Hideaki Ishibashi

Feasibility and limitations of magnetoencephalographic detection of epileptic discharges: Simultaneous recording of magnetic fields and electrocorticography

Abstract Magnetoencephalography (MEG) is considered clinically useful in localizing the epileptogenic focus in partial epilepsy. However, the relationship between the extent of the brain involved in paroxysmal activities and the magnetic field changes at the scalp has not been fully clarified. Furthermore, whether paroxysmal activities generated in deep brain structures such as the hippocampus can be detected magnetically is uncertain. Eight patients with temporal lobe epilepsy and two with extratemporal lobe epilepsy underwent chronic recording from subdural electrodes. Magnetic and electrocorticographic discharges representing epileptic activity were recorded simultaneously. MEG recorded magnetic field changes originating from paroxysmal activity in the superiolateral cerebral cortex when the amplitudes of the electrical paroxysmal activities exceeded 100 µ V and extended over more than 3 cm2 of cortical surface. MEG failed to record paroxysmal activity localized to the medial temporal lobe. MEG is often useful in identifying a spike focus in the superiolateral aspects of the cerebral hemisphere, but not discharges arising from the medial temporal lobe. Rapid decay of the magnetic field is likely to be the reason for this limited sensitivity to medial discharges. [Neurol Res 2002; 24: 531-536]

Differential interaction of somatosensory inputs in the human primary sensory cortex: a magnetoencephalographic study

OBJECTIVE Somatosensory evoked magnetic fields (SEFs) were recorded to investigate the interaction of the somatosensory inputs using the modality of electrical finger stimulation in 6 normal subjects. METHODS Electrical stimuli were given to the index (II), middle (III) or little (V) fingers individually, and also to pairs of either the II and III simultaneously, or the II and V simultaneously. The interaction ratio (IR) was calculated as the ratio of the SEF amplitude by simultaneous two-finger stimulation to the arithmetically summed SEF amplitudes of two individual-finger stimulations. RESULTS SEFs showed 3 major components: N22m, P30m and P60m. The N22m and P60m revealed a clear somatotopic organization in the primary sensory cortex (S1) in the sequence of II, III and V, while the P30m showed a cluster with medial location compared with N22m and P60m in S1. The N22m had a significantly greater IR in II and III stimulation compared to that in II and V stimulation. The P60m also showed a similar trend in the IR but was greater than that of N22m. In contrast, the IR in P30m showed no such tendency. CONCLUSION The interaction of S1 was most influenced when adjacent receptive fields were activated in the modality of electrical finger stimulation. Our results were consistent with the concept that the Brodmanns areas in S1 which produce the 3 components of the SEFs have different functional organization.

Multimodality neuroimaging evaluation improves the detection of subtle cortical dysplasia in seizure patients

Abstract The purpose of this study is to investigate if multimodality neuroimaging evaluation increases the detection of subtle focal cortical dysplasia as part of an epilepsy surgery evaluation. Three patients with normal magnetic resonance imaging and histopathological findings of focal cortical dysplasia were reviewed. Their magnetoencephalography recordings were performed on whole-head magnetoencephalography system. Magnetic resonance images were re-evaluated with special inspection in limited regions guided by magnetoencephalography spike localization. Two patients had ictal and interictal single photon emission computed tomography study after administration of Tc99m ECD. In two patients we found tiny focal abnormalities including slightly increased cortical thickness and blurred gray–white matter junction at the locations of interictal events after re-evaluation of the MR images indicating focal cortical dysplasia. The third patient showed focal atrophic change. All patients are seizure free after surgery. Both ictal and interictal single photon emission computed tomography showed hyperperfusion in the dysplastic cortex regions. Multimodality neuroimaging study can improve the detection of focal cortical dysplasia. Normal magnetic resonance images should be re-evaluated for subtle signs of focal cortical dysplasia especially when magnetoencephalography recording demonstrate focal epileptic discharges.

Localization of ictal and interictal bursting epileptogenic activity in focal cortical dysplasia : agreement of magnetoencephalography and electrocorticography

Abstract Focal cortical dysplasia (FCD) is often associated with severe partial epilepsy. In such cases, interictal frequent rhythmic bursting epileptiform activity (FBREA) on both scalp electroencephalography (EEG) and electrocorticography (ECoG) is generally accepted to be identical to the ictal epileptiform activity. We used magnetoencephalography (or Magnetic Source Imaging (MSI)) to determine the epileptogenic zone in a 6-year-old patient with histopathologically proven FCD and normal magnetic resonance imaging (MRI). MSI was used to localize the sources of both ictal activity and FRBEA, which was then compared with ECoG findings. The intracranial sources of both types of activity co-localized in the left inferior frontal and superior temporal gyri. The location and extent of the epileptogenic area determined by MSI was essentially identical to that determined directly through extra-operative ECoG. In the absence of structural abnormalities detectable on MRI, the noninvasive method of MSI provided valuable information regarding the location and extent of the primary epileptogenic area. This was critical for pre-surgical planning regarding placement of intracranial electrodes and for risk-benefit evaluation. [Neurol Res 2002; 24: 525-530]

