Magnetoencephalography and magnetic source imaging

Current brain imaging techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI), provide noninvasive, high-resolution images that depict fine anatomic structure and delineate pathology by control of image contrast and sensitivity to the physicochemical microenvironment. These methods, although invaluable for the identification, characterization, and localization of lesions, do not provide any assessment of the functional viability of brain tissues, nor of the spatial organization of sensory, motor, and cognitive systems. However, such additional functional information is of great significance to the clinician in the determination of treatment strategies and patient management.     

Continuous source imaging of scalp ictal rhythms in temporal lobe epilepsy

PURPOSE: We wished to determine whether continuous EEG source imaging can predict the location of seizure onset with sublobar accuracy in temporal lobe epilepsy (TLE). METHODS: We retrospectively analyzed the earliest scalp ictal rhythms, recorded with 23- to 27-channel EEG, in 40 patients with intractable TLE. A continuous source analysis technique with multiple fixed dipoles (Focus 1.1) decomposed the EEG into source components representing the activity of major cortical sublobar surfaces. For the temporal lobe, these were basal, anterior tip, anterolateral, and posterolateral cortex. Ictal EEG onset was categorized according to its most prominent and leading source component. All patients underwent intracranial EEG studies before epilepsy surgery, and all had a successful surgical outcome (follow-up >1 year). RESULTS: Most patients with ictal rhythms having a predominant basal source component had hippocampal-onset seizures, whereas those with seizures with prominent lateral source activity had predominantly temporal neocortical seizure origins. Seizures with a prominent anterior temporal tip source component mostly had onset in entorhinal cortex. Seizures in some patients had several equally large and nearly synchronous source components. These seizures, which could be modeled equally well by a single oblique dipole, had onset predominantly in either entorhinal or lateral temporal cortex. CONCLUSIONS: Multiple fixed dipole analysis of scalp EEG can provide information about the origin of temporal lobe seizures that is useful in presurgical planning. In particular, it can reliably distinguish seizures of mesial temporal origin from those of lateral temporal origin.     

Uses of magnetic resonance imaging (MRI) in presurgical evaluation of internal derangement of temporomandibular joint (TMJ)

Nine cases of unilateral internal derangement of TMJ indicated for surgical intervention, were selected for this study. The affected joints were imaged by MRI presurgically. The non-affected joints were also imaged as controls. MRI findings were correlated with intraoperative surgical findings. The results were absolutely accurate as regards degenerative joint pathosis and capsular rupture, and with high degree of accuracy (85.7%) in detection of disc displacement. Two cases of disc perforation were not detected by MRI. The results were promising and encouraging the use of this noninvasive imaging technique as presurgical aid in TMJ disorders.     

Correlation of functional magnetic source imaging with intraoperative cortical stimulation in neurosurgical patients

This study compared noninvasive preoperative functional imaging by using magnetoencephalography (MEG) with intraoperative direct cortical stimulation in ten patients undergoing neurosurgery. The goal was to assess the accuracy and reliability of MEG-based functional imaging in these patients as a possible replacement or adjunct for direct cortical stimulation with electrocorticography. Objective comparison of intraoperative mapping with preoperative MEG procedures was achieved by intraoperative recording of mapped cortical locations for motor responses using an interactive image-guided surgical device, the ISG viewing wand, with which mapping points could be marked on a previously acquired (MRI) set. In all ten patients, at least one stimulation site elicited a response during both MEG and intraoperative mapping. The central sulcus ipsilateral to the lesion was only directly visible on high-resolution MRIs in 3/10 cases and equivocally in 2/10. Coregistered with MRI to form magnetic source images (MSIs), MEG predictions of the postcentral gyrus were possible in all 10 cases. In all 10 cases, these were in agreement with intraoperative estimation of the precentral gyrus. Functional mapping of somatosensory cortex was achieved noninvasively in surgical patients by using MSI. The accuracy, compared with cortical stimulation, was always sufficient to define motor and somatosensory strips.     

A proposed modification of the Wada test for presurgical assessment in temporal lobe epilepsy

The distribution of prolactin receptors (PRL-R) in the rat brain was investigated for the first time with the immunohistochemical technique using monoclonal antibodies raised against PRL-R purified from rat liver. Granular immunostaining was observed in neurons and along their dendritic processes and fibers. PRL-R like immunoreactive neurons were found in a number of brain areas. There was a very dense labelling in the cerebral cortex (pyramidal cell layer), septal nuclei, amygdaloid complex as well as in the hypothalamus (suprachiasmatic, supraoptic, paraventricular and dorsomedial nuclei). A dense staining was seen in the substantia nigra, habenula and in the paraventricular thalamic nucleus. Immunostaining was also found in the choroid plexus and in the subcommissural organ. Comparison between the present distribution and that of PRL-like immunoreactivity indicates that the density of PRL-R generally corresponds to that of the fibers. However, in some regions densely stained by PRL-R antibody, there are very few PRL-immunoreactive fibers. These results are suggestive of different modes of action of PRL in the brain.     

