# 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
The angular speed over a period of 100 ms for each repetition and the idealised pulse are provided as potential input data for the validation of FEHM. The motion of the brain was tracked via tin targets placed throughout the specimen tissue, without the coatings used by [PERSON] et al. [35; 36], as the targets were small enough not to cause damage to the surrounding tissue. NDTs were inserted in each hemisphere of the brain, spaced according to fractions of the brain size with the intention of creating a scale-able approach to improve the consistency of validation across FEHM of various sizes. Six NDTs are implanted in the left hemisphere in a radial pattern, arching out from the centre of the Frankfurt plane while the right hemisphere had nine targets inserted according to a grid system. Figure 4 illustrates these spacing and the proportional measurements of the target locations. As with the previously discussed works [35; 36], the impact response is represented via motion traces of NDTs and their relative displacements parallel to the anterior-posterior and inferior-superior axes. The controlled nature of the impact meant that the behaviour of a given target could be compared across impact repeats. The NDTs had an average CORA score of 0.846 and normalised root mean squared difference of 9.0% between repetitions, indicating very good correlation between the impact conditions and responses. The frontal and occipital ICP response was also monitored by transducers fixed in the skull and the recordings for all four impulse repeats at each location are provided. At the time of writing, the authors are unaware of this experimental procedure being applied in any FEHM validation studies. Sonomicrometry was used as an alternative method to biplanar X-ray by [PERSON] et al. [40; 49] and applied during the validation of an updated version of the GHBMC FEHM [44; 50]. Sonomicrometry monitors the time ultrasound pulses take to travel between piezo-electric crystals. Knowing the speed of sound in a material allows distances to be calculated and setting up arrays of crystals allows their location to be determined. [PERSON] et al. [40] inserted 24 receiver crystals into the brain tissue, with a further eight fixed to the skull as transmitters. The location of each receiver was then monitored through a series of controlled rotational impulses. A pneumatic actuator was used to generate angular velocity impulses with a haversine shape profile. The peak velocity magnitude was set to either 20 and 40 rad/s and the pulse duration to 30 and 60 ms, respectively. The four possible magnitude/duration combinations were applied and the 40 rad/s, 30 ms test was repeated a second time in each direction. While impulses were applied about each anatomical axis, the impulse rotational velocity and acceleration profiles, and subsequent results are only provided for the coronal scenario. The repeated tests of the highest severity impulse were shown to have excellent similarity through both CORA analysis with an average score of 0.93 (see Section 3.3) and an average difference in root mean square velocity magnitudes of \(1.2\pm 0.68\) rad/s. The displacement response of the brain was again presented similarly to seen in earlier works [35; 36] with in situ motion traces for each receiver crystal from each impulse scenario. The relative displacement of three receivers are also presented across all the impact scenarios as displacement magnitudes against time. [PERSON] et al. [49] employ similar techniques, using 30 sonomicrometry crystals in the brain and six control crystals on the skull of six cadaver samples. These were subjected to controlled rotational impulses with similar properties to used in the 2018 study. However, analysis focused on the relationship between the impulse kinematics and the physical response of the brain and as such, the data provided is of limited potential when considering FEHM validation.