H. Kober

Integration of functional magnetic resonance imaging supported by magnetoencephalography in functional neuronavigation

OBJECTIVE: In this study, the intraoperative visualization of functional data provided by functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) leading to functional neuronavigation is demonstrated in surgery around the motor strip. METHODS: In seven patients with lesions adjacent to the central region, fMRI was performed with a 1.5-Tesla magnetic resonance system, using axial echo-planar imaging with a motor and a sensory task. Somatosensory and motor evoked fields were recorded with a biomagnetometer. fMRI and MEG were matched to an anatomic three-dimensional magnetic resonance image set by a contour fit. Then this three-dimensional image data set was transferred to the navigation microscope and displayed in the eyepieces of the microscope during surgery. Additionally, intraoperative recording of somatosensory evoked potentials was performed for verification of the central sulcus. RESULTS: In all cases, the projection of fMRI and MEG data into the operating viewing field allowed easy identification of the central region, which was confirmed by phase reversal of somatosensory evoked potentials in each case. fMRI and MEG measurements yielded corresponding results in each patient. CONCLUSION: Functional neuronavigation with integration of fMRI and MEG allows the fast identification of eloquent brain areas. The widespread availability of fMRI will result in a broad availability of functional neuronavigation, which will, in turn, contribute to the successful surgery of lesions in eloquent brain areas with lower morbidity.

Intraoperative magnetic resonance imaging combined with neuronavigation: a new concept.

OBJECTIVE Intraoperative image data may be used not only to evaluate the extent of a tumor resection but also to update neuronavigation, compensating for brain shift. To date, however, intraoperative magnetic resonance imaging (MRI) can be combined only with navigation microscopes that are separated from the magnetic field, thus requiring time-consuming intraoperative patient transport. To help solve this problem, we investigated whether a new navigation microscope can be used within the fringe field of the MRI scanner. METHODS The navigation microscope was placed at the 5-G line of a 0.2 MRI device. Patients were positioned lying down directly on the table of the scanner, with their heads placed approximately 1.5 m from the center of the magnet, fixed in an MRI-compatible ceramic head holder. Standard operating instruments were used. For intraoperative imaging, we slid the table into the center of the magnet in less than 30 seconds. RESULTS By use of this setup, we operated on 22 patients. In all patients, anatomic neuronavigation could be used in combination with intraoperative MRI. In addition, in 12 patients, functional data from magnetoencephalographic or functional MRI studies were integrated, resulting in functional neuronavigation. We did not encounter adverse effects of the low magnetic field during navigation. Moreover, intraoperative imaging was not disturbed by the navigation microscope and vice versa. CONCLUSION Functional neuronavigation and intraoperative MRI can be used essentially simultaneously without the need for lengthy intraoperative patient transport. The combination of intraoperative imaging with functional neuronavigation offers the opportunity for more radical resections and fewer complications.

Correlation of Sensorimotor Activation with Functional Magnetic Resonance Imaging and Magnetoencephalography in Presurgical Functional Imaging: A Spatial Analysis

In this study we investigated the spatial heterotopy of MEG and fMRI localizations after sensory and motor stimulation tasks. Both methods are frequently used to study the topology of the primary and secondary motor cortex, as well as a tool for presurgical brain mapping. fMRI was performed with a 1.5T MR system, using echo-planar imaging with a motor and a sensory task. Somatosensory and motor evoked fields were recorded with a biomagnetometer. fMRI activation was determined with a cross-correlation analysis. MEG source localization was performed with a single equivalent current dipole model and a current density localization approach. Distances between MEG and fMRI activation sites were measured within the same anatomical 3-D-MR image set. The central region could be identified by MEG and fMRI in 33 of 34 cases. However, MEG and fMRI localization results showed significantly different activation sites for the motor and sensory task with a distance of 10 and 15 mm, respectively. This reflects the different neurophysiological mechanisms: direct neuronal current flow (MEG) and secondary changes in cerebral blood flow and oxygenation level of activated versus non activated brain structures (fMRI). The result of our study has clinical implications when MEG and fMRI localizations are used for pre- and intraoperative brain mapping. Although both modalities are useful for the estimation of the motor cortex, a single modality may err in the exact topographical labeling of the motor cortex. In some unclear cases a combination of both methods should be used in order to avoid neurological deficits.

