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

Transcranial magnetic stimulation and magnetoencephalography are feasible alternatives to invasive methods in optimizing responsive neurostimulation device placement

Responsive neurostimulation (RNS) is a treatment option for patients with refractory epilepsy when surgical resection is not possible due to overlap of the irritative zone and eloquent cortex. Presurgical evaluations for RNS placement typically rely on invasive methods. This study investigated the potential of transcranial magnetic stimulation (TMS) and magnetoencephalography (MEG) to provide key presurgical information non-invasively. We hypothesized that these non-invasive methods may assist in optimizing RNS placement by providing useful information for seizure localization by MEG and eloquent cortex mapping by TMS. A retrospective chart review identified nine patients who underwent RNS placement (mean age = 20.4 years [SD = 5.6], two-thirds were female). Characterization of the irritative zone using MEG was successful in eight of nine patients. Non-invasive mapping of relevant eloquent cortex was attempted in all patients. TMS was successful in eight of nine patients, and MEG was successful in two of six patients. Importantly, patients mapped with non-invasive modalities experienced an average seizure reduction of 77 % at their most recent clinic visit, compared to 75 % seizure reduction in those with invasive evaluations, indicating appropriate RNS placement. These data demonstrate that TMS and MEG can provide key information for RNS and may be feasible alternatives to invasive methods for assisting in decision making regarding RNS placement. Non-invasive methods for determining RNS placement have a high rate of success when data from multiple non-invasive modalities converge and can inform more accurate placement of intracranial electrodes prior to RNS placement or mitigate their need.

Topographic Mapping of the Primary Sensory Cortex Using Intraoperative Optical Imaging and Tactile Irritation

The determination of exact tumor boundaries within eloquent brain regions is essential to maximize the extent of resection. Recent studies showed that intraoperative optical imaging (IOI) combined with median nerve stimulation is a helpful tool for visualization of the primary sensory cortex (PSC). In this technical note, we describe a novel approach of using IOI with painless tactile irritation to demonstrate the feasibility of topographic mapping of different body regions within the PSC. In addition, we compared the IOI results with preoperative functional MRI (fMRI) findings. In five patients with tumors located near the PSC who received tumor removal, IOI with tactile irritation of different body parts and fMRI was applied. We showed that tactile irritation of the hand in local and general anesthesia leads to reliable changes of cerebral blood volume during IOI. Hereby, we observed comparable IOI activation maps regarding the median nerve stimulation, fMRI and tactile irritation of the hand. The tactile irritation of different body areas revealed a plausible topographic distribution along the PSC. With this approach, IOI is also suitable for awake surgeries, since the tactile irritation is painless compared with median nerve stimulation and is congruent to fMRI findings. Further studies are ongoing to standardize this method to enable a broad application within the neurosurgical community.

Patient-Specific Network Connectivity Combined With a Next Generation Neural Mass Model to Test Clinical Hypothesis of Seizure Propagation

Dynamics underlying epileptic seizures span multiple scales in space and time, therefore, understanding seizure mechanisms requires identifying the relations between seizure components within and across these scales, together with the analysis of their dynamical repertoire. In this view, mathematical models have been developed, ranging from single neuron to neural population. In this study, we consider a neural mass model able to exactly reproduce the dynamics of heterogeneous spiking neural networks. We combine mathematical modeling with structural information from non invasive brain imaging, thus building large-scale brain network models to explore emergent dynamics and test the clinical hypothesis. We provide a comprehensive study on the effect of external drives on neuronal networks exhibiting multistability, in order to investigate the role played by the neuroanatomical connectivity matrices in shaping the emergent dynamics. In particular, we systematically investigate the conditions under which the network displays a transition from a low activity regime to a high activity state, which we identify with a seizure-like event. This approach allows us to study the biophysical parameters and variables leading to multiple recruitment events at the network level. We further exploit topological network measures in order to explain the differences and the analogies among the subjects and their brain regions, in showing recruitment events at different parameter values. We demonstrate, along with the example of diffusion-weighted magnetic resonance imaging (dMRI) connectomes of 20 healthy subjects and 15 epileptic patients, that individual variations in structural connectivity, when linked with mathematical dynamic models, have the capacity to explain changes in spatiotemporal organization of brain dynamics, as observed in network-based brain disorders. In particular, for epileptic patients, by means of the integration of the clinical hypotheses on the epileptogenic zone (EZ), i.e., the local network where highly synchronous seizures originate, we have identified the sequence of recruitment events and discussed their links with the topological properties of the specific connectomes. The predictions made on the basis of the implemented set of exact mean-field equations turn out to be in line with the clinical pre-surgical evaluation on recruited secondary networks.

A review of magnetoencephalography use in pediatric epilepsy: an update on best practice

Introduction: Magnetoencephalography (MEG) is a noninvasive technique that is used for presurgical evaluation of children with drug-resistant epilepsy (DRE).Areas covered: The contributions of MEG for localizing the epileptogenic zone are discussed, in particular in extra-temporal lobe epilepsy and focal cortical dysplasia, which are common in children, as well as in difficult to localize epilepsy such as operculo-insular epilepsy. Further, the authors review current evidence on MEG for mapping eloquent cortex, its performance, application in clinical practice, and potential challenges.Expert opinion: MEG could change the clinical management of children with DRE by directing placement of intracranial electrodes thereby enhancing their yield. With improved identification of a circumscribed epileptogenic zone, MEG could render more patients as suitable candidates for epilepsy surgery and increase utilization of surgery.

