Development of a miniaturized position sensing system for measuring brain motion during impact - biomed 2013

Since 2000, the Department of Defense has documented more than 253,000 cases of Traumatic Brain Injury (TBI). A significant portion of these injuries were attributed to explosive events, yet ninety-eight percent were non-penetrating. Understanding the response of the brain to blast events is critical, yet the mechanisms of brain injury from explosive trauma are poorly understood. This knowledge gap has led to an increased research focus on devices capable of investigating human brain response to non-penetrating, blast-induced loading. Furthermore, traumatic brain injury is a major issue for the civilian population as well with over 1.7 million cases of TBI per year in the US, primarily from falls and motor vehicle accidents. Current head surrogates and instrumentation are incapable of directly measuring critical parameters associated with TBI, such as brain motion, during dynamic loading. To this end, a novel sensor system for measuring brain motion inside of a human head surrogate was conceptualized and developed. The positioning system is comprised of a set of three fixed “generator” coils and a plurality of mobile, miniaturized “receiver” coil triads. Each generator coil transmits a sinusoidal electromagnetic signal at a unique frequency, and groups of three orthogonally arranged “receiver” coils detect these signals. Because of the oscillatory nature of these signals, the magnetic flux through the coil is always changing, allowing the application of Faraday’s Law of Induction and the point dipole model of an electric field to model the strength and direction of the field vector at any given point. Thus, the strength of the signal measured by a particular receiver coil depends on its position and orientation relative to the fixed position of the generators. These predictable changes are used to determine the six degrees of freedom (6-DOF) motion of the receiver. To calibrate and validate the system, a receiver coil was moved about in a controlled manner, and its actual position recorded by optical methods. Comparing the known position to the computed position at each time instance, a set of calibration constants were developed for each receiver triad. These constants were then utilized to convert receiver signal data into actual receiver position and orientation. Comparing this test case and several others like it, mean error was determined to be almost always less than 1.0 mm, and less than 0.5 mm >85% of the time. Additionally, high rate validation was conducted to confirm operation of the system in the impact domain. A coil was accelerated to approximately 15 m/sec along a fixed axis by ballistic impact and tracked by high speed video. The computed position was within 1 mm of the actual position 93% of the time and within 0.5 mm 83% of the time. The successful development and calibration of this sensing system now enables the direct measurements of brain displacement due to mechanical insults applied to a human head surrogate.