A technique for in situ intracranial strain measurement within a helmeted deformable headform.

Despite the broad use of helmets, incidence of concussion remains high. Current methods for helmet evaluation focus on the measurement of head kinematics as the primary tool for quantifying risk of brain injury. Though the primary cause of mild Traumatic Brain Injury (mTBI) is thought to be intracranial strain, helmet testing methodologies are not able to directly resolve these parameters. Computational injury models and impact severity measures are currently used to approximate intracranial strains from head kinematics and predict injury outcomes. Advancing new methodologies that enable experimental intracranial strain measurements in a physical model would be useful in the evaluation of helmet performance. This study presents a proof-of-concept head surrogate and novel helmet evaluation platform that allows for the measurement of intracranial strain using high-speed X-ray digital image correlation (XDIC). In the present work, the head surrogate was subjected to a series of bare and helmeted impacts using a pneumatically-driven linear impactor. Impacts were captured at 5,000 fps using a high-speed X-ray cineradiography system, and strain fields were computed using digital image correlation. This test platform, once validated, will open the door to using brain tissue-level measurements to evaluate helmet performance, providing a tool that can be translated to represent mTBI injury mechanisms, benefiting the helmet design processes.

The influence of the tertiary bronchi on dynamic lung deformation.

Finite element models of thoracic injury often treat the lung as a bulk homogeneous and isotropic material, which reduces the computational costs associated with such investigations. Ignoring the heterogeneous structure of the lung may be computationally expedient, but this simplification may inadvertently fail to capture the true lung strain dynamics. In the present work, a series of direct impact experiments were performed on porcine lungs, inflated to a relevant expiratory pressure, and monitored using high-speed X-ray imaging. The lungs were instrumented with radiopaque markers within the parenchyma and tertiary bronchi to monitor the resulting deformation mechanics. The deformation mechanics demonstrate a high degree of strain localization related to the structural heterogeneity of the lung. The relative motion of the tertiary bronchi was measured during the impact event, and used to estimate the parenchyma tissue strains in the inter-bronchial regions. These were shown to exceed the trans-lobe strains by a factor 3 to 5 times higher in their tensile, compressive, and shear strain responses. Our results demonstrate that the lung parenchyma and bronchial tissues form a heterogeneous structure with a substantial stiffness differential that cannot be appropriately modelled as a homogeneous and isotropic monolithic mass without loss of accuracy and predictive relevance.

Intracranial Displacement Measurements Within Targeted Anatomical Regions of a Postmortem Human Surrogate Brain Subjected to Impact.

The dynamic response of the human brain subjected to impulsive loading conditions is of fundamental importance to the understanding of traumatic brain injuries. Due to the complexity of such measurements, the existing experimental datasets available to researchers are sparse. However, these measurements are used extensively in the validation of complex finite element models used in the design of protective equipment and the development of injury mitigation strategies. The primary objective of this study was to develop a comprehensive methodology to measure displacement in specific anatomical regions of the brain. A state-of-the-art high-speed cineradiography system was used to capture brain motion in post-mortem human surrogate specimens at a rate of 7500 fps. This paper describes the methodology used to capture these data and presents measurements from these tests. Two-dimensional displacement fields are presented and analyzed based on anatomical regions of the brain. These data demonstrated a multi-modal displacement response in several regions of the brain. The full response of the brain consisted of an elastic superposition of a series of bulk rotations of the brain about its centre of gravity. The displacement field could be linked directly to specific anatomical regions. The methods presented mark an improvement in temporal and spatial resolution of data collection, which has implications for our developing understanding of brain trauma.

A Parametric Analysis of Embedded Tissue Marker Properties and Their Effect on the Accuracy of Displacement Measurements.

Datasets obtained from cadaveric experimentation are broadly used in validating finite element models of head injury. Due to the complexity of such measurements in soft tissues, experimentalists have relied on tissue-embedded radiographic or sonomicrometry tracking markers to resolve tissue motion caused by impulsive loads. Dynamic coupling of markers with the surrounding tissue has been a previous concern, yet a thorough sensitivity investigation of marker influences on tissue deformation has not been broadly discussed. Technological improvements to measurement precision have bolstered confidence in acquired data; however, precision is often conflated with accuracy; the inclusion of markers in the tissue may alter its natural response, resulting in a loss of accuracy associated with an altered displacement field. To gain an understanding of how marker properties may influence the measured response to impact, we prepared a set of nine marker designs using a Taguchi L9 array to investigate marker design choice sensitivity. Each of these designs was cast into a block of tissue simulant and subjected to repeated drop tests. Vertical displacement was measured and compared to the response of the neat material, which contained massless tracking markers. Medium density and medium stiffness markers yielded the least deviation from the neat material response. The results provide some design guidelines indicating the importance of maintaining marker matrix density ratio below 1.75 and marker stiffness below 1.0 MPa. These properties may minimize marker interference in tissue deformation. Overall, embedded marker properties must be considered when measuring the dynamic response of tissue.