Motor Cortical Network Plasticity in Patients With Recurrent Brain Tumors

Objective: The adult brain’s potential for plastic reorganization is an important mechanism for the preservation and restoration of function in patients with primary glial neoplasm. Patients with recurrent brain tumors requiring multiple interventions over time present an opportunity to examine brain reorganization. Magnetoencephalography (MEG) is a noninvasive imaging modality that can be used for motor cortical network mapping which, when performed at regular intervals, offers insight into this process of reorganization. Utilizing MEG-based motor mapping, we sought to characterize the reorganization of motor cortical networks over time in a cohort of 78 patients with recurrent glioma.

Methods: MEG-based motor cortical maps were obtained by measuring event-related desynchronization (ERD) in ß-band frequency during unilateral index finger flexion. Each patient presented at our Department at least on two occasions for tumor resection due to tumor recurrence, and MEG-based motor mapping was performed as part of preoperative assessment before each surgical resection. Whole-brain activation patterns from first to second MEG scan (obtained before first and second surgery) were compared. Additionally, we calculated distances of activation peaks, which represent the location of the primary motor cortex (MC), to determine the magnitude of movement in motor eloquent areas between the first and second MEG scan. We also explored which demographic, anatomic, and pathological factors influence these shifts.

Results: The whole-brain activation motor maps showed a subtle movement of the primary MC from first to second timepoint, as was confirmed by the determination of motor activation peaks. The shift of ipsilesional MC was directly correlated with a frontal-parietal tumor location (p < 0.001), presence of motor deficits (p = 0.021), and with a longer period between MEG scans (p = 0.048). Also, a disengagement of wide areas in the contralesional (ipsilateral to finger movement) hemisphere at the second time point was observed.

Conclusions: MEG imaging is a sensitive method for depicting the plasticity of the motor cortical network. Although the location of the primary MC undergoes only subtle changes, appreciable shifts can occur in the setting of a stronger and longer impairment of the tumor on the MC. The ipsilateral hemisphere may serve as a reservoir for functional recovery.

Evidence for age-related changes in sensorimotor neuromagnetic responses during cued button pressing in a large open-access dataset

Mu, beta, and gamma rhythms increase and decrease in amplitude during movement. This event-related synchronization (ERS) and desynchronization (ERD) can be readily recorded non-invasively using magneto- and electro-encephalography (M/EEG). In addition, event-related potentials and fields (i.e., evoked responses) can be elucidated during movement. There is some evidence that the frequency, amplitude and latency of the movement-related ERS/ERD changes with ageing, however the evidence surrounding this topic comes mainly from studies in sample sizes on the order of tens of participants. The objective of this study was to examine a large open-access MEG dataset for age-related changes in movement-related ERS/ERD and evoked responses. MEG data acquired at the Cambridge Centre for Ageing and Neuroscience during cued button pressing was used from 567 participants between the ages of 18 and 88 years. The characteristics movement-related ERD/ERS and evoked responses were calculated for each individual participant. Based on linear regression analysis, significant relationships were found between participant age and some response characteristics, although the predictive value of these relationships was low. Specifically, we conclude that peak beta rebound frequency and amplitude decreased with age, peak beta suppression amplitude increased with age, movement-related gamma burst amplitude decreased with age, and peak motor-evoked response amplitude increased with age. Given our current understanding of the underlying mechanisms of these responses, our findings suggest the existence of age-related changes in the neurophysiology of thalamocortical loops and local circuitry in the primary somatosensory and motor cortices.

Electrical Stimulation Mapping of the Brain: Basic Principles and Emerging Alternatives

The application of electrical stimulation mapping (ESM) of the brain for clinical use is approximating a century. Despite this long-standing history, the value of ESM for guiding surgical resections and sparing eloquent cortex is documented largely by small retrospective studies, and ESM protocols are largely inherited and lack standardization. Although models are imperfect and mechanisms are complex, the probabilistic causality of ESM has guaranteed its perpetuation into the 21st century. At present, electrical stimulation of cortical tissue is being revisited for network connectivity. In addition, noninvasive and passive mapping techniques are rapidly evolving to complement and potentially replace ESM in specific clinical situations. Lesional and epilepsy neurosurgery cases now offer different opportunities for multimodal functional assessments.

Biodistribution and clearance of magnetoelectric nanoparticles for nanomedical applications using energy dispersive spectroscopy

AIM

The biodistribution and clearance of magnetoelectric nanoparticles (MENs) in a mouse model was studied through electron energy dispersive spectroscopy.

MATERIALS & METHODS

This approach allows for detection of nanoparticles (NPs) in tissues with the spatial resolution of scanning electron microscopy, does not require any tissue-sensitive staining and is not limited to MENs.

RESULTS

The size-dependent biodistribution of intravenously administrated MENs was measured in vital organs such as the kidneys, liver, spleen, lungs and brain at four different postinjection times including 1 day, 1 week, 4 and 8 weeks, respectively.

CONCLUSION

The smallest NPs, 10-nm MENs, were cleared relatively rapidly and uniformly across the organs, while the clearance of the larger NPs, 100- and 600-nm MENs, was highly nonlinear with time and nonuniform across the organs.

