Intrasubject reliability and validity of somatosensory source localization using a large array biomagnetometer

Referral of touch and ownership between the hands and the role of the somatosensory cortices

Recent studies have shown that the feeling of body ownership can be fooled by simple visuo-tactile manipulations. Perceptual illusions have been reported in which participants sense phantom touch seen on a rubber hand (rubber hand illusion). While previous studies used homologous limbs for those experiments, we here examined an illusion where people feel phantom touch on a left rubber hand when they see it brushed simultaneously with brushes applied to their right hand. Thus, we investigated a referral of touch from the right to the left hand (across the body midline). Since it is known from animal studies that tactile illusions may alter early sensory processing, we expected a modulation of the primary somatosensory cortex (SI) corresponding to this illusion. Neuromagnetic source imaging of the functional topographic organization in SI showed a shift in left SI, associated with the strength of the referral of touch. Hence, we argue that SI seems to be closely associated with this perceptual illusion. The results suggest that the transfer of tactile information across the body midline could be mediated by neurons with bilateral tactile receptive fields (most likely BA2).

Decreased cortical somatosensory finger representation in X-linked recessive bulbospinal neuronopathy (Kennedy disease): a magnetoencephalographic study

BACKGROUND

Kennedy disease (KD) clinically presents as progressive lower motor neuron disease with minimal or no sensory impairment. However, electrophysiological studies found abnormal somatosensory-evoked potentials even in absence of clinical deficits. Little is known about possible influences of this sensory neuropathy on the central somatosensory processing.

METHODS

In this study, the cortical topography of index finger representation was studied in 7 patients with genetically proven KD compared to healthy control subjects by means of magnetoencephalography using an established stimulation paradigm. Data analysis was carried out with synthetic aperture magnetometry (SAM). Additionally, the latency and source amplitude of the earliest cortical somatosensory-evoked field (SEF) component were determined based on traditional single dipole source analysis.

RESULTS

In KD patients the latency of the SEF was prolonged (48.6 vs. 37.4 ms, P < .005). There was no significant difference in dipole source amplitude, but stimulus-related SAM activation of the contralateral sensorimotor cortex (pseudo-t-values -.107 vs. -.199, P < .05), including maximum activity (53.5%), was reduced.

CONCLUSIONS

These results implicate that even subclinical sensory neuropathy leads to possible functional reorganization of the sensorimotor cortex in KD patients and reinforces the view that in KD the somatosensory system is extensively involved.

Quantitative analysis of MEG using modified sLORETA for clinical application

OBJECTIVE

To determine whether standardised low-resolution brain electromagnetic tomography modified for a quantifiable method (sLORETA-qm) can be used for quantitative analysis in magnetoencephalography (MEG).

METHODS

Somatosensory evoked fields (SEFs) were obtained from 10 hemispheres of five healthy volunteers stimulated on the median nerve at 0.75, 1.0, 1.25, 1.5, 1.75 and 2.0 x threshold of thenar muscle twitch (TMT). N20 m intensity changes were analysed quantitatively using sLORETA-qm. Then, SEFs were measured with stimulation on the median nerve at 1.5 x TMT from 47 hemispheres in 24 subjects. sLORETA-qm intensity and the equivalent current dipole (ECD) moment of N20 m were calculated, and relationships between the values were evaluated.

RESULTS

sLORETA-qm intensity increased linearly with stimulus intensity between 0.75 and 1.5 x TMT, and tended to reach a plateau or decrease at higher stimulus intensities. The distribution of sLORETA-qm intensity after natural logarithmic transformation was normal and a close correlation was found between the ECD moment and sLORETA-qm intensity (r(s)=0.91, p<0.001).

CONCLUSIONS

The results of this study focusing on N20 m suggested that sLORETA-qm is reliable for quantitative analysis of MEG as well as ECD models.

SIGNIFICANCE

sLORETA-qm appears promising for quantitative analyses of MEG for which ECD models are inappropriate.

Combined EEG and MEG analysis of early somatosensory evoked activity in children and adolescents with focal epilepsies

OBJECTIVE

The study aimed to evaluate differences between EEG and MEG analysis of early somatosensory evoked activity in patients with focal epilepsies in localizing eloquent areas of the somatosensory cortex.

METHODS

Twenty-five patients (12 male, 13 female; age 4-25 years, mean 11.7 years) were included. Syndromes were classified as symptomatic in 17, idiopathic in 2 and cryptogenic in 6 cases. 10 patients presented with malformations of cortical development (MCD). 122 channel MEG and simultaneous 33-channel EEG were recorded during tactile stimulation of the thumb (sampling rate 769 Hz, band-pass 0.3-260 Hz). Forty-four hemispheres were analyzed. Hemispheres were classified as type I: normal (15), II: central structural lesion (16), III: no lesion, but central epileptic discharges (ED, 8), IV: lesion or ED outside the central region (5). Analysis of both sides including one normal and one type II or III hemisphere was possible in 15 patients. Recordings were repeated in 18 hemispheres overall. Averaged data segments were filtered (10-250 Hz) and analyzed off-line with BESA. Latencies and amplitudes of N20 and P30 were analyzed. A regional source was fitted for localizing S1 by MRI co-registration. Orientation of EEG N20 was calculated from a single dipole model.

RESULTS

EEG and MEG lead to comparable good results in all normal hemispheres. Only EEG detected N20/P30 in 3 hemispheres of types II/III while MEG showed no signal. N20 dipoles had a more radial orientation in these cases. MEG added information in one hemisphere, when EEG source analysis of a clear N20 was not possible because of a low signal-to-noise ratio. Overall N20 dipoles had a more radial orientation in type II when compared to type I hemispheres (p=0.01). Further N20/P30 parameters (amplitudes, latencies, localization related to central sulcus) showed no significant differences between affected and normal hemispheres. Early somatosensory evoked activity was preserved within the visible lesion in 5 of the 10 patients with MCD.

CONCLUSIONS

MEG should be combined with EEG when analyzing tactile evoked activities in hemispheres with a central structural lesion or ED focus.

SIGNIFICANCE

At time, MEG analysis is frequently applied without simultaneous EEG. Our results clearly show that EEG may be superior under specific circumstances and combination is necessary when analyzing activity from anatomically altered cortex.

