The most frequently used intraoperative brain mapping techniques are electrophysiological. These include electrocorticography for the identification of epileptic foci. Where cortical resections are indicated, evoked potential recordings with cortical electrodes are combined with peripheral nerve stimulation in anesthetized patients. Such an approach is frequently used to identify sensory and motor cortices. When more complex information is needed, particularly about language areas of the cortex, anesthesia is reversed and the patient is awakened during surgery. Direct electrical stimulation of the cortex is then used to reversibly disrupt local neuronal function while the patient performs behavioral tasks. In this setting, a patient must be psychologically able to accept the disturbing aspects of awake neurosurgery. Ideally, the examiner and the patient should have good rapport so that behavioral testing can proceed in an efficient and cooperative manner.
Optical intrinsic signal imaging, currently used in a research setting, may soon augment intraoperative electrophysiological measurements by directly providing visualization of cortical maps and functional responses seen ultimately through the operating microscope in real time. The high spatial and temporal resolution of this approach can be used to validate preoperative images of functional anatomy from individual patients. In addition, the functional brain maps can be updated as they become distorted from the preoperative state by changes in brain shape resulting from osmotic dehydration, ventricular
drainage, or local edema during the course of the operation.
As more interventional neuroradiology and neuro-surgery are performed within imaging devices, the reacquisition of structural and functional information as an invasive procedure unfolds will become possible and realistic. The most likely source of these data will be from MRI devices, where either patients are moved in and out of the device to update imaging data or the interventional procedure is performed within the magnetic field directly. In either case, direct updates of structural and, potentially, functional information can be provided to the clinical team performing the procedure.
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