Alexandra J Golby md and Kathleen A McConnell md Introduction

Minimally invasive intracranial neurosurgery depends on the ability to localize accurately the operative target and its relationship to any neighboring eloquent areas. There are several methods of mapping cortical function, including the intracarotid amytal injection test (IAT or Wada test), positron emission tomography (PET), magnetoencephalography (MEG), direct electro-cortical stimulation (ECS), transcranial magnetic stimulation (TMS), and functional magnetic resonance imaging (fMRI). Some of these tests (PET, FMRI, and MEG) are activation techniques, meaning they demonstrate brain activation, which occurs when the subject performs a particular behavioral task. In contrast, the tests traditionally used for the determination of eloquent cortex (ECS, IAT) are essentially deactivation techniques, meaning that a temporary brain lesion is created pharmacologically or electrically and behavioral effects are measured. Deacti-vation studies are well suited to the pre-resection evaluation of cortical function because they mimic the effects of resection of the tissue; however, their use is limited by their invasiveness. Transcranial magnetic stimulation is a promising and relatively less invasive deactivation technique that can be performed through an intact skull.

Each brain mapping modality offers different strengths and weaknesses in terms of spatial and temporal resolution, repeatability, and invasiveness. Many of these techniques are relatively new and unexplored; however, they promise to offer far more options in terms of functional mapping to neurosurgeons and their patients. It is likely that the most accurate and useful information will eventually be gained by combining techniques that are complementary. For example, the excellent spatial resolution of fMRI can be integrated with the fine temporal resolution obtained with MEG to get more information than with either technique alone. Combining these techniques with frameless stereotactic neuronavigation and coregistration of datasets will allow the integration of data from different modalities. Not only will this provide useful complementary information but this effort will also allow the cross-validation of methods that depend on different physiologic signals.

From: Minimally Invasive Neurosurgery, edited by: M.R. Proctor and P.M. Black © Humana Press Inc., Totowa, NJ

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