Magnetic source imaging combined with image-guided frameless stereotaxy: A new method in surgery around the motor strip

OBJECTIVE In this study, information about the localization of the central sulcus obtained by magnetic source imaging (MSI) was intraoperatively translated to the brain, using frameless image-guided stereotaxy. In the past, the MSI results could be translated to the surgical space only by indirect methods (e.g., the comparison of the MSI results, displayed in surface renderings, with bony landmarks or blood vessels on the exposed brain surface). METHODS Somatosensory evoked fields were recorded with a MAGNES II biomagnetometer (Biomagnetic Technologies Inc., San Diego, CA). Using the single equivalent current dipole model, the localization of the somatosensory cortex was superimposed on magnetic resonance imaging with a self-developed contour fit program. The magnetic resonance image set containing the magnetoencephalographic dipole was then transferred to a frameless image-guided stereotactic system. Intraoperatively, the gyrus containing the dipole was identified as the postcentral gyrus, using neuronavigation, and the next anterior sulcus was regarded as the central sulcus. With intraoperative cortical recording of somatosensory evoked potentials, this assumption was verified in each case. RESULTS In all cases, the preoperatively assumed localization of the central sulcus and motor cortex with MSI agreed with the intraoperative identification of the central sulcus using the phase reversal technique. CONCLUSION The combined use of MSI and a frameless stereotactic system allows a fast orientation of eloquent brain areas during surgery. This may contribute to a safer and more radical surgery in lesions adjacent to the motor cortex.

New approach to localize speech relevant brain areas and hemispheric dominance using spatially filtered magnetoencephalography

We used a current localization by spatial filtering‐technique to determine primary language areas with magnetoencephalography (MEG) using a silent reading and a silent naming task. In all cases we could localize the sensory speech area (Wernicke) in the posterior part of the left superior temporal gyrus (Brodmann area 22) and the motor speech area (Broca) in the left inferior frontal gyrus (Brodmann area 44). Left hemispheric speech dominance was determined in all cases by a laterality index comparing the current source strength of the activated left side speech areas to their right side homologous. In 12 cases we found early Wernicke and later Broca activation corresponding to the Wernicke‐Geschwind model. In three cases, however, we also found early Broca activation indicating that speech‐related brain areas need not necessarily be activated sequentially but can also be activated simultaneously. Magnetoencephalography can be a potent tool for functional mapping of speech‐related brain areas in individuals, investigating the time‐course of brain activation, and identifying the speech dominant hemisphere. This may have implications for presurgical planning in epilepsy and brain tumor patients. Hum. Brain Mapping 14:236–250, 2001.

Motor, somatosensory and auditory cortex localization by fMRI and MEG

FUNCTIONAL magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were performed in six subjects during self-paced finger movement performance, tactile somatosensory stimulation and binaural auditory stimulation using identical stimulation paradigms. Both functional imaging modalities localized brain activity in adjacent areas of anatomically correct cortex. The mean distances measured between fMRI activity and the corresponding MEG dipoles were 10.1 mm (motor), 10.7 mm (somatosensory), 13.5 mm (auditory right hemisphere) and 14.3 mm (auditory left hemisphere). The distances found may reflect the correlation between electrophysiological and hemodynamic responses due to the different underlying substrates of neurophysiology measured by fMRI and MEG: BOLD contrast vs neuronal biomagnetic activity.

Sources of spontaneous slow waves associated with brain lesions, localized by using the MEG.

SummaryElectric or magnetic slow wave brain activity can be associated with brain lesions. For an accurate source localization we transformed the magnetoencephalographic (MEG) coordinate system to the magnetic resonance imaging (MRI) system by using a surface fit of the digitally measured head surface and the reconstructed surface of the MRI scan. Furthermore we solved the problem to separate sources of focal activity from other multiple sources by introducing a spatial average, the Dipole Density Plot (DDP). The DDP shows in a quantified manner concentrations of dipoles across time. The DDP uses the single dipole model adequately, because only those signal sections will be analyzed, where one component contributes to the signal predominantly. In all cases, where multiple sources concurrently active are to be localized, a current distribution analysis will be used, the Current Localization by Spatial Filtering (CLSF). All source localization procedures were tested using structural brain lesions, which were verified by imaging techniques (MRI or CT), showing the results in close topographical relation to the lesions. The results so far let us assume, that the DDP and the CLSF are valuable tools to localize sources of focal spontaneous slow wave electrical brain activity.

