Traumatic brain injury
Hyperacute changes in the brain after sub-concussive impacts
Summery: Impacts in mixed martial arts (MMA) have been studied mainly in regard to the long-term effects of concussions. However, repetitive sub-concussive head impacts at the hyperacute phase (minutes after impact), are not understood. The head experiences rapid acceleration similar to a concussion, but without clinical symptoms. We utilize portable neuroimaging technology – transcranial Doppler (TCD) ultrasound and functional near infrared spectroscopy (fNIRS) – to estimate the extent of pre- and post-differences following contact and non-contact sparring sessions in nine MMA athletes. In addition, the extent of changes in neurofilament light (NfL) protein biomarker concentrations, and neurocognitive/balance parameters were determined following impacts. Athletes were instrumented with sensor-based mouth guards to record head kinematics. TCD and fNIRS results demonstrated significantly increased blood flow velocity (p = 0.01) as well as prefrontal (p = 0.01) and motor cortex (p = 0.04) oxygenation, only following the contact sparring sessions. This increase after contact was correlated with the cumulative angular acceleration experienced during impacts (p = 0.01). In addition, the NfL biomarker demonstrated positive correlations with angular acceleration (p = 0.03), and maximum principal and fiber strain (p = 0.01). On average athletes experienced 23.9 ± 2.9 g peak linear acceleration, 10.29 ± 1.1 rad/s peak angular velocity, and 1,502.3 ± 532.3 rad/s2 angular acceleration. Balance parameters were significantly increased following contact sparring for medial-lateral (ML) center of mass (COM) sway, and ML ankle angle (p = 0.01), illustrating worsened balance. These combined results reveal significant changes in brain hemody- namics and neurophysiological parameters that occur immediately after sub-concussive impacts and suggest that the physical impact to the head plays an important role in these changes.
Statement of significance: : Brain injuries sustained during sport participation have received much attention since it is a common occurrence among participants. Although protective technologies have been developed over the years, the mechanism of injury is still unclear. There is less focus on the repetitive exposure to sub-concussive impacts on the functional integrity of the brain. Sub-concussive impacts are defined as a lesser impact force resulting in acceleration of the head without symptoms of concussion. Diminished neurocognitive performance has been associated with increased sparring exposure in amateur MMA/boxers suggesting that repeated sub- concussive blows may be just as harmful. However, no one has studied the potential effect of repeated sub- concussive head impacts at the hyperacute level defined as within minutes after impact. We apply novel mo- bile sensing tools such as head impact sensors and portable neuroimaging devices that allow us to examine possible physiological effects taking place within minutes of sub-concussive impacts which are generally transient, and have not been captured before due to limitations with clinical imaging. Based on previous studies, we developed a protocol to test real-world sub-concussive head impact effects on cerebral blood flow and activation patterns and demonstrate that significant changes can be observed immediately after impacts occur, which could
lead to improved monitoring and management of injury risk in sport participation.
Low-Rank Representation of Head Impact Kinematics: A Data-Driven Emulator
Summary: Head motion induced by impacts has been deemed as one of the most important measures in brain injury prediction, given that the vast majority of brain injury metrics use head kinematics as input. Recently, researchers have focused on using fast approaches, such as machine learning, to approximate brain deformation in real time for early brain injury diagnosis. However, training such models requires large number of kinematic measurements, and therefore data augmentation is required given the limited on-field measured data available. In this study we present a principal component analysis-based method that emulates an empirical low-rank substitution for head impact kinematics, while requiring low computational cost. In characterizing our existing data set of 537 head impacts, each consisting of 6 degrees of freedom measurements, we found that only a few modes, e.g., 15 in the case of angular velocity, is sufficient for accurate reconstruction of the entire data set. Furthermore, these modes are predominantly low frequency since over 70% of the angular velocity response can be captured by modes that have frequencies under 40 Hz. We compared our proposed method against existing impact parametrization methods and showed significantly better performance in injury prediction using a range of kinematic-based metrics—such as head injury criterion (HIC), rotational injury criterion (RIC), and brain injury metric (BrIC)—and brain tissue deformation-based metrics—such as brain angle metric (BAM), maximum principal strain (MPS), and axonal fiber strains (FS). In all cases, our approach reproduced injury metrics similar to the ground truth measurements with no significant difference, whereas the existing methods obtained significantly different values as well as substantial differences in injury classification sensitivity and specificity. This emulator will enable us to provide the necessary data augmentation to build a head impact kinematic data set of any size.
