# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 2 Experimental Procedures Used in FEHM Validation
### In Vivo Experimentation
Further tagged MRI experimentation has been conducted, alongside the development of new FEHM by [PERSON] et al. [63]. These authors recreated an occipital impact following an equivalent method to the one proposed by [PERSON] et al. [52], with 12 impact repetitions per image series. Relative brain displacements are presented for both an axial and a sagittal plane (although exact locations are not provided) via pairs of colour plots indicating the displacement magnitude within a \(-3\) to \(+3\) mm range, along each axis in the displayed plane (e.g., for the sagittal plane, one series of colour plots indicates the inferior-superior brain displacement an the second the anterior-posterior). The displacement maps are then used to further validate an FEHM, previously developed and validated against the results of [PERSON] et al. [34] for intracranial pressure response [64]. At the time, this was believed to be the first attempt to validate an FEHM against in vivo impact response data. The numerical response was further analysed to obtain the shear and principal strains in each plane. The development and validation of subject-specific head models against the tagged MRI response of the same subject has been conducted by [PERSON] et al. [65] and [PERSON] et al. [66]. The former [65] used data directly from [PERSON] et al. [55], while the latter [66] subjected a new volunteer to both rotational accelerations in the axial plane (similar to [PERSON] et al. [55]) and a neck flexion-extension acceleration (similar to [PERSON] et al. [52]). The rotational acceleration histories for both impact types are provided and applied to the FEHM as boundary conditions during validation. The radial-circumferential strain response in the rotational plane is considered for each impact, across 145 ms (8 time-steps) and through five brain slices separated by 10 mm. The maximum shear strain is also provided for three slices across the five time steps describing the main impulse. Peak radial-circumferential and maximum shear strains are also reported for the cerebral grey and white matters, the thalamus, caudate and putamen. In both these cases [65; 66] however, the head models have been developed using methods closer to Material Point Method [67] than more conventional FEHM. Employing MRI to monitor the dynamic response of the head to impacts is an evolving practice. As well as alternative theories to process tagged-MRI data (e.g., [PERSON] et al. [68]), different imaging techniques are available. [PERSON] et al. [69; 70] employed magnetic resonance elastography techniques, previously used to determine tissue properties [71], to consider the harmonic response of the brain to repeated oscillations. In the first study [69], a driven actuator oscillated the heads of six volunteers for 100 ms at 50 Hz within an MRI scanner. Accelerometers fitted to a mouthpiece measured the rigid body kinematics of the skull, allowing its motion to be separated from that of the brain. [PERSON] et al. [69] use colour plots for a single time-step to illustrate the total displacement, \"wave displacement\" (relative brain motion) and \"curl\" (brain displacement from the shear wave only). The RMS averages of these measurements and the resulting strain are compared in the inner, middle and outer regions of the brain. The following study [70] used the same methods but expanded the analysis to study the relationships between the scalp, skull and brain motions.