In Vivo Tumour Mapping Using Electrocorticography Alterations During Awake Brain Surgery: A Pilot Study

During awake brain surgery for tumour resection, in situ EEG recording (ECoG) is used to identify eloquent areas surrounding the tumour. We used the ECoG setup to record the electrical activity of cortical and subcortical tumours and then performed frequency and connectivity analyses in order to identify ECoG impairments and map tumours. We selected 16 patients with cortical (8) and subcortical (8) tumours undergoing awake brain surgery. For each patient, we computed the spectral content of tumoural and healthy areas in each frequency band. We computed connectivity of each electrode using connectivity markers (linear and non-linear correlations, phase-locking and coherence). We performed comparisons between healthy and tumour electrodes. The ECoG alterations were used to implement automated classification of the electrodes using clustering or neural network algorithms. ECoG alterations were used to image cortical tumours.Cortical tumours were found to profoundly alter all frequency contents (normalized and absolute power), with an increase in the δ activity and a decreases for the other bands (P < 0.05). Cortical tumour electrodes showed high level of connectivity compared to surrounding electrodes (all markers, P < 0.05). For subcortical tumours, a relative decrease in the γ1 band and in the alpha band in absolute amplitude (P < 0.05) were the only abnormalities. The neural network algorithm classification had a good performance: 93.6 % of the electrodes were classified adequately on a test subject. We found significant spectral and connectivity ECoG changes for cortical tumours, which allowed tumour recognition. Artificial neural algorithm pattern recognition seems promising for electrode classification in awake tumour surgery.

Awake craniotomy for supratentorial gliomas: why, when and how?

Awake craniotomy has become an increasingly utilized procedure in the treatment of supratentorial intra-axial tumors. The popularity of this procedure is partially attributable to improvements in intraoperative technology and anesthetic techniques. The application of awake craniotomy to the field of neuro-oncology has decreased iatrogenic postoperative neurological deficits, allowed for safe maximal tumor resection and improved healthcare resource stewardship by permitting early patient discharge. In this article, we review recent evidence for the utility of awake craniotomy in the resection of gliomas and describe the senior author's experience in performing this procedure. Furthermore, we explore innovative applications of awake craniotomy to outpatient tumor resections and the conduct of neurosurgery in resource-poor settings. We conclude that awake craniotomy is an effective and versatile neurosurgical procedure with expanding applications in neuro-oncology.

Clinical use of diffusion tensor image-merged functional neuronavigation for brain tumor surgeries: review of preoperative, intraoperative, and postoperative data for 123 cases

PURPOSE

To achieve maximal safe resection during brain tumor surgery, functional image-merged neuronavigation is widely used. We retrospectively reviewed our cases in which diffusion tensor image (DTI)-merged functional neuronavigation was performed during surgery.

MATERIALS AND METHODS

Between November 2008 and May 2010, 123 patients underwent surgery utilizing DTI-merged neuronavigation. Anatomical magnetic resonance images (MRI) were obtained preoperatively and fused with DTI of major white matter tracts, such as the corticospinal tract, optic radiation, or arcuate fasciculus. We used this fused image for functional neuronavigation during brain tumor surgery of eloquent areas. We checked the DTI images together with postoperative MRI images and evaluated the integrity of white matter tracts.

RESULTS

A single white matter tract was inspected in 78 patients, and two or more white matter tracts were checked in 45 patients. Among the 123 patients, a grossly total resection was achieved in 90 patients (73.2%), subtotal resection in 29 patients (23.6%), and partial resection in 4 patients (3.3%). Postoperative neurologic outcomes, compared with preoperative function, included the following: 100 patients (81.3%) displayed improvement of neurologic symptoms or no change, 7 patients (5.7%) experienced postoperative permanent neurologic deterioration (additional or aggravated neurologic symptoms), and 16 patients (13.0%) demonstrated transient worsening.

CONCLUSION

DTI-merged functional neuronavigation could be a useful tool in brain tumor surgery for maximal safe resection. However, there are still limitations, including white matter tract shift, during surgery and in DTI itself. Further studies should be conducted to overcome these limitations.

Intraoperative cortico–subcortical stimulations in surgery of low-grade gliomas

In order to increase the impact of surgery on the natural history of low-grade glioma, resection should be of maximum importance. Nevertheless, since low-grade gliomas are frequently located in eloquent structures, function needs to be preserved. Therefore, studying the functional organization of the brain is mandatory for each patient due to the inter-individual anatomofunctional variability, increased in tumors due to cerebral plasticity. This strategy enables performance of a resection according to functional boundaries. However, preoperative neurofunctional imaging only allows the study of the gray matter. Consequently, since low-grade glioma invades cortical and subcortical structures and shows an infiltrative progression along the fibers, the goal of this review is to focus on the techniques able to map both cortical and subcortical regions. In addition to diffusion tensor imaging, which gives only anatomical information and still needs to be validated, intraoperative direct cortico-subcortical electrostimulation is the sole current method allowing a reliable study of the individual anatomofunctional connectivity, concerning sensorimotor, language and other cognitive functions. Its actual contribution is detailed, both in clinical issues, especially the improvement of the benefit/risk ratio of low-grade glioma resection, and in fundamental applications--namely, a new door to the connectionism and cerebral plasticity.

Reappraisal of field dynamics of motor cortex during self-paced finger movements

BACKGROUND

The exact origin of neuronal responses in the human sensorimotor cortex subserving the generation of voluntary movements remains unclear, despite the presence of characteristic but robust waveforms in the records of electroencephalography or magnetoencephalography (MEG).

AIMS

To clarify this fundamental and important problem, we analyzed MEG in more detail using a multidipole model during pulsatile extension of the index finger, and made some important new findings.