Localization analysis of neuronal activities in benign rolandic epilepsy using magnetoencephalography

Benign epilepsy of childhood with rolandic spikes (BECRS) is an electroclinical syndrome characterized by partial sensorimotor seizures with centrotemporal spikes. We report a detailed localization analysis of spontaneous magnetic brain activities in seven BECRS patients using magnetoencephalography (MEG). All patients had BECRS diagnosis with typical seizures and electroencephalographic findings and five patients had minor psychomotor deficits. MEG was recorded over both parieto-temporal regions using a 2x37-channel biomagnetic system. The collected data were digitally bandpass-filtered (2-6, 14-30, or 1-70 Hz) to analyze slow- and fast-wave magnetic activities and rolandic spikes. Slow-wave activity was increased in four hemispheres of three patients. Increased fast-wave activity was found in all five patients with minor neuropsychological deficits. The presence of increased fast-wave magnetic brain activity appeared to cause functional anomalies in the higher brain function processes. In the spike analysis, the dipoles of rolandic spikes which constantly manifested anterior positivity in direction were concentrated in the superior rolandic region in four cases and the inferior rolandic region in three cases. The localizations of increased slow- and fast-wave activities were identical with those of the spikes. The seizure profiles were frequently characterized by the spike locations. Source localizations of the focal brain activities and rolandic spikes by MEG will contribute to the different diagnosis and pathophysiological elucidation of BECRS.

A combined study of tumor-related brain lesions using MEG and proton MR spectroscopic imaging

The purpose of this study is to localize, in cases of brain tumors, pathological magnetic brain activities and to analyze metabolic alterations in functionally abnormal lesions using magnetoencephalography (MEG) and proton magnetic resonance spectroscopic imaging (1H MRSI). The study focused on 10 healthy volunteers and seven patients with common brain tumors, namely astrocytic tumor and meningioma. In spontaneous MEG, the pathological brain activities (slow, fast waves and spikes) were localized using a single equivalent dipole model. After the results of MEG and 1H MRSI were superimposed onto the corresponding MR images, the signal intensities of spectroscopically visible metabolites were analyzed in the regions where the dipoles of the pathological activities were concentrated. Increased slow wave activity was observed in four cases and fast wave or spike activity was significantly increased in one case. These pathological activities were localized in surrounding regions of the bulk of tumors, where mild reduction of N-acetyl aspartate (NAA) and slight accumulation of lactate (Lac) consistently existed. Preserved cortical areas, which are indicated by residual NAA, might be able to generate pathological magnetic activities under lactic acidosis. Such areas could be understood as a border zone between normal and seriously damaged brain tissue by tumors or associated brain edema. This combined technique with the different modalities gives insight into functional as well as metabolic aspects of pathological brain conditions.

Responses to silent Kanji reading of the native Japanese and German in task subtraction magnetoencephalography

The neuromagnetic activities evoked by semantic processing were localized by magnetoencephalography (MEG). We observed distinct time courses of the activities in native speaking Japanese subjects (Japanese speaker) and German subjects (German speaker) during silent reading of Japanese letters; Kanji and meaningless figures made by deforming the Arabian letters. There were significant differences in amplitude of the activities between Kanji and meaningless figure stimuli. The responses with meaningless figure stimuli were subtracted from those with Kanji stimuli to demonstrate the semantic responses. Earlier responses peaked at about 273.3+/-50. 8 and 245.0+/-23.8 ms (mean+/-S.D.) and were mainly located in the right fusiform gyrus (FuG) in the Japanese and German speakers, respectively. All the Japanese speakers constantly showed additional later responses in the left superior temporal gyrus (STG) and the supramarginal gyrus (SmG) at approximately 616.1+/-105.5 ms, whereas no further activity was observed in the German speakers who did not know the meaning of each Kanji. Because the later responses in the STG and SmG in the Japanese speakers were only observed in their dominant hemisphere, we believe the source of these responses to be part of the neural basis of Kanji semantic processing. The task subtraction MEG analysis could be a powerful method to discriminate distinct responses and visualize the neural networks involved in semantic processing.