Dynamics of human brain and skull
Summary: Although safety standards have reduced fatal head trauma due to single severe head impacts, mild trauma from repeated head exposures may carry risks of long-term chronic changes in the brain's function and structure. To study the physical sensitivities of the brain to mild head impacts, we developed the first dynamic model of the skull–brain based on in vivo MRI data. We showed that the motion of the brain can be described by a rigid-body with constrained kinematics. We further demonstrated that skull–brain dynamics can be approximated by an under-damped system with a low-frequency resonance at around 15 Hz. Furthermore, from our previous field measurements, we found that head motions in a variety of activities, including contact sports, show a primary frequency of less than 20 Hz. This implies that typical head exposures may drive the brain dangerously close to its mechanical resonance and lead to amplified brain–skull relative motions. Our results suggest a possible cause for mild brain trauma, which could occur due to repetitive low-acceleration head oscillations in a variety of recreational and occupational activities.
Measure pathophysiology immediately following head trauma using photoacoustic and ultrasound imaging
Summary: Within minutes following head trauma, a dynamic collection of physiologic and cellular alterations rapidly emerges to initiate a course of pathology and protection that will ultimately define patient outcomes in terms of survival, recovery and long-term disability. Currently, the pre-hospital management of severe TBI – which accounts for 288,000 of the nearly 2.5 million TBI cases annually in the United States and over 57,000 deaths – lacks both diagnostic imaging markers and effective immediate intervention. Thus, rapid, on-site mapping of vulnerable brain tissue, prediction of clinical outcomes close to time of injury (the Golden Hour) can greatly reduce morbidity and mortality following head trauma.
Neuroimaging in biomechanics
Summary: We measure and model in vivo brain motion and investigate how we can minimize sports brain trauma. Designing protective devices for the head has long focused primarily on better helmet padding material to prevent skull deformation (occurring at ~100Hz), while overlooking the dynamics of skull-brain. As such, helmets are effective in protecting the skull. However, we have shown that brain’s motion is dominated by slow dynamics (~20Hz) and it can be driven dangerously close to its resonance frequency in typical head impacts in sports such as soccer and football. This observation undermines the effectiveness of current helmets in repeated low-energy head impacts and necessitates systematic investigation of brain motion dynamics.
Multi-directional dynamic model for traumatic brain injury detection
Summary: Given the worldwide adverse impact of traumatic brain injury (TBI) on the human population, its diagnosis and prediction are of utmost importance. Recently, there has been a push toward using computationally expensive finite element (FE) models of the brain to create tissue deformation metrics of brain injury. Here, we develop a new brain injury metric, the brain angle metric (BAM), based on the dynamics of a 3 degree-of-freedom lumped parameter brain model. The brain model is built based on the measured natural frequencies of an FE brain model simulated with live human impact data. We show that it can be used to rapidly estimate peak brain strains experienced during head rotational accelerations that cause mild TBI. In our data set, the simplified model correlates with peak principal FE strain and fiber-oriented peak strain in the corpus callosum on a data set of head kinematics from 27 clinically diagnosed injuries and 887 non-injuries. Our brain model can be used to rapidly approximate the peak strain resulting from mild to moderate head impacts and to quickly assess brain injury risk.
New perspectives on protective equipment
Summary: Brain injury occurs either due to (1) focal trauma from concentrated forces causing skull fracture/deformation that impinges on the brain, or (2) diffuse trauma from acceleration causing inertial loading deep within the brain. Focal brain trauma is largely a solved problem due to ubiquitous helmet use as concentrated forces are effectively distributed and therapeutics are on the horizon for meningeal injury. However, diffuse inertial injury is still highly prevalent despite helmet use. Whether helmeted or not, millions of people suffer from mTBI each year, which could be prevented if we address a basic problem: the human neck is the weak link in the chain. Helmets attempt to reduce the contact force on the head but run up against practical and theoretical limits, ultimately requiring the neck to do the critical work to prevent high inertial accelerations and resulting diffuse injury. Among primates, humans have the smallest neck/head area ratio, rendering the relatively weak human neck ineffective at restraining head acceleration during collision. When exposed to high loading inputs, the 5kg human head will remain suscep- tible to inertial injury until the neck is augmented with a restraint system that can provide safe operation while not impeding normal voluntary motion. We are convinced that a neck restraint can address the problem of inertial brain injury. Using an acceleration-triggered restraint system, we have invented an approach to selectively restrain head motion by transferring force to the torso. Similar to the action of neck muscles, tension in the restraint system results in compressive force on the cervical spine which must be maintained within safe levels (4.5kN based on reports by the National Highway Traffic Safety Administration (NHTSA). Our invention can feasibly prevent diffuse brain trauma by achieving three design goals: (1) reduce amplitude and duration of acceleration impulse, (2) permit voluntary movement, and (3) maintain safe cervical spine compression loads.
