# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 4 Further Research into Validation  
### Retrospective Validation and Comparison Investigations  
The range of representations in terms of geometry, material models and included features across different FEHM is well documented and discussed in literature. As such, (particularly prominent) FEHM have been subjected to further investigations into their behaviour and comparisons to other models. [PERSON] et al. [31] looked to objectively and quantitatively compare the performance of FEHM against relative brain displacement data. Five tests (C755-T2, C383-T1, T3, T4 from [PERSON] et al. [35] and C291-T1 from [PERSON] et al. [36]) were selected to represent a range of impact locations and kinematic properties as well as to represent cases typically used during FEHM validation. Existing results from simulating these cases were compared for the ABM [83], SIMon (Simulated Injury Monitor) [76], GHBMC [44], THUMS [25], KTH (Kungliga Tekniska Hogskolan) [47; 110] and WHIM [56; 81] FEHM (KTH and WHIM had partial data sets available). The relative displacements at each NDT in both directions considered were analysed using CORA. These ratings were averaged to provide a model rating for each impact scenario. These were averaged again to provide an overall rating, for the available scenarios. The ranking of the six FEHM was also considered and it was found that the FEHM ranked differently according to CORA rating for each test. The KTH FEHM [47] was stated to have the highest overall rating (\(0.413\pm 0.059\)), although the results for WHIM [56; 81] are higher (\(4.15\pm 0.035\)). The WHIM also had the most consistent rating (lowest standard deviation). However, these two FEHM only had data available for three and two of the impact scenarios considered, respectively, which is a limitation of the study. Of those with data for all the scenarios considered, the ABM [83] had the best overall rating (\(0.376\pm 0.053\)). [PERSON] and [PERSON] [33] compared the response of the GHBMC [44], THUMS [25] and SIMon [76] FEHM to the brain strain response recorded during tagged MRI experiments from [PERSON] et al. [52]. The impacts were generated by simulating the FHEM droppig onto a rubber pad, replicating the conditions experienced by in vivo subjects. Despite being subject to identical conditions, the three FEHM experienced different head kinematics, particularly rotational accelerations, after the point of impact. As well as comparing the distinct models, THUMS then had the brain material models from GHBMC and SIMon applied to it, and the responses were again compared. Compared to the tagged MRI response, all three FEHM underestimated the strain response, but by differing magnitudes, although there were consistencies in the strain distribution pattern across the monitored brain slice. Furthermore, there were discrepancies when just the brain material changed. [PERSON] and [PERSON] [33] surmise that the brain material model, the skull-brain interface and the presence/properties of the neck all influence these discrepancies. [PERSON] et al. [28] investigated the influence properties of NDTs have on the simulated brain response compared to impacts from [PERSON] et al. [36]. A mass replicating the NDTs used in the empirical method was modelled within the FEHM brain volume. Compared to the original FEHM, relative brain displacements \begin{table}
\begin{tabular}{l c} \hline \hline
**Classification** & **Correlation Score range** \\ \hline Excellent & \(86\leq\text{CS}<100\) \\ Good & \(65\leq\text{CS}<86\) \\ Fair & \(44\leq\text{CS}<65\) \\ Marginal & \(26\leq\text{CS}<44\) \\ Unacceptable & \(0\leq\text{CS}<26\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Bio-fidelity classifications adapted from [102]. The classifications have an equivalent distribution but the range is increased by a factor of 10 (from a 0 to 10 range, to 0 to 100). CS is the Correlation Score. magnitudes changed by less than 2.5%. The effects remained less than 10% when the mass of the modelled target was increased ten fold.