Functional Magnetic Resonance Imaging in Mild Traumatic Brain Injury

Abstract

Patients with concussion (mild traumatic brain injury (mTBI)) frequently complain of both cognitive and emotional disturbances in the days to weeks after their injury, with a percentage of patients (5-20 %) remaining chronically symptomatic. Relative to other static neuroimaging techniques, functional MRI (fMRI) offers great promise for elucidating the underlying neuropathology associated with dynamic processes such as higher-order cognition. Not surprisingly, the majority of mTBI studies have focused on working memory and attention, with results suggesting a complex relationship between cognitive load/attentional demand and functional activation. More recently researchers have used functional connectivity analyses to investigate how injury may affect intrinsic neuronal activation. Several groups have reported that connectivity within the default-mode network is disrupted following injury, which may also contribute to patient reports of increased distract-ibility. The general benefi ts and drawbacks of the two methods (evoked versus con-nectivity studies) are discussed in the context of the injury literature. Mood disturbances are also prevalent following concussion, but fewer studies (evoked or connectivity) have been conducted to investigate the integrity of the emotional processing network. Finally, fMRI can also be used as a surrogate biomarker of 250 pharmacological and cognitive rehabilitation treatment effi cacy, although only preliminary work has been conducted in this area to date. The chapter also discusses the methodological challenges of performing and evaluating fMRI research with brain-injured patients, including clinical heterogeneity in patient selection criteria and variations in scan time post-injury. Finally, the chapter concludes with a discussion of the physiological underpinnings of the blood oxygen level-dependent (BOLD) response and the many ways in which trauma can affect this complex signal. We conclude that the fMRI signal represents a complex fi lter through which researchers can more directly measure the physiological correlates of concussive symptoms, an important goal for this burgeoning fi eld.