Reproducibility of measures of neurophysiological activity in Wernicke's area: a magnetic source imaging study

OBJECTIVE

The purpose of this study was to evaluate the reproducibility of estimates of neurophysiological activity obtained with Magnetic Source Imaging.

METHODS

Split-half data sets were obtained from 14 healthy volunteers during performance of a continuous recognition task for spoken words. The concurrent validity of spatiotemporal activation maps obtained with this task has been previously verified through comparisons with the Wada test and electrocortical stimulation mapping. Consecutive late activity sources (> 200 ms after stimulus onset) were modeled independently as equivalent current dipoles (ECDs) and used to identify the location of language-specific cortex in the left hemisphere (Wernicke's area).

RESULTS

Linear displacement of the geometric center of the cluster of ECDs in this region ranged between 2 and 8 mm across subjects. Intraparticipant variability (range) in the onset latency of activity was +/-50 ms, while the range of change in global field power for the entire set of ECDs in Wernicke's area was less than 17% in all cases.

CONCLUSIONS

The results indicate that despite its many conceptual limitations the ECD model can provide reliable estimates of regional cortical activity associated with the engagement of linguistic processes.

SIGNIFICANCE

The results highlight the need for reproducibility studies when research questions pose particular requirements for precision of estimates of regional neurophysiological activity.

Localization of the human female breast in primary somatosensory cortex

RATIONALE

Despite an extensive body of research on the topography of the primary somatosensory cortex (S1) little is known about the representation of the trunk.

AIM

The aim of this study was to determine the representation of the breast in S1 in human females.

RESULTS

The representation of the human breast in primary somatosensory cortex was determined in ten healthy female subjects. Non-painful electrical stimulation of the mammilla (Th4 dermatome), groin (L1 dermatome) and the first digit of both sides of the body activated cutaneous receptors and thus elicited somatosensory evoked potentials. The representation of these body parts in primary somatosensory cortex (S1) was determined using neuroelectric source imaging. Equivalent current dipole localizations were overlaid with individual structural magnetic resonance images to account for individual cortical differences. The breast representation was localized between the representation of the groin and the first digit. In the medial-lateral direction the representation of the breast was approximately 15 mm lateral of the longitudinal fissure in the contralateral hemisphere. Source localizations were stable across subjects. However, one subject showed ipsilateral representation of the breast, which might be related to bilateral receptive fields of the ventral body midline representation. This study confirms the Penfield and Rasmussen (1950) invasive data by use of noninvasive source imaging.

Magnetoencephalography (MEG) and magnetic source imaging (MSI)

BACKGROUND

Real-time, direct assessment of brain electrophysiology is critical for noninvasive functional mapping and for the identification of paroxysmal epileptiform abnormalities in the evaluation of patients for epilepsy surgery. Historically, electroencephalography (EEG) and evoked potentials (EPs) have performed these functions. However, both often required direct intracranial recording for precise localization. Magnetoencephalography (MEG) takes advantage of the fact that neuromagnetic signals penetrate the skull and scalp without distortion. The magnetic source image (MSI) is created when the MEG data is superimposed on a magnetic resonance image (MRI).

REVIEW SUMMARY

MEG performs noninvasive functional imaging by recording the magnetic flux on the head surface associated with electrical currents in activated sets of neurons. MEG has rapidly evolved in the last 2 decades because of the introduction of whole head systems and advances in computer technology. MEG is now the imaging modality of choice where a precise and high degree of localization is required. MEG can be used to localize the primary sensory cortices (visual, auditory, or somatosensory), areas involved with receptive language function, the irritative zone in epilepsy patients, and identify children with anomalous language development. This article reviews the basis of MEG, the instrumentation used, the clinical applications and current limits of the technology.

CONCLUSION

MEG studies can now be performed on a routine basis as a clinical tool. MEG is now indicated for: 1) localization of the irritative zone in lesional and nonlesional epilepsy surgery patients, 2) functional mapping of eloquent cortex, and 3) assessment of normal and abnormal language development. In the future MEG may help the understanding of normal development and reorganization after brain injury. The neurologist can use MEG data to complement structural and metabolic imaging techniques.

An integrative MEG-fMRI study of the primary somatosensory cortex using cross-modal correspondence analysis

We develop a novel approach of cross-modal correspondence analysis (CMCA) to address whether brain activities observed in magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) represent a common neuronal subpopulation, and if so, which frequency band obtained by MEG best fits the common brain areas. Fourteen adults were investigated by whole-head MEG using a single equivalent current dipole (ECD) and synthetic aperture magnetometry (SAM) approaches and by fMRI at 1.5 T using linear time-invariant modeling to generate statistical maps. The same somatosensory stimulus sequences consisting of tactile impulses to the right sided: digit 1, digit 4 and lower lip were used in both neuroimaging modalities. To evaluate the reproducibility of MEG and fMRI results, one subject was measured repeatedly. Despite different MEG dipole locations and locations of maximum activation in SAM and fMRI, CMCA revealed a common subpopulation of the primary somatosensory cortex, which displays a clear homuncular organization. MEG activity in the frequency range between 30 and 60 Hz, followed by the ranges of 20-30 and 60-100 Hz, explained best the defined subrepresentation given by both MEG and fMRI. These findings have important implications for improving and understanding of the biophysics underlying both neuroimaging techniques, and for determining the best strategy to combine MEG and fMRI data to study the spatiotemporal nature of brain activity.

Hemodynamic evoked response of the sensorimotor cortex measured noninvasively with near-infrared optical imaging

We have performed a noninvasive bilateral optical imaging study of the hemodynamic evoked response to unilateral finger opposition task, finger tactile, and electrical median nerve stimulation in the human sensorimotor cortex. This optical study shows the hemoglobin-evoked response to voluntary and nonvoluntary stimuli. We performed measurements on 10 healthy volunteers using block paradigms for motor, sensory, and electrical stimulations of the right and left hands separately. We analyzed the spatial/temporal features and the amplitude of the optical signal induced by cerebral activation during these three paradigms. We consistently found an increase (decrease) in the cerebral concentration of oxy-hemoglobin (deoxy-hemoglobin) at the cortical side contralateral to the stimulated side. We observed an optical response to activation that was larger in size and amplitude during voluntary motor task compared to the other two stimulations. The ipsilateral response was consistently smaller than the contralateral response, and even reversed (i.e., a decrease in oxy-hemoglobin, and an increase in deoxy-hemoglobin) in the case of the electrical stimulation. We observed a systemic contribution to the optical signal from the increase in the heart rate increase during stimulation, and we made a first attempt to subtract it from the evoked hemoglobin signal. Our findings based on optical imaging are in agreement with results in the literature obtained with positron emission tomography and functional magnetic resonance imaging.