RESULTS

Movement-related cerebral fields (MRCFs) were confirmed over the sensorimotor region contralateral to the movement, consisting of a temporal succession of the first premovement component termed motor field, followed by two or three postmovement components termed movement evoked fields. A source analysis was applied to separately model each of these field components. Equivalent current diploes of all components of MRCFs were estimated to be located in the same precentral motor region, and did not differ with respect to their locations and orientations. The somatosensory evoked fields following median nerve stimulation were used to validate these findings through comparisons of the location and orientation of composite sources with those specified in MRCFs. The sources for the earliest components were evoked in Brodmann's area 3b located lateral to the sources of MRCFs, and those for subsequent components in area 5 and the secondary somatosensory area were located posterior to and inferior to the sources of MRCFs, respectively. Another component peaking at a comparable latency with the area 3b source was identified in the precentral motor region where all sources of MRCFs were located.

CONCLUSION

These results suggest that the MRCF waveform reflects a series of responses originating in the precentral motor area.

Multimodal neuroimaging in presurgical evaluation of drug-resistant epilepsy

Intracranial EEG (icEEG) monitoring is critical in epilepsy surgical planning, but it has limitations. The advances of neuroimaging have made it possible to reveal epileptic abnormalities that could not be identified previously and improve the localization of the seizure focus and the vital cortex. A frequently asked question in the field is whether non-invasive neuroimaging could replace invasive icEEG or reduce the need for icEEG in presurgical evaluation. This review considers promising neuroimaging techniques in epilepsy presurgical assessment in order to address this question. In addition, due to large variations in the accuracies of neuroimaging across epilepsy centers, multicenter neuroimaging studies are reviewed, and there is much need for randomized controlled trials (RCTs) to better reveal the utility of presurgical neuroimaging. The results of multiple studies indicate that non-invasive neuroimaging could not replace invasive icEEG in surgical planning especially in non-lesional or extratemporal lobe epilepsies, but it could reduce the need for icEEG in certain cases. With technical advances, multimodal neuroimaging may play a greater role in presurgical evaluation to reduce the costs and risks of epilepsy surgery, and provide surgical options for more patients with drug-resistant epilepsy.

CortiQ - clinical software for electrocorticographic real-time functional mapping of the eloquent cortex

Planning for epilepsy surgery depends substantially on the localization of brain cortical areas responsible for sensory, motor, or cognitive functions, clinically also known as eloquent cortex. In this paper, we present the novel software package 'cortiQ' that allows clinicians to localize eloquent cortex, thus providing a safe margin for surgical resection with a low incidence of neurological deficits. This software can be easily used in addition to traditional mapping procedures such as the electrical cortical stimulation (ECS) mapping. The software analyses task-related changes in gamma activity recorded from implanted subdural electrocorticography electrodes using extensions to previously published methods. In this manuscript, we describe the system's architecture and workflow required to obtain a map of the eloquent cortex. We validate the system by comparing our mapping results with those acquired using ECS mapping in two subjects. Our results indicate that cortiQ reliably identifies eloquent cortex much faster (several minutes compared to an hour or more) than ECS mapping. Next-neighbour analyses show that there are no false positives and an average of 1.24% false negatives.

Intraoperative visualization for resection of gliomas: the role of functional neuronavigation and intraoperative 1.5 T MRI

OBJECTIVE

To investigate how functional neuronavigation and intraoperative high-field magnetic resonance imaging (MRI) influence glioma resection.

METHODS

One hundred and thirty-seven patients [World Health Organization (WHO) grade I: 20; II: 19; III: 41; IV: 57] underwent resection for supratentorial gliomas in an operative suite equipped with intraoperative high-field MRI and microscope-based neuronavigation. Besides standard anatomical image data including T1- and T2-weighted sequences, various functional data from magnetoencephalography (n=37), functional MRI (n=65), positron emission tomography (n=8), MR spectroscopy (n=28) and diffusion tensor imaging (n=55) were integrated in the navigational setup.

RESULTS

Intraoperative MRI showed primary complete resection in 27% of all patients (I: 50%; II: 53%; III: 2%; IV: 28%). In 41% of all patients (I: 40%; II: 26%; III: 66%; IV: 28%) the resection was extended owing to intraoperative MRI increasing the percentage of complete resections to 40% (I: 70%; II: 58%; III: 17%; IV: 40%). Integrated application of functional navigation resulted in low post-operative morbidity with a transient new neurological deficit in 10.2% (paresis: 8.8% and speech disturbance: 1.4%) decreasing to a permanent deficit in 2.9% (four of 137 patients with a new or increased paresis).

CONCLUSIONS

The combination of intraoperative MRI and functional navigation allows safe extended resections in glioma surgery. However, despite extended resections, still in the majority of the grade III and IV gliomas no gross total resection could be achieved owing to the extension of the tumor into eloquent brain areas. Intraoperative MRI data can be used to localize the tumor remnants reliably and compensate for the effects of brain shift.

Source analysis of the magnetic field evoked during self-paced finger movements

OBJECTIVE

The aim of this study is to investigate a source of cortical magnetic fields evoked by index finger movements.

METHODS

We analysed both movement-related cortical fields (MRCFs) and somatosensory-evoked fields (SEFs) by single equivalent current dipole (ECD) method in six healthy subjects. Dipole locations were superimposed on MR images of each individual subject.

RESULTS

The first component after finger movement (movement-evoked field I, MEFI) was observed in all subjects. The dipole of MEFI was oriented posteriorly, and was located on the posterior wall of the central sulcus of the hemisphere contralateral to the movement. The SEFs showed three major components: N20m, P30m and P60m. The dipoles of P30m and P60m were orientated posteriorly, similarly to the MEFI dipole, while that of N20m was orientated anteriorly. The dipole location of MEFI was closely located to P60m, not to N20m and P30m. The mean location of the MEFI dipole was significantly (p<0.05) superior to N20m.

