Technological advances in the last century have allowed us unprecedented access to brain structure and function. Structural imaging techniques such as skull X rays, computed tomography (CT), and magnetic resonance imaging (MRI) have proved immensely helpful in assessment of extent of brain injury and in following the medical sequelae of traumatic brain injury (TBI), such as edema, intracranial bleeding, and degeneration. These tools provide increasing detail about bone and tissue injury sustained in TBI and many other medical conditions. However, these methods cannot assess the "function" or underlying cerebral metabolic rate (CMR) and cerebral blood flow (CBF) in the brain. Subtle brain changes after traumatic brain injury (TBI), although sufficient to affect a patient's ability to function at a normal level, may not be visible on structural imaging. The majority of mild TBI patients have normal CT and MRI scans (for review, see Belanger et al. 2007). Functional imaging techniques promise to help elucidate brain injury in these particularly challenging cases.