Somatosensory Representation of the Digits and Clinical Performance in Patients with Focal Hand Dystonia

OBJECTIVE

The purpose of this study was to incorporate magnetoencephalography and clinical testing to describe differences in somatosensory organization and sensorimotor function of the hand in patients with focal hand dystonia, a target-specific disorder of voluntary movement that interferes with fine motor control during the performance of rapid, repetitive, skilled movements.

DESIGN

This descriptive study included prospective, quasi-experimental comparisons between groups.

RESULTS

Patients with focal hand dystonia demonstrated deficits in physical variables, sensory processing, and motor control when compared with age- and sex-matched controls. They also had altered patterns of firing (amplitude and latency integrated over time) and abnormal somatosensory representations on magnetoencephalography.

CONCLUSIONS

These study findings suggest that there are alterations in both somatosensory representation of the digits and clinical performance in patients with focal hand dystonia. Future studies to determine if alterations in the sensorimotor feedback loop contribute to the development of focal hand dystonia are indicated. If so, intervention strategies may need to include specific types of somatosensory retraining as part of the rehabilitation program for patients with focal hand dystonia.

Preoperative magnetic source imaging for brain tumor surgery: a quantitative comparison with intraoperative sensory and motor mapping

Object

The aim of this study was to compare quantitatively the methods of preoperative magnetic source (MS) imaging and intraoperative electrophysiological cortical mapping (ECM) in the localization of sensorimotor cortex in patients with intraaxial brain tumors.

Methods

Preoperative magnetoencephalography (MEG) was performed while patients received painless tactile somatosensory stimulation of the lip, hand, and foot. The early somatosensory evoked field was modeled using a single equivalent current dipole approach to estimate the spatial source of the response. Three-dimensional magnetic resonance image volume data sets with fiducials were coregistered with the MEG recordings to form the MS image. These individualized functional brain maps were integrated into a neuronavigation system. Intraoperative mapping of somatosensory and/or motor cortex was performed and sites were compared.

In two subgroups of patients we compared intraoperative somatosensory and motor stimulation sites with MS imaging–based somatosensory localizations. Mediolateral projection of the MS imaging source localizations to the cortical surface reduced systematic intermodality discrepancies. The distance between two corresponding points determined using MS imaging and ECM was 12.5 ± 1.3 mm for somatosensory–somatosensory and 19 ± 1.3 mm for somatosensory–motor comparisons. The observed 6.5 mm increase in site separation was systematically demonstrated in the anteroposterior direction, as expected from actual anatomy. In fact, intraoperative sites at which stimulation evoked the same patient response exhibited a spatial variation of 10.7 ± 0.7 mm.

Conclusions

Preoperative MS imaging and intraoperative ECM show a favorable degree of quantitative correlation. Thus, MS imaging can be considered a valuable and accurate planning adjunct in the treatment of patients with intraaxial brain tumors.

Magnetoencephalography in stroke: a 1-year follow-up study

Recovery after stroke is closely linked to cerebral plasticity. Magnetoencephalography (MEG) is a non-invasive technique, which allows location of cerebral cells activities. In the present work, a cohort of patients has been studied with MEG. Twelve patients with a recent ischemic or hemorragic stroke were included as soon as possible after onset of stroke. Neurologic assessment, including standard neurologic examination, functional independence measure (FIM) and Orgogozo's scale was performed for 1 year in addition to a study of the somatosensory evoked field (SEF) using a 37-channel Biomagnetometer system. No response could be recorded in five patients at the first SEF exploration. In three cases, no response was ever recorded during the study. All these patients had a bad recovery. The location of the SEF sources was always in the normal non-infarcted cortex of the postcentral gyrus. Sensory recovery seemed to be linked to the reorganization of the persistent functional cortex, which was a limiting factor for recovery. These observations confirm the experimental results obtained in animal models. After stroke it can be assumed that in the case of incomplete lesion, an intensive sensory peripheral stimulation could maximize the use of residual sensory function and then contribute to improve the sensory deficit. In case of total sensory loss other techniques have to be used, such as visual monitoring of hand activity in order to improve hand function.

Consistency of interictal and ictal onset localization using magnetoencephalography in patients with partial epilepsy

OBJECT

The aim of this study was to evaluate the spatial accuracy of interictal magnetoencephalography (MEG) in localizing the primary epileptogenic focus in comparison with alternative MEG-derived estimates such as ictal onset recording or sensory mapping of the periphery where seizures manifest.

METHODS

During this retrospective study of 12 patients with epilepsy who had undergone successful magnetic source (MS) imaging with the aid of a dual 37-channel biomagnetometer as well as simultaneous MEG/electroencephalography (EEG) recordings, ictal events were observed in five patients and quantitative comparisons of interictal spike and ictal seizure onset source localizations were made. In the eight patients who had presented with sensorimotor seizure, source localization of cortical sites concordant with seizure foci was determined using somatosensory functional mapping, and the results were quantitatively compared with interictal spike source localizations. Interictal spike sources demonstrated on MEG localized to the same region as the corresponding ictal event or somatosensory source localizations. The mean distance between the ictal foci and interictal spike sources was 1.1 +/- 0.3 cm. Results of functional somatosensory mapping in patients with sensorimotor seizures demonstrated that seizure sources consistently colocalized with interictal MEG spike sources, with a mean distance of 1.5 +/- 0.4 cm. No systematic directional bias was observed. Interictal sources tended to be tightly clustered, and the mean ellipsoid volume, defined by one standard deviation of the source spatial coordinates, was 1 cm3.

CONCLUSIONS

Interictal spike localizations on MEG were concordant with ictal and, where relevant, functional somatosensory mapping localizations. These findings support the interpretation of interictal spikes on MEG as a useful and effective noninvasive method for localizing primary seizure foci.

