Defense Date

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Medical Physics

First Advisor

Kenneth Kraft

Abstract

Blast wave induced traumatic brain injury (bTBI) is a modality of injury that has come into prominence at the current time due to the large number of military and civilian personnel who have experienced the localized shock wave produced by explosive devices. The shock wave will travel concentrically outward from the explosive center, being absorbed and transmitted thru soft objects, such as tissue, and reflecting off stationary obstructions. Transmission and absorption in tissues can result in a number of physiological measureable injuries, the most common of which being what is frequently called “blast lung”. Blast lung involves the spalling effect at air-tissue interfaces. Another documented effect involves the asynchronous motion of tissue, particularly in the cranium, as the shock wave passes by. This predominately manifests itself in what is believed to be diffuse axonal injury and initiation of secondary injury mechanism. This study is designed to explore the relationship between shock waves and bTBI. A blast device was constructed for generating a free field shock wave through the high pressure rupture of a polycarbonate membrane. Air pressure in a small chamber is increased to a value several orders of magnitude greater than ambient air pressure and is held in place with the polycarbonate member. At the rupture of this membrane a shock wave is created. Measurements of this blast event, carried out with a piezoelectric pressure transducer, have shown that this shock wave is reproducible for the different membrane materials tested and is symmetrical with respect to the central axis of the high pressure chamber and exit nozzle. Having characterized the shock wave properties in the blast field, a location was chosen at which maximum shock wave pressure could be applied to the cranium for inducing bTBI. Experiments involving blast wave exposure were performed on two separate groups of animals in an attempt at establishing injury. One group was placed at a fixed distance directly below the blast nozzle, thereby experiencing both the shock wave and the associated air blast from the residual air in the chamber, and one placed at a defined distance off-axis to avoid the air blast, yet receiving two sequential blast exposures. All animal studies were approved by the VCU Institutional Animal Care and Use Committee. The degree of injury was then assessed with the use of magnetic resonance imaging (MRI) and spectroscopy (MRS). Image Data was acquired on a 2.4 Tesla magnet for assessing changes in either the total percent water concentration or the apparent diffusion coefficients (ADC) of selected regions of interest in the brain of rats. Localized proton spectroscopic data was acquired from a voxel placed centrally in the brain. The baseline values of these parameters were established before the induction of bTBI. After the blast exposure, the animals were followed up with MRI and MRS at defined intervals over a period of one week. The first group of animals received blast exposure directly underneath the blast device nozzle and the MR data does suggest changes in some of the measureable parameters from baseline following blast exposure. This blast wave data though is confounded with additional and undesirable characteristics of the blast wave. The second group of animals that received a pure shock wave blast exposure revealed no remarkable changes in the MR data pre- to post- blast exposure. The percent water concentration, ADC and spectroscopic parameters were for statistical purposes identical before and after the blast. The resolution of this negative result will require reconsideration of the free field blast exposure concept.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

May 2011

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