Anatomy of an Explosion
In Iraq, an IED is often buried near a road or hidden in a car and then triggered remotely. Detonating the device sets off a chemical reaction in which anywhere from a few to hundreds of kilograms of explosive expel their energy in a microsecond, compressing the surrounding air into a powerful shock wave. The explosion can also produce an electromagnetic pulse, a wave of electric and magnetic fields that may cause surges in current and voltage. Though blasts and the resulting injuries have been part of warfare for a long time–after the Napoleonic wars, some speculated that people who mysteriously died near firing cannons were injured by excessive vibration in the air–little is known about exactly how a blast wreaks havoc on the brain. (Before newer types of body armor were available, soldiers exposed to blasts often died of lung injury when pressure waves ruptured air-filled tissue; so blast research has largely been concerned with the lungs rather than the brain.)
Most studies of concussion have focused on blunt trauma, as in a blow to the head, not the effects of blasts. To complicate matters, an explosion can cause multiple types of brain injury. For example, when Kinney’s Humvee was blown up, his brain endured the type of rapid acceleration and rotational forces typically seen in a car crash. Such forces, which can send the brain bouncing around inside the skull, can twist or tear axons–the long, thin fibers that connect nerve cells–and induce bleeding and swelling in the brain. But Kinney also felt the forces unique to blasts: the massive pressure wave, the electromagnetic pulse, and the light, heat, and sound from the explosion, all of which may ravage the brain in ways that haven’t been fully documented.
To better understand what a blast does to the brain, Raul Radovitzky, an associate professor of aeronautics and astronautics at MIT, and David F. Moore, a neurologist at Walter Reed Army Medical Center who has a doctorate in fluid dynamics, developed a software model incorporating both the physics of pressure waves and the variable properties of the brain’s tissues. Through magnetic resonance imaging, Moore modeled 11 features of the head, including the skull, the cerebrospinal fluid, the brain’s fluid-filled ventricles, the sinuses, the brain’s layer of white matter, and even the fat layer surrounding the eyes. The researchers used that information to create a computer model of the head, which they subjected to a simulated blast, observing how energy transferred from the air to the head affects the different structures. The model highlights the parts of the brain that endure the greatest stress and are thus most vulnerable to injury.
A movie of one simulation shows a rainbow-colored pressure wave propagating through a cross-sectional slice of the head, ricocheting off the skull, and rippling through the brain seemingly at random. So far, using values approximating a pressure wave that would damage the lungs, the model indicates that pressure from a blast far exceeds the minimum level thought to induce impact-related brain injuries. The researchers have also determined that tissue interfaces, such as the boundary between bone and brain, reflect the waves, so those areas are at greater risk. The pressure wave appears to enter the brain predominantly through the eyes and sinuses, and to a lesser extent through the skull, an observation that could influence the design of protective gear. Radovitzky and Moore are testing a new version of the model that includes a helmet, to evaluate how well it shields against the blast wave. “Blast protection for the head has not been a consideration in the design of body armor,” says Radovitzky. “Maybe a small change to the armor could mediate the damage.”