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Traumatic brain injury (TBI) is a major public health and socio economic problem in the United States and throughout the world. The US Centers for Disease Control and Prevention esti mates that approximately 1.7 million Americans sustain a TBI annually, resulting in 1.3 million visits to the emergency de partment, 275,000 hospitalizations, and more than 50,000 deaths related to TBI annually. 1 Clearly, TBI is a leading cause of death among young adults in the United States. These numbers underestimate the actual magnitude of TBI because the statis tics are unknown for those who do not seek medical care or who have no access to care. Moreover, long-term disability as a result of TBI is being increasingly recognized as a public health issue. It is estimated that at least 5.3 million Americans (up to 2% of the US population) suffer from long-term or lifelong disability as a result of TBI. 2

The common causes of TBI are falls, motor-vehicle crashes, bike accidents, sports-related injuries, assault and battery, and most recently blast injuries among soldiers returning from the Iraq and Afghanistan wars. 3 In Seattle, a city of motorcycle and bicycle enthusiasts, accidents are frequent and occasionally fatal among youth, despite mandatory bicycle-helmet-use laws in King County and Washington State. 4 The vast majority of sports-related TBIs are so-called mild traumatic brain injury (mTBI), or concussion. 5 Concussion presents unique features of TBI in which emotional and be havioral psychological symptoms persist in some of the victims. Conventional imaging does not usually demonstrate abnormality, and advanced imaging has been tested with var …