Preoperative magnetic source imaging for brain tumor surgery: a quantitative comparison with intraoperative sensory and motor mapping

OBJECT

The aim of this study was to compare quantitatively the methods of preoperative magnetic source (MS) imaging and intraoperative electrophysiological cortical mapping (ECM) in the localization of sensorimotor cortex in patients with intraaxial brain tumors.

METHODS

Preoperative magnetoencephalography (MEG) was performed while patients received painless tactile somatosensory stimulation of the lip, hand, and foot. The early somatosensory evoked field was modeled using a single equivalent current dipole approach to estimate the spatial source of the response. Three-dimensional magnetic resonance image volume data sets with fiducials were coregistered with the MEG recordings to form the MS image. These individualized functional brain maps were integrated into a neuronavigation system. Intraoperative mapping of somatosensory and/or motor cortex was performed and sites were compared. In two subgroups of patients we compared intraoperative somatosensory and motor stimulation sites with MS imaging-based somatosensory localizations. Mediolateral projection of the MS imaging source localizations to the cortical surface reduced systematic intermodality discrepancies. The distance between two corresponding points determined using MS imaging and ECM was 12.5 +/- 1.3 mm for somatosensory-somatosensory and 19 +/- 1.3 mm for somatosensory-motor comparisons. The observed 6.5 mm increase in site separation was systematically demonstrated in the anteroposterior direction, as expected from actual anatomy. In fact, intraoperative sites at which stimulation evoked the same patient response exhibited a spatial variation of 10.7 +/- 0.7 mm.

CONCLUSIONS

Preoperative MS imaging and intraoperative ECM show a favorable degree of quantitative correlation. Thus, MS imaging can be considered a valuable and accurate planning adjunct in the treatment of patients with intraaxial brain tumors.

Paradoxical lateralization of parasagittal spikes revealed by back averaging of EEG and MEG in a case with epilepsia partialis continua

Our aim was to localize the generator site of parasagittal epileptiform discharges in a patient with epilepsia partialis continua (EPC) in the right leg. We examined a 32-year-old woman with EPC whose conventional EEG did not show any epileptic discharge. We performed the jerk-locked back averaging (JLA) of EEG and magnetoencephalography (MEG) to localize the dipole source of sharp transients. The myoclonic discharges in the right soleus muscle were used as a trigger pulse. JLA revealed consistent EEG and MEG sharp transients that coincided consistently and constantly preceded the myoclonic jerks. JLA of EEG demonstrated sharp waves paradoxically distributed over the vertex and right hemisphere. However, the estimated dipoles of MEG were localized in a restricted area in the primary leg motor area in the left hemisphere, which was closely located in the abnormal lesion on the brain MRI. JLA of MEG is considered to be a useful non-invasive method for localizing the epileptogenic area in EPC even when paradoxical lateralization of electroencephalographic discharges was noted.

Somatotopic organization of the ventral and dorsal finger surface representations in human primary sensory cortex evaluated by magnetoencephalography

Cortical reorganization of the subtly differentiated hand map after peripheral nerve injury might be better understood if there was a topographic conception of the homuncular representation of the dorsal finger surfaces in humans, in addition to the well-established sequential rostrocaudal array of the ventral finger aspects in cortical area 3b. In the present magnetoencephalographic study, tactile pneumatic stimulation was delivered to the fingertip and to the ventral and dorsal proximal phalanx of each digit of the dominant hand in 20 right-handed volunteers. Source localization of equivalent current dipoles underlying the recorded somatosensory evoked magnetic field was performed using a Cartesian coordinate system established by the anatomical landmarks nasion and preauricular points. Of the first major peak of each somatosensory evoked field, the region with the maximum field power (root-mean-square across channels) was selected for source reconstruction. Analysis of variance for repeated measures yielded significant results with respect to the arrangement of digits along the vertical coordinate axis, demonstrating a sequential array from the most inferiorly located D1 to the most superiorly located D5 for all different stimulus positions. This is the first study providing evidence for a sequential topographical arrangement of not only the ventral but also the dorsal surface representations of the individual digits in the human somatosensory cortex. The study contributes to a better understanding of the somatosensory hand representation in human primary cortex and provides useful information with regard to cortical plasticity studies in patients with peripheral nerve injuries at the upper extremity.

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.

Brain plasticity for sensory and linguistic functions: a functional imaging study using magnetoencephalography with children and young adults

In this report, the newest of the functional imaging methods, magnetoencephalography, is described, and its use in addressing the issue of brain reorganization for basic sensory and linguistic functions is documented in a series of 10 children and young adults. These patients presented with a wide variety of conditions, ranging from tumors and focal epilepsy to reading disability. In all cases, clear evidence of reorganization of the brain mechanisms of either somatosensory or linguistic functions or both was obtained, demonstrating the utility of magnetoencephalography in studying, completely noninvasively, the issue of plasticity in the developing brain.

Magnetoencephalographic investigation of somatosensory homunculus in patients with peri-Rolandic tumors

In order to investigate functional topography of the hand somatosensory cortex in five patients with peri-Rolandic tumors (four frontal lobes and one parietal lobe), we recorded somatosensory evoked fields (SEFs) using magnetoencephalography (MEG) after stimulation of the median nerve (MN) and the five digits. The results obtained were compared with those of five normal healthy subjects. In all five patients, SEFs following MN and digit stimulation showed the previously described respective N20m and N22m components of primary sensory response. Single dipole modeling was applied to determine the three dimensional cortical representations of the N20m and N22m components. The cortical representations of the hand were identical to those of normal subjects, arranging in an orderly somatotopic way from lateral inferior to medial superior in the sequence thumb, MN, index, middle, ring, and little fingers. This sensory homunculus was confirmed by cortical recording of the somatosensory evoked potentials (SEPs) at the time of surgery. Thus, we demonstrate that SEFs, recorded on MEG in conjunction with source localization techniques, are useful to non-invasively investigate the functional topography of the human hand somatosensory cortex in pathological conditions.

