# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 2 Experimental Procedures Used in FEHM Validation
### In Vivo Experimentation
A partial FEHM developed during a preliminary study before developing infant FEHM and validation tools [59] was validated against anterior-posterior brain displacements measured in the coronal plane by [PERSON] et al. [55]. Tagged MRI techniques have continued to develop. For example, [PERSON] et al. [60] studied the tagged MRI responses of a significantly larger cohort of volunteers (\(n=33\)) than previously seen for rotational accelerations, reporting a mean peak acceleration of 195 rad/s\({}^{2}\). While these experiments do not appear to have been applied to FEHM validation yet, they present potential in vivo impact response data sources. The impulse conditions are described through plots of the mean angular position, rotational velocity and rotational acceleration across the full cohort against time. The strain behaviour in the brain is subsequently depicted via colour plots of the shear strain magnitude across 13 axial slices spread between the inferior and superior brain limits, as seen in Figure 11a, for 145 ms after the point of impact, through nine time-steps. Additionally, the maximum recorded shear, first (tensile) and second (impressive) principal strains are plotted for each brain slice. The area fraction of the brain experiencing each strain type above a 3% threshold is further analysed. At each time-step, the mean percentage of brain tissue exceeding 3% strain across the full cohort is provided for the whole brain and in each segmented part, namely the cortical grey matter, white matter and deep grey matter. The distribution of effected areas is also plotted through spider diagrams, providing an idealised measure of the expected strain patterns. Figure 11 provides an example of this alongside the \"raw\" shear strain colour plots. Increasing the range of strain data available, [PERSON] et al. [61] recently proposed combining tagged MRI techniques with simulations constrained to the MRI data to exp  
Figure 11: Example of brain strain responses reported by [PERSON] et al. [60], 54 ms after volunteer trigger release. **(a)** Maximum recorded shear strains through 13 axial brain slices moving from inferior to superior regions. The superior most slice does not register any strains above 1% so appears blank. **(b)** Distribution of the area fraction of the brain experiencing strains above 3% for each of the tensile principal (E\({}_{1}\)), compressive principal (E\({}_{2}\)) and shear (\(\gamma_{\max}\)) strains. The shaded area indicates the mean proportion across the cohort of volunteers, with the confidence interval indicated via the bordering line. The spiderplot is orientated with the frontal region of the brain (F) at the top, occipital (O) bottom and left (L) and right (R) as indicated. The segments between represent the temporal and parietal regions respectively. response analysis to 3D, known as HARP-FE. Both linear and rotational acceleration scenarios were considered, providing potential data for FEHM validation across a greater spacial range, especially when considering small deformations. This approach has been expanded by [PERSON] et al. [62] on 20 MRI data-sets captured during controlled rotational impulses and mild occipital impacts, using similar apparatus to previous tagged MRI methods. The 3D displacement data from HARP-FE analysis allows the maximum principle strain (MPS) to be calculated across the brain tissue, which is presented as colour plots in the central slice of each anatomical plane for both impulse regimes for 10 time-steps of 18 ms. Furthermore, in regions of white matter where there are high levels of anisotropy, the fibre-strain is calculated, a prevalent metric when considering more severe traumatic brain injuries. These strain responses are also analysed in terms of the fraction of the complete brain/white matter volume experiencing MPS/fibre strains over thresholds of 0.1, 0.2 and 0.3 at each time-step. While less applicable to FEHM validation, the relationship between these strain responses and the head kinematics are also discussed.