CONCLUSION

These findings suggest that MEFI would be generated in the sensory area (area 3b) affected by multiple afferents and activities, and that the source of the MEFI is not identical to that of the N20m component.

Determining surgical candidacy in temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common form of adult epilepsy that is amenable to surgical treatment. In the carefully selected patient, excellent seizure outcome can be achieved with minimal or no side effects from surgery. This may result in improved psychosocial functioning, achieving higher education, and maintaining or gaining employment. The objective of this paper is to discuss the surgical selection process of a patient with TLE. We define what constitutes a patient that has medically refractory TLE, describe the typical history and physical examination, and distinguish between mesial TLE and neocortical TLE. We then review the role of routine (ambulatory/sleep-deprived electroencephalography (EEG), video EEG, magnetic resonance imaging (MRI), neuropsychological testing, and Wada testing) and ancillary preoperative testing (positron emission tomography, single-photon emission computed tomography (SPECT), subtraction ictal SPECT correlated to MRI (SISCOM), magnetoencephalography, magnetic resonance spectroscopy, and functional MRI) in selecting surgical candidates. We describe the surgical options for resective epilepsy surgery in TLE and its commonly associated risks while highlighting some of the controversies. Lastly, we present teaching cases to illustrate the presurgical workup of patients with medically refractory TLE.

Clinical magnetoencephalography for neurosurgery

Noninvasive neuroimaging aids in surgical planning and in counseling patients about possible risks of surgery. Magnetoencephalography (MEG) performs the most common types of surgical planning that the neurosurgeon faces, including localization of epileptic discharges, determination of the hemispheric dominance of verbal processing, and the ability to locate eloquent cortex. MEG is most useful when it is combined with structural imaging, most commonly with structural magnetic resonance (MR) imaging and MR diffusion imaging. This article reviews the history of clinical MEG, introduces the basic concepts about the biophysics of MEG, and outlines the basic neurosurgical applications of MEG.

Intraoperative acquisition of fMRI and DTI

Multimodal functional navigation enables removing a tumor close to eloquent brain areas with low postoperative deficits, whereas additional intraoperative imaging ensures that the maximum extent of the resection can be achieved and updates the image data compensating for the effects of brain shift. Intraoperative imaging beyond standard anatomic imaging, that is, intraoperative functional magnetic resonance imaging (fMRI) and especially intraoperative diffusion tensor imaging (DTI), add further safety for complex tumor resections. This article discusses the acquisition of intraoperative fMRI, DTI, and imaging.

Development of a clinical functional magnetic resonance imaging service

One of the limitations of anatomy-based imaging approaches is its relative inability to identify whether specific brain functions may be compromised by the location of brain lesions or contemplated brain surgeries. Of the many techniques available to the surgeon, functional magnetic resonance imaging (fMRI) has become the primary modality of choice because of the ability of MRI to serve as a "one-stop shop" for assessing both anatomy and functionality of the brain. This article discusses the specific requirements for establishing an fMRI program, including specific software and hardware requirements. In addition, the nature of the fMRI CPT codes is discussed.

Multimodal navigation integrated with imaging

Intraoperative high-field MRI in combination and close integration with microscope-based navigation serving as a common interface for the presentation of multimodal data in the surgical field seems to be one of the most promising surgical setups allowing avoiding unwanted tumor remnants while preserving neurological function. Multimodal navigation integrates standard anatomical, structural, functional, and metabolic data. Navigation achieves visualizing the initial extent of a lesion with the concomitant identification of neighboring eloquent brain structures, as well as, providing a tool for a direct correlation of histology and multimodal data. With the help of intraoperative imaging navigation data can be updated, so that brain shift can be compensated for and initially missed tumor remnants can be localized reliably.

Proceedings of the first international workshop on advances in electrocorticography

In October 2009, a group of neurologists, neurosurgeons, computational neuroscientists, and engineers congregated to present novel developments transforming human electrocorticography (ECoG) beyond its established relevance in clinical epileptology. The contents of the proceedings advanced the role of ECoG in seizure detection and prediction, neurobehavioral research, functional mapping, and brain-computer interface technology. The meeting established the foundation for future work on the methodology and application of surface brain recordings.

A practical procedure for real-time functional mapping of eloquent cortex using electrocorticographic signals in humans

Functional mapping of eloquent cortex is often necessary prior to invasive brain surgery, but current techniques that derive this mapping have important limitations. In this article, we demonstrate the first comprehensive evaluation of a rapid, robust, and practical mapping system that uses passive recordings of electrocorticographic signals. This mapping procedure is based on the BCI2000 and SIGFRIED technologies that we have been developing over the past several years. In our study, we evaluated 10 patients with epilepsy from four different institutions and compared the results of our procedure with the results derived using electrical cortical stimulation (ECS) mapping. The results show that our procedure derives a functional motor cortical map in only a few minutes. They also show a substantial concurrence with the results derived using ECS mapping. Specifically, compared with ECS maps, a next-neighbor evaluation showed no false negatives, and only 0.46 and 1.10% false positives for hand and tongue maps, respectively. In summary, we demonstrate the first comprehensive evaluation of a practical and robust mapping procedure that could become a new tool for planning of invasive brain surgeries.