Tracking functions of cortical networks on a millisecond timescale

The human cerebral cortex, consisting of six layers and billions of neurons and synapses, processes sensory input from numerous sensory receptors. Noninvasive magnetoencephalographic (MEG) recordings provide a view through the skull to electrophysiological signals of the cortex on a millisecond timescale. For example, magnetic somatosensory evoked fields (SEFs) to a given peripheral somatosensory stimuli, reflect sequential activation of an extensive cortical network. Several cortical areas contributing to the SEFs can be evaluated in time and space by using source modeling. This brief review focuses on MEG studies of the human somatosensory networks with a special emphasis on tactile stimulation.

The somatosensory evoked magnetic fields

Averaged magnetoencephalography (MEG) following somatosensory stimulation, somatosensory evoked magnetic field(s) (SEF), in humans are reviewed. The equivalent current dipole(s) (ECD) of the primary and the following middle-latency components of SEF following electrical stimulation within 80-100 ms are estimated in area 3b of the primary somatosensory cortex (SI), the posterior bank of the central sulcus, in the hemisphere contralateral to the stimulated site. Their sites are generally compatible with the homunculus which was reported by Penfield using direct cortical stimulation during surgery. SEF to passive finger movement is generated in area 3a or 2 of SI, unlike with electrical stimulation. Long-latency components with peaks of approximately 80-120 ms are recorded in the bilateral hemispheres and their ECD are estimated in the secondary somatosensory cortex (SII) in the bilateral hemispheres. We also summarized (1) the gating effects on SEF by interference tactile stimulation or movement applied to the stimulus site, (2) clinical applications of SEF in the fields of neurosurgery and neurology and (3) cortical plasticity (reorganization) of the SI. SEF specific to painful stimulation is also recorded following painful stimulation by CO(2) laser beam. Pain-specific components are recorded over 150 ms after the stimulus and their ECD are estimated in the bilateral SII and the limbic system. We introduced a newly-developed multi (12)-channel gradiometer system with the smallest and highest quality superconducting quantum interference device (micro-SQUID) available to non-invasively detect the magnetic fields of a human peripheral nerve. Clear nerve action fields (NAFs) were consistently recorded from all subjects.

Magnetic source imaging and brain surgery: presurgical and intraoperative planning in 26 patients

OBJECT

The availability of large-array biomagnetometers has led to advances in magnetoencephalography that permit scientists and clinicians to map selected brain functions onto magnetic resonance images. This merging of technologies is termed magnetic source (MS) imaging. The present study was undertaken to assess the role of MS imaging for the guidance of presurgical planning and intraoperative neurosurgical technique used in patients with intracranial mass lesions.

METHODS

Twenty-six patients with intracranial mass lesions underwent a medical evaluation consisting of MS imaging, a clinical history, a neurological examination, and assessment with the Karnofsky Performance Scale. Magnetic source imaging was used to locate the somatosensory cortex in 25 patients, the visual cortex in six, and the auditory cortex in four. The distance between the lesion and the functional cortex was determined for each patient. Twenty-one patients underwent a neurosurgical procedure. As a surgical adjunct, a frameless stereotactic navigational system was used in 17 cases and a standard stereotactic apparatus in four cases. Because of the results of their MS imaging examination, two patients were not offered surgery, four underwent a stereotactic biopsy procedure, 10 were treated with a subtotal surgical resection, and seven were treated with complete surgical resection. One patient deteriorated before a procedure could be scheduled and, therefore, was not offered surgery, and two patients were offered surgery but declined. Three patients experienced surgery-related complications.

CONCLUSIONS

Magnetic source imaging is an important noninvasive neurodiagnostic tool that provides critical information regarding the spatial relationship of a brain lesion to functional cortex. By providing this information, MS imaging facilitates a minimum-risk management strategy and helps guide operative neurosurgical technique in patients with intracranial mass lesions.

Cortical asymmetries of the human somatosensory hand representation in right- and left-handers

Hemispheric asymmetry is known for higher brain functions like language and attention. We tested whether such an asymmetry also exists in the representation of elementary sensory functions. Magnetic source imaging was used to compare the cortical somatosensory hand representation in seven right- and five left-handed individuals. In all right-handers the representation of the dominant hand was larger than the contralateral one in the corresponding hemispheres. In contrast, only two out of five left-handers revealed a larger representation of the dominant left hand compared to the right one. In agreement with previous findings on the lateralization of language and attention, there is a strong correlation between handedness and the extent of the cortical hand representation in right-, but not in left-handers. We conclude that a profound functional hemispheric asymmetry also exists in primary sensory cortices.

Functional neuronavigation with magnetoencephalography: outcome in 50 patients with lesions around the motor cortex

OBJECT

The authors conducted a study to evaluate the clinical outcome in 50 patients with lesions around the motor cortex who underwent surgery in which functional neuronavigation was performed.

METHODS

The sensorimotor cortex was identified in all patients with the use of magnetoencephalography (MEG). The MEG-source localizations were superimposed onto a three-dimensional magnetic resonance image and the image data set was implemented into a neuronavigation system. Based on this setup, the surgeon chose the best surgical strategy. During surgery, the pre- and postcentral gyri were identified by neuronavigation and, in addition, the central sulcus was localized using intraoperative recording of somatosensory evoked potentials. In all cases MEG localizations of the sensory or motor cortex were correct. In 30% of the patients preoperative paresis improved, in 66% no additional deficits occurred, and in only 4% (two patients) deterioration of neurological function occurred. In one of these patients the deterioration was not related to the procedure.

CONCLUSIONS

The method of incorporating functional data into neuronavigation systems is a promising tool that can be used in more radical surgery to lessen morbidity around eloquent brain areas.

Functional neuronavigation with magnetoencephalography: outcome in 50 patients with lesions around the motor cortex

The authors conducted a study to evaluate the clinical outcome in 50 patients with lesions around the motor cortex who underwent surgery in which functional neuronavigation was performed.

The sensorimotor cortex was identified in all patients with the use of magnetoencephalography (MEG). The MEG-source localizations were superimposed onto a three-dimensional magnetic resonance image, and the image data set was then implemented into a neuronavigation system. Based on this setup, the surgeon chose the best surgical strategy. During surgery, the pre- and postcentral gyrus were identified by neuronavigation, and in addition, the central sulcus was localized using intraoperative recording of somatosensory evoked potentials. In all cases MEG localizations of the sensory or motor cortex were correct. In 30% of the patients preoperative paresis improved, in 66% no additional deficits occurred, and in only 4% (two patients) deterioration of neurological function occurred. In one of these patients the deterioration was not related to the method.

