# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 2 Experimental Procedures Used in FEHM Validation
### In Vivo Experimentation
The levels experienced by S1 and S2 [53] are presented, again with additional data showing the mean between the cases, indicated as “Av”, calculated by the authors for the purposes of this review. Figure 8: Brain response to a mild frontal impact of subject S1 44.8 ms after the head is released into free-fall, extracted from [PERSON] et al. [54]. (**a**) An example of the displacement vector fields published, largely indicating the direction the brain is displaced relative to the skull. (**b**) Colour plot indicating the magnitude of the relative displacements. (**c**) Ellipsoid plot indicating the principal strain across the brain slice. The colour and degree of stretch in the ellipsoid (compared to a circle) indicate magnitude. The direction of the elongation of the ellipsoid denotes the vector along which the principal strain acts. These images show the response of subject S1. Similar images are provided by [PERSON] et al. [54] for all three subjects (S1, S2, S3) 39.2, 44.8, 50.4, 56.0 and 61.6 ms from the point of trigger release. Figure 9: Relative brain displacement magnitude responses to mild frontal impacts conducted by [PERSON] et al. [54] at four discrete points. The chosen locations relate to where NDTs were inserted in specimens during cadaver experimentation [35; 36]. Displacements recorded for each of the three subjects (S1, S2, S3) are presented at each location in the original publication and replicated here [54]. As with previous examples, the authors have calculated and present the mean response across all three subjects. Both [PERSON] et al. [53] and [PERSON] et al. [54] build on the tagged MRI work of [PERSON] et al. [52]. In their study, [PERSON] et al. [52] subjected three volunteers to mild occipital impacts. The volunteers lay supine with their heads raised 2 cm (the same height [54] then used to generate frontal impacts) and neck in flexion. Releasing a trigger caused their head to drop via neck extension before hitting a stop, with a peak acceleration of \(2g-3g\) over 40 ms. A single slice in each of the sagittal and axial planes is considered. Contour plots depict the strain magnitudes in the anterior-posterior and inferior-superior directions and the principal strain magnitude after impact. Four time-steps 6 ms apart are provided and compared to the state of the brain prior to release. Area fraction measures above thresholds between 0.2-0.5 are also provided against time. [PERSON] et al. [55] optimised MRI acquisition settings during rotational accelerations, similar to those conducted by [PERSON] et al. [53]. A gelatin phantom and an updated trigger system were used to reduce the number of rotations the volunteer had to be subjected to to determine the strain response across a given brain slice to as few as eight rotations, from the 72-144 rad/s\({}^{2}\) reported in previous studies. The mean of the peak acceleration magnitudes was 260 rad/s\({}^{2}\), which is slightly higher than those recorded by [PERSON] et al. [53], generated by a 32\({}^{\circ}\) rotation. Both maximum shear strain and radial-circumferential shear strain contour plots are available in the supplementary material of the paper across two axial slices for each of the three subjects. Across existing FEHM, the Worcester Head Injury Model (WHIM) [56] has been validated against brain displacement and deformation from cadaver-based experimental data (see Table 4) and the in vivo experimentation conducted by [PERSON] et al. [53], whil  
Figure 10: Principal strain magnitudes recorded during mild frontal impacts conducted by [PERSON] et al. [54] at the same four discrete points as seen in Figure 9. The strains recorded from each subject, extracted from the original publication [54], are presented with a newly calculated group mean. and [PERSON] and [PERSON] [58] validated a new FEHM against the observations of [PERSON] et al. [54].