Clinical applications of magnetoencephalography

Magnetoencephalography (MEG), in which magnetic fields generated by brain activity are recorded outside of the head, is now in routine clinical practice throughout the world. MEG has become a recognized and vital part of the presurgical evaluation of patients with epilepsy and patients with brain tumors. We review investigations that show an improvement in the postsurgical outcomes of patients with epilepsy by localizing epileptic discharges. We also describe the most common clinical MEG applications that affect the management of patients, and discuss some applications that are close to having a clinical impact on patients.

Presurgical Localization of Primary Motor Cortex in Pediatric Patients with Brain Lesions by the Use of Spatially Filtered Magnetoencephalography

Objective:

The objective of this study was to confirm the efficacy of spatially filtered magnetoencephalography for the preoperative localization of primary motor cortex in pediatric patients with focal lesions in the region of the sensorimotor cortex.

Methods:

We recorded movement-related magnetoencephalographic activity in 10 pediatric patients (age range, 7–18 years; mean age, 12.5 years) undergoing presurgical evaluation for focal brain lesion resection. Participants made transient movements of the right and left index finger in response to a visual cue. The premovement motor field component in the averaged brain response was localized with a newly developed beamformer spatial filter algorithm. Cortical mapping of motor cortex intraoperatively was conducted in 5 of the 10 patients.

Results:

The motor field time-locked to electromyography onset was successfully localized to cortical areas corresponding to the hand region primary motor cortex in 95% of cases (9 of 10 from nonlesional hemisphere; 10 of 10 from lesional hemisphere). Intraoperative electrocortical stimulation activated the expected muscles at motor field coregistered cortical source locations in all cases tested (n = 5). Using these methods, we also found that displacement of the sensorimotor cortex by space-occupying tumors did not interfere with the localization of motor cortex.

Conclusion:

We conclude that noninvasive localization of the primary motor cortex can be reliably performed by using spatially filtered magnetoencephalography techniques, which provide a robust and accurate measurement of motor cortical function for the purpose of surgical guidance.

Aberrant cortical functionality and somatosensory deficits after stroke

Damage and/or disconnection of the primary somatosensory cortex (SI) after stroke leads to deficits in touch perception. We used magnetoencephalography to test whether specific patterns of functionality of the somatosensory cortex are associated with different degrees of postacute somatosensory deficit. Nineteen postacute unilateral stroke patients suffering different degrees of somatosensory deficit (six nonexistent, six moderate, and seven severe) and eight aged-matched controls underwent high-resolution MRI and whole-head magnetoencephalography recordings of somatosensory-evoked fields and of spontaneous slow oscillatory activity. Amplitude of SI activation after tactile stimulation in the affected and nonaffected hemispheres and delta dipole density (DDD) in the postcentral areas were estimated and compared across the four groups. Severe postacute somatosensory deficit was accompanied, in all cases, with absence of SI responses to stimulation in the affected hand and a significant asymmetry in postcentral DDD toward the affected hemisphere. Patients with moderate sensory loss showed asymmetry in their postcentral DDD (four cases toward the affected hemisphere and two toward the unaffected) but no atypical amplitudes in SI activation. Recordings in stroke patients without somatosensory deficit did not differ from those obtained in controls for SI amplitude or postcentral DDD. In stroke patients, amplitude of SI responses and postcentral DDD show a negative correlation. Lack of activation of SI cortex after stimulation of the affected hand and spontaneous slow oscillatory activity in postcentral areas are neurophysiological correlates of somatosensory deficit in the postacute phase of stroke.

Mapping functional connectivity in patients with brain lesions

OBJECTIVE

The spatial distribution of functional connectivity between brain areas and the disturbance introduced by focal brain lesions are poorly understood. Based on the rationale that damaged brain tissue is disconnected from the physiological interactions among healthy areas, this study aimed to map the functionality of brain areas according to their connectivity with other areas.

METHODS

Magnetoencephalography recordings of spontaneous cortical activity during resting state were obtained from 15 consecutive patients with focal brain lesions and from 14 healthy control subjects. Neural activity in the brain was estimated using an adaptive spatial filtering technique. The mean imaginary coherence between brain voxels was then calculated as an index of functional connectivity.

RESULTS

Imaginary coherence was greatest in the alpha frequency range corresponding to the human cortical idling rhythm. In healthy subjects, functionally critical brain areas such as the somatosensory and language cortices had the highest alpha coherence. When compared with healthy control subjects, all lesion patients had diffuse or scattered brain areas with decreased alpha coherence. Patients with lesion-induced neurological deficits displayed decreased connectivity estimates in the corresponding brain area compared with intact contralateral regions. In tumor patients without preoperative neurological deficits, brain areas showing decreased coherence could be surgically resected without the occurrence of postoperative deficits.

INTERPRETATION

Resting state coherence measured with magnetoencephalography is capable of mapping the functional connectivity of the brain, and can therefore offer valuable information for use in planning resective surgeries in patients with brain lesions, as well as investigations into structural-functional relationships in healthy subjects.

Intraoperative confirmation of hand motor area identified preoperatively by magnetoencephalography

BACKGROUND

Presurgical functional mapping using magnetoencephalography (MEG) has been performed for somatosensory, auditory and visual functions; however, the traditional analysis method utilizing dipole source analysis has some inherent limitations when applied to the mapping of cortical motor areas. Recently, a novel source reconstruction algorithm [event-related synthetic aperture magnetometry (erSAM)] has demonstrated success for the localization of motor function in healthy adults. We applied this technique to preoperatively map motor function in a young patient. We then confirmed our mapping with direct cortical stimulation intraoperatively.