The method of incorporating functional data into neuronavigation systems is a promising tool that can be used in more radical surgery to cause less morbidity around eloquent brain areas.

[High-resolution magnetoencephalography--studies with a small-volume phantom]

To investigate the spatiotemporal organisation of neuronal processes in an animal model using magnetoencephalography (MEG), a high temporal resolution (ms) and an appropriate spatial resolution of about 1 mm is necessary. With the aim of determining the localization error and the resolution power of high-resolution MEG systems, we developed a phantom capable of simulating the characteristics of animal models. The phantom enables us to variably position at least two magnetic field sources to within 0.1 mm. For source localization on the basis of the magnetic field data, a spatial filtering algorithm was used. The investigation of a 16-channel micro SQUID-MEG system with a current dipole orientated tangentially to the phantom surface produced the following localization data (min ... max, x, y--horizontal plane, z--depth); systematic localization error e(x) = 1.16 ... 1.67 mm, e(y) = -1.01 ... -1.28 mm, e(z) = -5.22 ... -7.64 mm, standard deviation of the individual measurements perpendicular to the dipole axis s(perp) = 0.05 ... 0.22 mm, along this axis s(long) = 0.20 ... 1.73 mm, in the depths sz = 0.17 ... 3.17 mm. The "goodness of fit" was > 95%. Separation of two dipoles was still possible for parallel dipoles at a distance apart of d(parallel) = 0.03 mm and for those oriented perpendicularly to each other at a distance apart of d(perp) = 0.10 mm. On the basis of these results we conclude that the MEG system can achieve a resolution sufficient to permit the investigation of neuronal microstructures. The spatial errors detected were related to sensor position in the cryostatic vessel as well as to external low-frequency noise.

Estimation of the accuracy of a surface matching technique for registration of EEG and MRI data

OBJECTIVES

We developed a method to register EEG and MRI data used for the source reconstruction of electric brain activity.

METHODS

The method is based on matching of the head surfaces as obtained by 3D scanning after the EEG recording, and by segmentation of MRI data. The registration accuracy was estimated by calculating the residual error of the surface matching and its intra-individual and inter-individual variability. In addition, the test-retest reliability concerning the transformation of electrode positions was studied, to estimate how inaccuracies resulting from the 3D scanning of the head surface translate into registration uncertainty.

RESULTS

For 61 measurements, performed on 20 subjects, the average root mean square of the Euclidean distances between the 3D-scanned and the MRI-derived head surfaces amounted to 3.4 mm. An inter-individual standard deviation of 0.24 mm, and an intraindividual standard deviation of 0.003-0.31 mm proved a high inter- and intra-subject stability of the surface matching technique. The variation of transformation results when studying the test-retest reliability amounted to 1.6 mm on average. The maximum error of transformation was smaller than the diameter of the electrodes.

CONCLUSIONS

The findings suggest that the surface matching technique is a precise method for determination of the transformation of electrode positions and MRI data into a single co-ordinate system and can successfully be used in a routine laboratory setting.

Somatosensory homunculus as drawn by MEG

We studied a detailed somatosensory representation map of the human primary somatosensory cortex using magnetoencephalography. Somatosensory-evoked magnetic fields following tactile stimulation of multiple points in the right hemibody (including the tongue, lips, fingers, arm, trunk, leg, and foot) were analyzed in five normal subjects. We were able to estimate equivalent current dipoles (ECDs) following stimulation of the tongue, lips, fingers, palm, forearm, elbow, upper arm, and toes in most subjects and those following the stimulation of the chest, ankle, and thigh in one subject. The ECDs were located in the postcentral gyrus and generally arranged in order along the central sulcus, which is compatible with the somatosensory "homunculus." Linear distances, averaged in five subjects, from the receptive area of the thumb to that of the tongue, little finger, forearm, upper arm, and toes were estimated to be 2.42 +/- 0.28, 1.25 +/- 0.28, 2.21 +/- 0.72, 2.75 +/- 0.63, and 5.29 +/- 0.48 cm, respectively. The moment of each ECD, which suggested the size of the cortical areas responsive to the stimulation, was also compatible with the bizarre proportion of the homunculus with a large tongue, lips, and fingers. According to these results, we were able to reproduce a large part of the somatosensory homunculus quantitatively on an individual brain MRI.

Fingertip representation in the human somatosensory cortex: an fMRI study

Eight right-handed adult humans underwent functional magnetic resonance imaging (fMRI) of their brain while a vibratory stimulus was applied to an individual digit tip (digit 1, 2, or 5) on the right hand. Multislice echoplanar imaging techniques were utilized during digit stimulation to investigate the organization of the human primary somatosensory (SI) cortex, cortical regions located on the upper bank of the Sylvian fissure (SII region), insula, and posterior parietal cortices. The t test and cluster size analyses were performed to produce cortical activation maps, which exhibited significant regions of interest (ROIs) in all four cortical regions investigated. The frequency of significant ROIs was much higher in SI and the SII region than in the insula and posterior parietal region. Multiple digit representations were observed in the primary somatosensory cortex, corresponding to the four anatomic subdivisions of this cortex (areas 3a, 3b, 1, and 2), suggesting that the organization of the human somatosensory cortex resembles that described in other primates. Overall, there was no simple medial to lateral somatotopic representation in individual subject activity maps. However, the spatial distance between digit 1 and digit 5 cortical representations was the greatest in both SI and the SII region within the group. Statistical analyses of multiple activity parameters showed significant differences between cortical regions and between digits, indicating that vibrotactile activations of the cortex are dependent on both the stimulated digit and cortical region investigated.

