BOLD functional MRI may overlook activation areas in the damaged brain
Clinical applications of blood-oxygenation-level-dependent contrast functional MRI (BOLD-fMRI) have been rapidly moving toward routine non-invasive cortical mapping in the patients with brain disorders. However, it is not yet clear whether the damaged brain shows same cerebral blood oxygenation (CBO) changes during neuronal activation as those in the normal adult. We compared the activation mapping obtained by BOLD-fMRI and the evoked-CBO changes measured by near infrared spectroscopy (NIRS) in normal adults (6 cases) and patients with damaged brain (6 cases of cerebral ischemia and 10 cases of brain tumors in or adjacent to the motor cortex). BOLD-fMRI demonstrated robust activation areas in the primary sensorimotor cortex (PSMC) during contralateral hand grasping tasks in all of the normal adults; however, in the cerebral ischemia (6 cases) and the brain tumors (2 cases), BOLD-fMRI demonstrated only limited activation areas in the PSMC on the lesion side during the task. NIRS demonstrated an increase of focal concentration of oxyhemoglobin and total hemoglobin at the PSMC during the task in all of the normal adults and the patients, indicating the presence of rCBF increase in response to neuronal activation. A focal concentration of deoxyhemoglobin decreased during the task in the normal adults, however, in the patients that showed limited activation areas by BOLD-fMRI, deoxyhemoglobin concentrations increased during the entire course of the task. In summary, the evoked-CBO changes occurring in the damaged brain differed from those in the normal brain. This indicates that BOLD-fMRI may overlook activation areas in the damaged brain.
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