gms | German Medical Science

In surgery near eloquent brain areas detailed knowledge about the topographic relation of the lesion to the functional brain area is crucial to avoid postoperative neurological deficits. While presurgical mapping of the sensorimotor cortex is possible on a routine basis in many centers with fMRI presurgical mapping of higher cognitive brain functions is more complex. We present our experience using a multimodal approach for the evaluation of memory and other higher cognitive functions using fMRI and MEG and its integration into the neurosurgical operation field.

We used a 2 x 37 channel biomagnetometer for MEG and a 1.5 T MR scanner with echo planar imaging for fMRI. For the localization of language related brain areas we used visual stimulation and a silent reading and silent naming task. In addition we developed a memory (encoding and recall) and calculation task. A third experiment is based on semantic decision making. These experiments were designed to stimulate activity from Broca's and Wernicke's area, from the angular and supramarginal gyrus and from memory related structures. This test battery was adjusted to the individual pathology and the concerned anatomical structures. After averaging data processing was done using the equivalent current dipole model (ECD) and a current density reconstruction algorithm based on spatial filtering (CLSF). For fMRI we used a block design experiment and processed linear correlation maps. All functional maps were superimposed onto anatomical MR data sets. The localization results were transferred to a neuronavigation system.

In total we evaluated language MEG and fMRI in 65 patients and used the activation maps intraoperatively. In cases where fMRI and MEG were both performed the localization results for language were congruent. However in the case of large tumors fMRI activation may be distorted and not congruent with MEG because of pathological tumor vascularization and thus may lead to false results. In the memory task during encoding we found activation in the posterior hippocampus. During recall in the memory task activation was found in the posterior and frontal hippocampus and parahippocampal gyrus. Latencies of memory activation ranged from 600 to 1400 ms. We also found activity after object recognition in fusiform gyrus after 200 ms. Corresponding activity in the calculation task could be localized in the supramarginal gyrus. All 50 patients underwent resective surgery or in cases where the tumor had infiltrated eloquent areas a biopsy was performed. No neurological deterioration was observed.

We have developed a test battery for the localization of higher cognitive functions for presurgical diagnostic evaluation and intraoperative visualization of these areas with functional neuronavigation. MEG and fMRI are both useful tools and give complementary information. Ideally by applying both methods in conjunction a comprehensive map of higher cognitive functions in patients with brain tumors can be achieved. Congruent activation in fMRI and MEG contributes to a higher degree of reliabilty in presurgical brain mapping.