Replicability of MEG and EEG measures of the auditory N1/N1m-response

We investigated the replicability of the source location, amplitude and latency measures of the auditory evoked N1 (EEG) and N1m (MEG) responses. Each of the 5 subjects was measured 6 times in two recording sessions. Responses to monaural stimuli were recorded from 122 MEG and 64 EEG channels simultaneously. The EEG data were modeled with a symmetrically-located dipole pair. For the MEG data, one dipole in each hemisphere was located independently using a subset of channels. Standard deviation (SD) was used as a measure for replicability. The average SD of the x, y and z coordinates of the contralateral N1m dipole was about 2 mm, whereas the corresponding figures for the ipsilateral N1m and the contra- and ipsilateral N1 were about twice as large. The SDs of the dipole amplitudes and latencies were almost equal with MEG and EEG. The amplitude and latency measures of the MEG field gradient waveforms were almost as replicable as those of the dipole models. The results suggest that both MEG and EEG can be used for investigating the simultaneous activity of the left and right auditory cortices independently, MEG being superior in certain experimental setups.

Visual evoked cortical magnetic responses to checkerboard pattern reversal stimulation: a study on the neural generators of N75, P100 and N145

In an attempt to elucidate the neural generators of pattern reversal visual evoked potentials (PR-VEPs), we measured the visual evoked magnetic fields (PR-VEFs) using a 37-channel magnetoencephalography in six healthy young adults. A half-field checkerboard pattern was phase-reversed at a rate of 1 Hz to stimulate the right or left visual half-field, thus yielding 12 PR-VEFs in total from the six subjects. The simultaneously recorded scalp PR-VEPs showed three distinct components of N75, P100 and N145. Three corresponding components were also identified in the PR-VEFs with similar peak latencies (N75m, P100m and N145m). P100m and N145m were clearly identified in all 12 PR-VEFs, whereas N75m was observed in only nine of 12 PR-VEFs. The equivalent current dipoles (ECDs) of N75m, P100m and N145m were located closely to each other in the occipital cortex around the calcarine fissure contralateral to the stimulated visual field, when they were overlaid on the MRI. The reliability of dipole estimation was highest in P100m, followed by N145m while N75m showed the least reliability. The direction of the current flow of ECDs of N75m and N145m was from the medial to the lateral in the occipital cortex when viewed in a coronal section, whereas that for P100m was toward the medial. The ECD location of P100m changed according to the retinotopic organization when the upper or lower quadrant of the visual field was stimulated, with the ECDs being located in the lower or upper part, respectively, of the visual cortex. Our results therefore indicate that the neural origins of N75m, P100m and N145m of PR-VEFs are in the primary visual cortex on the contralateral side of the stimulated visual half-field, while the three components are physiologically distinct.

Jerk-locked back averaging and dipole source localization of magnetoencephalographic transients in a patient with epilepsia partialis continua

In order to localize the generator site of epileptiform discharges, we applied the techniques of jerk-locked back averaging (JBA) of magnetoencephalographic (MEG) activities and dipole source localization in a patient with epilepsia partialis continua (EPC), who showed continuous, focal myoclonic jerks in the right arm. The myoclonic discharges in the right thenar muscle were used as a trigger pulse. JBA revealed consistent EEG and MEG transients that coincided consistently and constantly preceded the myoclonic jerks. The estimated dipoles of MEG were localized in a restricted area in the left precentral area, which closely correlated with the area of epileptic discharges recorded in electrocorticography. Therefore, JBA of MEG is considered to be a useful non-invasive method for localizing the epileptogenic area in EPC.

Functional and metabolic analysis of cerebral ischemia using magnetoencephalography and proton magnetic resonance spectroscopy

The details of the relationship between brain function and metabolism in brain infarcts have not been studied. Using magnetoencephalography (MEG) and proton magnetic resonance spectroscopic imaging (1H MRSI), we localized sources of abnormal magnetic activities in ischemic brain regions and biochemical changes in suspected lesions showing pathological characteristics. Twelve patients with ischemic stroke were examined and the results of MEG and 1H MRSI were superimposed onto the corresponding MR images. The signal intensities of N-acetyl (NA) and lactate (Lac) were measured in the lesions with highly concentrated dipoles of slow wave activity. Eleven of 12 cases had increased slow wave activity in the cortical areas adjacent to the infarcts; 1 case was excluded because the infarct was too small (<1 cm in diameter). The signal intensity of NA in the regions with the highest slow wave activity was significantly reduced and was well correlated with the dipole density of slow waves. Though Lac was mildly accumulated in the lesions, the Lac level had no correlation with slow wave magnetic activity. The remaining and metabolically active cortical tissue showing NA signal produced the abnormal slow wave activity under lactic acidosis (mild accumulation of Lac).

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.

Magnetic source imaging of abnormal low-frequency magnetic activity in presurgical evaluations of epilepsy

PURPOSE

Regional cortical dysfunction associated with epileptogenic activity was predicted from interictal localized abnormal low frequency neuromagnetic activity (ALFMA) using Magnetic Source Imaging (MSI). ALFMA can be detected in patients who show no interictal spikes.

METHODS

A large array biomagnetometer was used in a blinded, rapid screening protocol. The MSI procedure required no alteration in epileptic medications. MSI results were compared with the presumed epileptogenic region as determined by a consensus of standard techniques, which included MR and electroclinical monitoring.

RESULTS

One or more sites of localized abnormality were detected by MSI ALFMA in 29 of the 33 epileptic patients. ALFMA mapped with MSI showed a 48.5% specificity with respect to the presumed epileptogenic region. MSI ALFMA was in agreement with the final consensus as often as was ictal noninvasive video EEG monitoring, and was exceeded in specificity overall only by invasive ictal video EEG monitoring, which was required for conventional localization in 21 of the 33 patients tested with MSI.

CONCLUSIONS

ALFMA measurements with MSI may augment the array of noninvasive methods used for reaching a consensus for epilepsy surgery.

Evaluation of MRI-MEG/EEG co-registration strategies using Monte Carlo simulation

We present a Monte Carlo analysis method for evaluating MRI-MEG/EEG co-registration techniques. The method estimates the error in co-registration as a function of position within the brain. Using this analysis technique, we demonstrate the limitations of conventional head-based fiducial point methods, and propose a new strategy utilising a dental bite-bar incorporating accurately machined fiducial markers. Results presented demonstrate the improved accuracy of MEG/EEG to MRI co-registration using the bite-bar.