METHODS

This is a case report of an 8-year-old girl with right hand and arm weakness and poor right hand motor control secondary to a left peri-rolandic tumor. Preoperatively, whole-head MEG was recorded while the patient performed a self-paced button pressing task. Cortical activity associated with the onset of movement was localized to the right hand precentral gyrus superior and medial to the tumor using erSAM, while sensory function was localized posterior to the tumor on the postcentral gyrus.

RESULTS

Intraoperative direct cortical stimulation of the motor area identified by MEG resulted in electromyographic activation of intrinsic muscles of the contralateral hand exclusively.

CONCLUSIONS

This is the first report of a case where direct cortical stimulation has confirmed a motor cortical location identified by the erSAM method.

MAGNETOENCEPHALOGRAPHIC STUDY OF POSTERIOR TIBIAL NERVE STIMULATION IN PATIENTS WITH INTRACRANIAL LESIONS AROUND THE CENTRAL SULCUS

OBJECTIVE

To study interhemispheric differences of somatosensory evoked field (SEF) characteristics and the spatial distribution of equivalent current dipole sources in patients with unilateral hemispheric lesions around the central sulcus region.

METHODS

In 17 patients with perirolandic lesions, averaged somatosensory responses after posterior tibial nerve stimulation at the ankle were recorded with magnetoencephalography. Dipole source solutions in the affected (AH) and unaffected (UH) hemispheres were analyzed and compared for latency, equivalent current dipole strength, root mean square, and spatial distribution in relation to clinical findings.

RESULTS

Three main SEF components, P45m, N60m, and P75m, were identified in the hemisphere contralateral to the stimulated nerve. Dipole strength for the P45m component was significantly higher in the AH compared with the UH. SEF characteristics in the AH and UH showed no significant differences with respect to component latency or dipole strength of the N60m and P75m components. Interdipole location asymmetries exceeded 1.0 cm in 71% of the patients. Comparison of the posterior tibial nerve evoked responses (P45m and N60m) in patients with motor deficits and patients without deficits showed that these responses are enlarged in the AH when perirolandic lesions are present. Patients with motor deficits also showed an increased response for P45m in the UH.

CONCLUSION

The results of posterior tibial nerve SEFs suggest spatial and functional changes in the somatosensory network as a result of perirolandic lesions with a possible relationship with clinical symptoms. The results can provide further basis for the evaluation of cortical changes in the presence of perirolandic lesions.

Diffusion tensor imaging and white matter tractography in patients with brainstem lesions

BACKGROUND

Diffusion tensor imaging (DTI) and white matter tractography (WMT) are promising techniques for estimating the course, extent, and connectivity patterns of the white matter (WM) structures in the human brain. In this study, we investigated the ability of DTI and WMT to visualize white matter tract involvement for the preoperative surgical planning and postoperative assessment of brainstem lesions.

METHODS

Preoperative and postoperative DTI data (echo-planar, 1.5T) were retrospectively analyzed in 10 patients with brainstem lesions (3 diffuse, 7 focal). WMT applying a tensor deflection algorithm was used to reconstruct WM tracts adjacent to the lesions. Reconstructed tracts included corticospinal tracts and medial lemnisci. The clinical and imaging follow-up data were also compared and analyzed.

FINDINGS

WMT revealed a series of tract alteration patterns including deviation, deformation, infiltration, and apparent tract interruption. WMT reconstructions showed that the major WM tracts were preserved during surgery and improved in position and appearance postoperatively. These findings correlated with the improvement or preservation of neurological function as determined by clinical assessment.

CONCLUSIONS

Compared with the information provided by conventional MR imaging, DTI and WMT provided superior quantification and visualization of lesion involvement in eloquent fibre tracts of the brainstem. Moreover, DTI and WMT were found to be beneficial for white matter recognition in the neurosurgical planning and postoperative assessment of brainstem lesions.

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.

Robust source analysis of oscillatory motor cortex activity with inherently variable phase delay

This study evaluated quantitatively the synchronization between the magnetoencephalography (MEG) and electromyography (EMG) signals and developed a novel method for the determination of the synchronization in order to increase the reliability of the source analysis of the oscillatory motor cortex activity. The new method is based on our observation that there are large variances in the time lag due to relatively low muscle-cortex synchronization which reduces the signal-to-noise ratio of the MEG signal when averaged in direct synchrony with the rectified EMG peaks. To improve the localization of the motor cortex activity, time-frequency analysis was performed for each epoch coinciding with an EMG peak to reject the weak oscillatory activity and artifacts. In addition, the MEG signals were shifted to maximize the coherence between MEG and rectified EMG by determining for each accepted epoch the time lag resulting in a maximum cross-correlation. An experiment was carried out using 30 subjects in order to determine the applicability of this method to a real situation. The synchronization and the results of the corresponding source analysis based on the novel method were compared with the data obtained using the non-phase-shift method and Hilbert approach detecting EMG phase. The results showed that the synchronization was significantly enhanced and the signal-to-noise ratio of the MEG signals improved, and that the localized dipoles of all subjects were well clustered at the motor cortex. This method, based on shifting the MEG epochs according to the simultaneously measured time lag, considerably improves the performance of the averaging and localization of the rhythmic activity of the motor cortex.

Strategies for resection of lesions in the motor area: preliminary results in 42 surgical patients

In recent years considerable technological advances have been made with the purpose of improving the surgical results in the treatment of eloquent lesions. The overall aim of this study is to evaluate the postoperative surgical outcome in 42 patients who underwent surgery to remove lesions around the motor cortex, in which preoperative planning by using neuroimaging exams, anatomical study, appropriate microsurgery technique and auxiliary methods such as cortical stimulation were performed. Twenty-two patients (52.3%) presented a normal motor function in the preoperative period. Of these, six developed transitory deficit. Twenty patients (47.6%) had a motor deficit preoperatively, nevertheless 90% of these improved postoperatively. Surgery in the motor area becomes safer and more effective with preoperative localization exams, anatomical knowledge and appropriate microsurgery technique. Cortical stimulation is important because it made possible to maximize the resection reducing the risk of a motor deficit. Stereotaxy method was useful in the location of subcortical lesions.

