# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 2 Experimental Procedures Used in FEHM Validation
### Cadaver Experimentation
#### 2.1.2 Relative Brain Displacement Response  
FEHM have frequently been validated against the relative motion between the brain and skull compared to cadaver experiments of [PERSON] et al. [35; 36]. These authors use biplanar X-ray technology to track target points in the brain and skull during at total of 45 impacts at velocities between 2.5 and 3.9 m/s. The kinematic response of each impact was recorded through accelerometers fitted to each cadaver's skull. The target points in the brain were Neutral Density Targets (NDTs): Tin granules encased in polystyrene, providing an X-ray opaque node of similar density to the surrounding tissue that could be traced during impacts without damaging the tissue. In their earlier study [35], three cadavers (C731, C755 and C383) were impacted in the frontal or occipital region. cadavers C731 and C755 had usable data collected from six impacts between them, mainly to the occipital region, with the exception of C755-T5 which was a frontal impact. These impacts were created by accelerating a static cadaver by striking it with a piston. The six cases had comparable kinematic responses with average peak resultant accelerations of \(22g\pm 5g\) and \(1718\pm 583\) rad/s\({}^{2}\) (mean\(\pm\)SD). C383 was subjected to four deceleration impacts, three of which on the frontal region and test C383-T4 on the occipital area. More variation was seen across these cases where the sample was decelerated against a fixed block, with peak kinematic responses of \(69g\pm 24g\) and \(4539\pm 3439\) rad/s\({}^{2}\). The difference between acceleration and deceleration responses was attributed to the impact method [35]. The only occipital deceleration impact (test C383-T4) displayed significantly higher peak resultant accelerations (108\(g\) and 10,482 rad/s\({}^{2}\)), contributing to the spread in deceleration kinematics. In this study by [PERSON] et al. [35], two vertical columns of NDTs were inserted into the anterior and posterior regions of a single hemisphere of the brain of each cadaver sample with 7-12 mm between each NDT in each column. In this arrangement, the key NDTs refer to the inferior-most and superior-most target of each column. The 2007 study [36] focused only on deceleration impacts, but generated a greater range of kinematic responses. During deceleration impacts, cadaver specimens were suspended from a carriage on rails. The cadaver was mounted at the neck such that it could rotate about its mounting but translation was restricted to along the rails, creating an oblique impact. The initial orientation of the cadaver was adjusted to create impacts in different directions. A piston then brought the complete apparatus to the desired speed before impact. As well as introducing lateral motion by impacting the temporal and parietal regions of two cadavers (C380 and C393), approximately half the impacts across the study were offset from the anatomical axes, altering the linear and rotational motion of the cadaver. Of the 25 impacts conducted on cadavers suitably instrumented for validation, 15 had a helmet fitted to the sample. As well as enabling investigations into the effects of protection, this further altered the impulses experienced by the cadaver heads. The properties of each impact across both studies are summarised in Table 3. Impacts conducted on two cadavers were not deemed suitable for FEHM validation so are not included in the 25 cases considered. Cadaver C427 had instrumentation failure preventing meaningful data collection in terms of simulating replications, and cadaver C408 was instrumented to study the brain motion at the cortex periphery, near the brain-skull interface during occipital impacts. Uncertainty around the behaviour in this area compared to deeper regions of the brain, as discussed by [PERSON] et al. [36], render this example less well suited to FEHM validation compared to other similar impact cases provided within the same work. The NDT configuration was different in this investigation [36] from the earlier work [35].