Approximately 4.3 million (1.8%) adults and 750,000 (1.0%) children in the United States have been diagnosed with epilepsy or other seizure disorders; approximately one-third have seizures refractory to medical therapies.1,2 MRI is essential for the assessment of these patients with medically refractory epilepsy, as it allows identification of structural abnormalities which, when resected, may lead to seizure freedom. Task-based functional MRI (fMRI) has been shown to aid structural MRI in the evaluation of epilepsy by determining the relationship between epileptogenic lesions and the normal brain. It is being used more frequently in the presurgical evaluation of these patients, including the localization of motor and language function. Recently, several studies have used resting-state functional MRI (rs-fMRI) to describe cortical network alterations in patients with epilepsy.
Briefly, blood oxygen level–dependent (BOLD) fMRI detects neural activity by imaging changes in local oxygen metabolism in the brain. fMRI contrast relies on the different magnetic properties between diamagnetic oxygenated (HbO) and paramagnetic deoxygenated hemoglobin (HbR). During task-based fMRI, neural activity in the parts of the brain involved in performing the task causes a localized increase in oxygen consumption. However, cerebral blood flow increases within seconds, creating a surplus of HbO in these same regions. This results in an overall decrease in regional HbR concentration, a decrease in susceptibility effects, and an increase in the measured T2* BOLD signal.
Conversely, rs-fMRI evaluates the brain in the absence of a stimulus (or at rest) and studies synchronous fluctuations … more »