Non-rigid alignment of pre-operative MRI, fMRI, and DT-MRI with intra-operative MRI for enhanced visualization and navigation in image-guided neurosurgery

OBJECTIVE

The usefulness of neurosurgical navigation with current visualizations is seriously compromised by brain shift, which inevitably occurs during the course of the operation, significantly degrading the precise alignment between the pre-operative MR data and the intra-operative shape of the brain. Our objectives were (i) to evaluate the feasibility of non-rigid registration that compensates for the brain deformations within the time constraints imposed by neurosurgery, and (ii) to create augmented reality visualizations of critical structural and functional brain regions during neurosurgery using pre-operatively acquired fMRI and DT-MRI.

MATERIALS AND METHODS

Eleven consecutive patients with supratentorial gliomas were included in our study. All underwent surgery at our intra-operative MR imaging-guided therapy facility and have tumors in eloquent brain areas (e.g. precentral gyrus and cortico-spinal tract). Functional MRI and DT-MRI, together with MPRAGE and T2w structural MRI were acquired at 3 T prior to surgery. SPGR and T2w images were acquired with a 0.5 T magnet during each procedure. Quantitative assessment of the alignment accuracy was carried out and compared with current state-of-the-art systems based only on rigid registration.

RESULTS

Alignment between pre-operative and intra-operative datasets was successfully carried out during surgery for all patients. Overall, the mean residual displacement remaining after non-rigid registration was 1.82 mm. There is a statistically significant improvement in alignment accuracy utilizing our non-rigid registration in comparison to the currently used technology (p<0.001).

CONCLUSIONS

We were able to achieve intra-operative rigid and non-rigid registration of (1) pre-operative structural MRI with intra-operative T1w MRI; (2) pre-operative fMRI with intra-operative T1w MRI, and (3) pre-operative DT-MRI with intra-operative T1w MRI. The registration algorithms as implemented were sufficiently robust and rapid to meet the hard real-time constraints of intra-operative surgical decision making. The validation experiments demonstrate that we can accurately compensate for the deformation of the brain and thus can construct an augmented reality visualization to aid the surgeon.

Periventricular nodular heterotopia: A challenge for epilepsy surgery

Pharmacoresistant focal epilepsies due to periventricular nodular heterotopia are a diagnostic and therapeutic challenge because of the need of invasive presurgical diagnostics and the selection of an optimal surgical approach. Invasive investigations in previous studies showed that focal epileptic activity can be correlated predominantly either with one of the nodular heterotopia or with neocortical epileptogenic zones distant to the periventricular nodules. Up to now, invasive recordings were required for localization of epileptic activity and its correlation to heterotopia. The following case presentation reports on a non-invasive approach using magnetic source imaging (MSI) combined with intraoperative ECoG. MSI combines preoperative data from magnetic resonance imaging (MRI) with magnetoencephalography (MEG). The MSI data for definition of the localization of the epileptic activity and functional important areas were coregistered with the intraoperative high-field-MRI and diffusion tensor imaging-based fiber tracking (DTI) of the visual pathway using a neuronavigational system. A neuronavigation-guided surgical resection of the epileptogenic area was performed leaving the heterotopia and the visual tract fibers intact. Postoperatively preservation of the visual fields was documented and the frequency of seizures was markedly reduced.

Sensorimotor Cortex Localization: Comparison of Magnetoencephalography, Functional MR Imaging, and Intraoperative Cortical Mapping

PURPOSE

To prospectively evaluate magnetoencephalography (MEG) and functional magnetic resonance (MR) imaging, as compared with intraoperative cortical mapping, for identification of the central sulcus.

MATERIALS AND METHODS

Fifteen patients (six men, nine women; age range, 25-58 years) with a lesion near the primary sensorimotor cortex (13 gliomas, one cavernous hemangioma, and one meningioma) were examined after institutional review board approval and written informed consent from each patient were obtained. At MEG, evoked magnetic fields to median nerve stimulation were recorded; at functional MR imaging, hemodynamic responses to self-paced palmar flexion of the wrist were imaged. General linear model analysis with contextual clustering (P < .01) was used to analyze functional MR imaging data, and dipole modeling was used to analyze MEG data. MEG and functional MR localizations were compared with intraoperative cortical mappings. The distance from the area of functional MR imaging activation to the tumor margin was compared between the patients with discordant and those with concordant intraoperative mapping findings by using unpaired t testing.

RESULTS

MEG depicted the central sulcus correctly in all 15 patients, as verified at intraoperative mapping. The functional MR imaging localization results agreed with the intraoperative mappings in 11 patients. In all four patients with a false localization, the primary activation was in the postcentral sulcus region, but it did not differ significantly from the primary activation in the patients with correct localization with respect to proximity to the tumor (P = .38). Furthermore, at functional MR imaging, multiple nonprimary areas were activated, with considerable interindividual variation.

CONCLUSION

Although both MEG and functional MR imaging can provide useful information for neurosurgical planning, in the present study, MEG proved to be superior for locating the central sulcus. Activation of multiple nonprimary cerebral areas may confound the interpretation of functional MR imaging results.