# 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 In Vivo response of the brain to impulsive loads can be monitored using tagged Magnetic Resonance Imaging (MRI) techniques. [PERSON] et al. [52], [PERSON] et al. [53] and [PERSON] et al. [54] conducted a series of experiments on live volunteer subjects, monitoring the response of the brain during mild frontal [54] and occipital [52] impacts and rotational impulses [53] in MRI scanners. [PERSON] et al. [55] conducted further experimentation into rotational impulses, looking to optimise data acquisition. Since these works have been published, the potential of tagged MRI techniques has been explored in a number of ways. This section reviews the experimental methods and data that have already been used in relation to FEHM validation and research. Other works with similar potential are also mentioned. Tagged MRI uses radio-frequency pulses to superimpose a grid of \"tag lines\" onto an MRI scan, which then move with the brain tissue. The locations of intersections between tag lines can be mapped into a regular mesh by comparing the location of each intersection to its neighbours. This mesh deforms as the tag lines move with the brain tissue during impacts. Comparisons of mesh configurations at each time-step to the original mesh allow a deformation gradient tensor to be calculated for a complete slice through the brain. This can be used to calculate the deformation and displacement response of a complete brain slice. [PERSON] et al. [53] and [PERSON] et al. [54] each subjected three live 22 to 45 years old male volunteers to mild impacts while acquiring MRIs of their brains. [PERSON] et al. [53] considered rotational impacts by having the subjects lie supine, with their head in a cradle within an MRI scanner. The subjects released a weight causing the cradle to rotate about their inferior-superior axis. A stopper was then impacted, causing a rotational deceleration impulse, approximately 200 ms after the initial release. [PERSON] et al. [54] considered predominantly linear motion by having subjects lie prone with a cradle lifting their forehead and chin. Subjects released their head supports causing a drop of approximately 2 cm before the forehead impacted a rubber stop 40-50 ms after release, creating a frontal impact. In both studies, each impact was repeated to maximise the temporal resolution of the acquired data. A sample of complete angular acceleration repeats recorded during one set of rotational impacts is provided by [PERSON] et al. [53] and shown in Figure 5. The repetitions are also shown, normalised to have time reset at each trigger release. [PERSON] et al. [54] provide estimated rigid body displacements linearly in the anterior-posterior (\(x\)) and inferior-superior (\(y\); others have defined this direction as \(z\)[35; 36; 38]) directions and rotational displacement about the medial-lateral axis for each subject of the head-drop experiment. These are illustrated in Figure 6, along with the average across the subjects. In both studies, the subject initiating the impact motion also triggered the MRI capture, which recorded an image approximately every 6 ms.  
[PERSON] et al. [53] considered the response of the brain to the rotational impulse across four axial planes. The \"zero plane\" passed through the genu and splenium of the corpus callosum and the remaining three were 2 and 4 cm superior and 1 cm inferior of the reference. The radial-circumferential strain in each plane was recorded for 60 ms after the impact in 6 ms time-steps, and reported using contour plots mapping the strain levels across each slice. The response of subject S1 was depicted at each plane and compared to the response of either S2 or S3. An example of this for the \(+\)2 mm plane is shwon in Figure 7a, featuring two subjects (labelled S1 and S2), along with the average between the two volunteers. The fraction of area experiencing strains above the 0.02, 0.04 and 0.06 thresholds is shown in Figure 7b for the \(+\)2 mm plane for subjects S1 and S2, along with the average between subjects.