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

Abstract In order to investigate functional topography of the hand somatosensory cortex in five patients with periRolandic 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. [Neurol Res 2001; 23: 29-38]

MAGNETIC SOURCE IMAGING OF THE SENSORY CORTEX ON THE SURFACE ANATOMY MR SCANNING

Surgery for lesions either within or close to the central sulcus of the brain always carries the risk of inducing iatrogenic motor or sensory deficits. We performed advanced magnetic source imaging (MSI) of the somatosensory cortex combined with 3-dimensional (3-D) surface anatomy scanning (SAS) of magnetic resonance imaging on 9 patients who had peri-Rolandic lesions, including 7 tumor cases, 1 arteriovenous malformation and 1 focal cortical dysplasia. 3-D MSI mapped out the entire somatosensory homunculus and localized the lesion on a cortical surface image. The results were then used for developing an appropriate surgical strategy and also as a reference in the operating room. This relationship and deduced localization were both confirmed by a cortical recording of the somatosensory evoked potentials at the time of surgery. Case histories of selected patients are briefly reported to demonstrate how the method can be used to improve the safety of surgical excisions of peri-Rolandic lesions. MSI on SAS including the cortical veins, was thus found to provide a readily interpretable presurgical road map of the cortical surface which compares favorably to that of intraoperative brain mapping.

Multimodality functional imaging evaluation in a patient with Rasmussen's encephalitis

Rasmussens encephalitis (RE) is a cryptogenic progressive inflammatory disorder of the brain that causes severe neurological problems, including intractable focal epilepsy. In select patients, aggressive treatment using cerebral hemispherectomy may ameliorate the devastating cognitive decline that accompanies this disease, even if the epileptic focus appears broadly distributed. We present a case of histopathologically-confirmed RE evaluated using a multimodal process that explored the physical and functional aspects of the associated epilepsy. This process included magnetic resonance imaging, single photo emission computed tomography, electroencephalography, and magnetoencephalography (MEG). The findings indicate that functional brain imaging data may greatly assist the surgical treatment decision-making process in RE, especially when structural imaging fails to reveal definitive localizing information. In addition, MEG may provide insights about the cortical reorganization of somatosensory cortex following hemispherectomy.

Magnetoencephalographic evaluation of anterior corpus callosotomy for intractable epilepsy in a patient with lennox-gastaut syndrome

Abstract We analyzed the preoperative and postoperative interictal magnetoencephalographic (MEG) patterns in a 32-year-old woman with Lennox-Gastaut syndrome who underwent an anterior corpus callosotomy (CC). A 37-channel biomagnetometer was used for the simultaneous recording of electroencephalogram (EEG) and MEG. Preoperatively, interictal EEG demonstrated a bilaterally synchronous spike and slow wave bursts with a maximum amplitude on the bifrontal region. The waveforms of the MEG were similar but did not completely correspond to those of the EEG. The estimated equivalent current dipoles (ECDs) originating from bilaterally synchronous paroxysmal discharges formed a dense cluster on the bilateral frontal lobes, especially on the deep midfrontal region. After anterior CC, MEG disclosed a marked reduction of the number of ECDs. The preoperative MEG patterns were thought to be representative of interhemispheric synchrony as a large dipole orientation, and these MEG paterns were partly disrupted by anterior CC.

Posterior fossa chondroma arising from the tentorium: a case report.

Intracranial chondroma is a rare tumor with an estimated incidence of 0.1–0.2 percent of all intracranial tumors [9]. The most common site of origin is the synchondrosis of the skull base. Some authors have reported intracerebral [2] or meningeal chondroma [1,3,8,14,15]. However, posterior fossa chondroma arising from the tentorium is rare. Here we report a case of a tentorial chondroma in the posterior fossa.

Do different writing systems involve distinct profiles of brain activation? A magnetoencephalography study

Abstract Receptive language-specific cortical maps have been repeatedly verified through normative and clinical magnetoencephalography studies. However, different writing systems may entail distinct neuro-anatomical substrates, hence different brain activation patterns for reading the various types of script. The project presented here is an attempt to describe the brain mechanisms mediating printed word recognition in languages with complex writing systems, such as Japanese, in view of the implications for a neurolinguistic model of language processing.