Integration of functional brain mapping in image-guided neurosurgery

Magnetoencephalographic (MEG) brain mapping was performed in 90 patients with lesions associated with eloquent sensorimotor cortex. The MEG-derived sensorimotor mapping information was utilised for risk analysis and planning. Subsequently, these patients underwent either stereotactic volumetric resection, stereotactic biopsy or non-surgical management of their lesions. In seventeen patients, the MEG sensorimotor localization was integrated into an operative stereotactic database (consisting of CT, MRI and digital angiography) to be used in an interactive fashion during computer-assisted stereotactic volumetric resection procedures. The spatial relationship between the MEG derived functional anatomy, the structural/radiological anatomy and the pathology could then be viewed simultaneously, thereby affording a safer trajectory and approach. In addition, the real-time availability of functional mapping information in an interactive fashion helped reduce surgical risk and minimise functional morbidity. All of these patients had resection of their lesions with no change in their neurological status. In conclusion, MEG is a non-invasive, accurate, and reproducible method for pre-operative assessment of patients with lesions associated with eloquent sensory and motor cortex. The interactive use of MEG functional mapping in the operating room can allow for a safer approach and resection of these eloquent cortex lesions.

Generators for human P300 elicited by somatosensory stimuli using multiple dipole source analysis

Cognitive event-related potentials, such as P300, are sensitive to manipulations of psychological variables and may provide evidence to support theories of brain mechanisms involved in cognition. However, the relationship between event-related potentials and the active neural structures is not yet understood. Electrical stimulation of the index and little fingers of the left hand in the context of a somatosensory target discrimination task, performed by healthy human subjects, elicited the middle-latency component of somatosensory evoked potentials, N60, the long-latency component, N140, and the P300 component. Identification of the generators for both the earlier components and P300, using equivalent electrical dipole modeling, was performed. Individual spatiotemporal seven-dipole models were developed in order to suggest locations of the sources generating each subject's scalp-recorded wave forms. Three dipoles with fairly weak moments, located in the primary and secondary sensory areas, explained the middle- and long-latency somatosensory evoked potential components, and the remaining four dipoles (4-7), with stronger dipole moments, were active during P300. There was a clear temporal separation of dipole activity between the somatosensory evoked potential components and the P300 component. Dipoles 4 and 5 were found quite symmetrically in the parahippocampal areas of the two hemispheres, while dipoles 6 and 7 were slightly asymmetrical. Dipole 7 was found in the left hippocampal area. Dipole 6 appeared in the right insular cortex. The locations of the four dipoles implicated in the generation of the somatosensory P300 were compared with the locations of four dipoles accounting for the auditory evoked P300 described in our previous paper [Tarkka et al. (1995) Electroenceph. clin. Neurophysiol. 96, 538-545]. No substantial difference in source locations of the P300 was found between auditory and somatosensory modality other than an asymmetrical activity in the somatosensory modality contralateral to the stimulated hand.

The interactive use of magnetoencephalography in stereotactic image-guided neurosurgery

OBJECTIVE

To expand the use of magnetoencephalography (MEG) functional mapping in the operating room as well as preoperatively, a method of integrating the MEG sensorimotor mapping information into a stereotactic database, using computed tomographic scans, magnetic resonance imaging scans, and digital angiography, was developed. The combination of functional mapping and the stereotactic technique allows simultaneous viewing of the spatial relationship between the MEG-derived functional mapping, the radiological/structural anatomic characteristics, and the pathological abnormality.

METHODS

MEG data were collected using a MAGNES II Biomagnetometer and were incorporated into the COMPASS frame-based and REGULUS frameless stereotactic systems. The transformation process, by calculating a translational vector and a rotation matrix, integrates functional and anatomic information that is then directly available intraoperatively in the stereotactic database. This procedure was employed in 10 patients undergoing computer-assisted stereotactic volumetric resections for lesions involving the sensorimotor cortex. The principles of coregistration and coordinate transformation are reviewed in the context of preoperative functional mapping. We introduce innovations to apply these techniques to intraoperative stereotactic systems.

RESULTS

Tests of the accuracy of the intraoperative integration of functional information in patients and calibration phantoms indicated close agreement with earlier preoperative methods. The intraoperative availability of functional information was a significant aid to the surgeon because it provided more accurate information on the location of functional tissue than could be derived solely by radiological criteria.

CONCLUSION

The real-time availability of functional mapping information in an interactive fashion can reduce surgical risk and minimize functional morbidity. Within the ever-expanding realm of functional mapping and image-guided neurosurgery, further progress and integration of these methods is critical for resection of lesions involving eloquent cortex.

A technique for sequential measurements of DC neuromagnetic fields

Magnetoencephalography (MEG) has the capability of detecting DC neuromagnetic fields which might arise from migraine, stroke, head trauma, etc. However, such fields are not readily measured in serial fashion for technical reasons, including arbitrary neuromagnetometer baselines, residual DC fields which vary from place to place inside conventional magnetically shielded rooms, and especially the high sensitivity of such measurements to small changes in distance between the probe and head. A technique is described to carry out such measurements by raising and lowering the subject under the probe and calibrating the DC shift against the amplitude of spontaneous activity.

Potential contribution of bilateral magnetic source imaging to the evaluation of epilepsy surgery candidates

The current procedures that are used to evaluate candidates for epilepsy surgery are time-consuming, costly, and often invasive. Magnetic source imaging (MSI), the combination of magnetoencephalography and anatomic imaging modalities, has shown promise as an efficient noninvasive means of localizing and characterizing seizure sources for possible resection. However, MSI has been limited by the inability to conduct simultaneous bilateral monitoring. In this study, a newly developed dual-magnetometer system was employed to record bilaterally the interictal activity in 30 candidates for epilepsy surgery. A standard monitoring protocol that included concurrent electroencephalographic recording and required a 2- to 3-hour examination period for each patient was developed. As a first step in a series of studies, the resultant MSI indications were compared with the information available from standard magnetic resonance imaging and concurrent electroencephalographic results. In 83% of the cases, this MSI protocol provided new information about the location of interictal epileptic activity that could be directive for subsequent patient care. Based on these results, it seems that MSI may become a cost-effective early step in epilepsy surgery evaluation. To continue the development on this basis, a study intended to validate the accuracy of MSI indicated by comparison with invasive electroencephalography has been initiated.