# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 2 Experimental Procedures Used in FEHM Validation
### In Vivo Experimentation
Figure 5: Summary of acceleration data recorded by [PERSON] et al. [53] during a series of controlled mild rotational impacts experienced by subject S2 to analyse brain deformation through tagged MRI. **(a)** Sequential rotational acceleration from a series of six impact cycles. The discrete bars (turquoise) indicate the time of each trigger release (approximately 1.8 s apart). **(b)** The same six acceleration cycles normalised to each trigger release, showing the complete motion until the next trigger. Each repeat is shown, however the authors have calculated and added the mean trend across the complete set (mean peak, 187 rad/s2). **(c)** Rotational accelerations across the six impact repeats encompassing the impulse and subsequent brain response analysis period. Figure 6: Rigid body (skull) displacements in the anterior-posterior (\(x\)), inferior-superior direction (\(y\)) and about the medial-lateral axis (\(z\)-rot) recorded by [PERSON] et al. [54] during mild frontal impacts to three subjects (S1, S2, S3). The authors have calculated and also present the average motion across the three subjects, resulting in mean peak displacements of 8.0 mm (\(x\)), 1.0 mm (\(y\)) and 0.1 rad (\(z\)). Figure 7: Cont. [PERSON] et al. [54] focused on a single slice in the sagittal plane of the left hemisphere of each subject during the frontal impact experiments. In order to separate the motion of the brain from the skull, these authors manually identified 10 landmarks at tag line intersections within the skull. The skull was considered a rigid body and the displacement vectors of the new landmarks were used to realign each image to a constant anatomical frame. The relative displacements of the tag line intersections within the brain tissue were monitored, giving the relative motion of the brain compared to the skull across the complete brain slice. Deformation of the mesh between intersects was also analysed to consider the principal strains across the slice. Both vector fields (largely depicting direction) and contour plots (indicating magnitudes) of the relative brain displacement are presented for each of the three volunteer subjects (S1, S2, S3) for 22.4 ms after the point of impact (61.6 ms from trigger release), of which examples are seen in Figure 8a,b. The principal strain is similarly presented, although with an extra level of detail. Each element of the tag line mesh has a circle drawn within its grid square. When the deformation tensor is applied to to the mesh, the circles are similarly stretched into ellipsoids, as shown in Figure 8c. The direction and degree of the elongation indicate the direction of the principal strain and its magnitude, along with the colour gradients seen in other works. The relative displacement and principal strain magnitudes of four discrete points within the brain at similar locations to the NDTs tracked during frontal and occipital impacts conducted by [PERSON] et al. [35; 36] are plotted for the full 168 ms of the impact test. These can be seen in Figures 9 and 10. Figure 7: Examples of brain deformation measures adapted from [PERSON] et al. [53]. (**a**) Colourplots indicating the brain deformation levels from the point of impact (216 ms from trigger release) as radial-circumferential strain for subject S1 and S2, adapted from [53]. Again the authors have presented additional data depicting the average response between the subjects, calculated by digitising the published contour plots into strain levels for each subject, averaging them at each pixel and re-plotting into a new contour plot on the same scale as the original image. Only pixels where both subjects have brain tissue (contour plot is coloured) are included in the average. The slice shown is in the axial plane, 2 mm above the plane between genu and splenium of the corpus callosum. (**b**) Area fraction of the brain experiencing radial-circumferential strains above thresholds of 0.02, 0.04 and 0.06 in the axial plane, 2 mm above the plane between genu and splenium of the corpus callosum.