the tissue [19].
During translational acceleration, the samples were immersed in artificial cerebrospinal fluid (aCSF) to prevent dehydration of the tissue. Furthermore, in order to obtain a representative model of the relative motion of the brain inside the skull during acceleration, the sample together with the aCSF layer was encapsulated in an almost rigid cavity with the shape of the actual brain slice. The motion was recorded by a high-speed video camera and displacements and strain fields were obtained using digital image correlation. The strain patterns obtained were qualitatively compared with the grey and white matter composition of the slice and the positions of the sulci.
Section snippets
Materials and methods
Planar brain slices were prepared from female pig brains (Dutch landrace hybrid) of 4–6 months old, obtained at a local slaughterhouse. During transport and preparation, the brains were cooled and stored in porcine based aCSF [20] to prevent dehydration and to slow down the degradation and swelling process of the tissue. Sagittal slices of 4 mm in thickness were made using a standard slicing machine (Bizerba) from the region about 2 cm outwards from the mid-sagittal plane, see Fig. 1. This
Strain fields
Two brain slices, obtained from different brains, were used and 17 measurements were conducted in total. Field information was obtained from either the lateral or the medial side of the slice. The results are shown from both the lateral and medial sides of one slice, at the most pronounced moments in time. The displacements are presented as vector fields, plotted on an image of the corresponding brain slice, taken before spraying a pattern on the surface. The vectors were scaled corresponding
Discussion and conclusions
Strain fields were measured in porcine brain tissue slices, during translational acceleration. Although the sampleholder was accelerated translationally, a rotation of brain tissue relative to it was found. This may have been induced by a small rotation of the sampleholder during the onset of the acceleration pulse. However, this rotation was limited to about 2, compared to a maximum translation of about 23 mm. Instead, the rotational displacement field could also be due to variations in slice
Acknowledgements
This work was conducted within the APROSYS Integrated Project supported by the European Commission.
Conflict of interest
None declared.
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