Prolonged video EEG monitoring in differential diagnosis of seizures and in presurgical epilepsy diagnosis]

Prolonged video-EEG-monitoring facilitates a correlation of clinical seizure semiology and corresponding EEG changes. Indications for prolonged video-EEG-monitoring comprise differential diagnosis of epileptic and psychogenic seizures, correct classification of epileptic syndromes and presurgical evaluation of patients with medically refractory focal epilepsies. 6000 patients in Austria would benefit from epilepsy surgery with an additional 150 bis 200 new patients appearing each year. Presurgical evaluation consists of a non-invasive Phase I and an invasive Phase II. During Phase I each patient is evaluated with a prolonged video-EEG-monitoring with scalp-EEG, a MRI-scan, a SPECT- and/or PET-scan, a neuropsychological evaluation and a Wada-test. If the epileptogenic zone cannot be localized adequately with these methods, invasive electrophysiological techniques with intracranial (epidural peg-electrodes, foramen-ovale electrodes, subdural strip or grid electrodes) or intracerebral electrodes (stereotaxically implanted depth electrodes) have to be applied. Epilepsy surgery renders 70 to 80% of patients seizure free and thus can regarded an effective and safe treatment option for patients with medically refractory focal epilepsies.     

Neuron loss localizes human temporal lobe epilepsy by in vivo proton magnetic resonance spectroscopic imaging

Temporal lobe epileptogenic foci were blindly localized in 8 patients with medically refractory unilateral complex partial seizures using noninvasive in vivo proton magnetic resonance spectroscopic imaging (1H-MRSI) with 4-ml effective voxel size. The brain proton metabolite signals in 8 matched normal controls were bilaterally symmetrical within +/- 10%. The hippocampal seizure foci had 21 +/- 5% less N-acetyl aspartate signal than the contralateral hippocampal formations (p < 0.01). The focal N-acetyl aspartate reductions were consistent with pathology findings of mesial temporal sclerosis with selective neuron loss and gliosis in the surgically resected epileptogenic foci. Proton MRSI correctly localized the seizure focus in all 8 cases. By comparison, MR imaging correctly localized 7 of 8 cases and single photon emission computed tomography correctly localized 2 of 5 cases. No lactate was detected in these interictal studies. No significant changes in choline or creatine were observed. In conclusion, 1H-MRSI is a useful tool for the noninvasive clinical assessment of intractable focal epilepsy. These preliminary results suggest that 1H-MRSI can accurately localize temporal lobe epileptogenic foci.     

Volumetric analysis of epilepsy surgery resections using high resolution magnetic imaging: technical report

A method is described for accurately measuring the volume and site of epilepsy surgery resections utilizing magnetic resonance imaging. Accuracy has been assessed using post-mortem studies, and both the intra- and interobserver variability is consistently less than 5%. The technique has so far been applied to 25 patients following a variety of operations for medically intractable epilepsy. It provides the crucially accurate baseline required for meaningful follow-up outcome studies of epilepsy surgery. Consequently, it should allow the development of more precise prognostic indices for such operations.     

Preliminary experimental evaluation of an inverse source imaging procedure using a decoupled coil detector array in magnetic resonance imaging

Results of treatment for 104 forearm shaft fractures in 70 patients have been presented. In all cases included in this study an open reduction of the fracture was followed by AO plate stabilization. Functional and radiological assessment was carried out according to the criteria of Anderson et al. Excellent and good functional results were achieved in 48 cases, fair in 10 and poor in 12 cases. The ulna united in 75%, the radius in 78% (delayed union included). Cross- union occurred in three patients, one case of destabilization at fracture site was observed, no infection has been noted. AO plate osteosynthesis proved to be still valuable mode of treatment for forearm shaft fracture.     

Incidence of lesions described by magnetic resonance imaging in the clarification of focal epilepsy of frontal and temporal origin

AIM: Today, MRI is an integral part of the presurgical evaluation of patients suffering from partial epilepsy. These patients frequently show focal morphological abnormalities with potential epileptogenic character and surgical resection of these lesions is associated with superior postsurgical outcome as to seizure frequency. Apart from easily detectable defects, such as post-traumatic lesions or cerebral infarction, as wide variety of mainly small abnormalities can be detected using MRI. METHODS: In this study, 484 patients suffering from partial epilepsy of temporal or frontal onset were evaluated for the incidence of different lesions in this population. RESULTS: All lesions found were included without evaluating their potential epileptogenicity, which remains to be proven using other procedures (EEG, SPECT, PET, etc.). Involvement of the hippocampal formation was a major finding in temporal lobe epilepsy, which could be detected as sclerosis (T2w-images), atrophy (T2w-TSE or T1w-IR-images) or both (15%). In addition and in declining frequency various tumors (14%), post-traumatic lesion (-5%), and focal cortical dysplasia or other disturbances of cortical integrity (-4%) were found. These lesions are detectable with best contrast on different sequences. As a consequence it is suggested to acquire sequences in 3 dimensions including a T1w-SE, two (coronal and axial) double-echo-SE sequences and similarly two T1w-IR-sequences. The application of contrast media can be restricted to special questions, derived either from the first imaging results or from the patients history. CONCLUSION: Using qualitative data for interpretation, the sensitivity as to the detection of any focal pathology of a recent-generation MRI in this population was 75%, with 79% for temporal lobe epilepsies and 67% for frontal lobe epilepsies. Quantitative measurements of hippocampal volume or signal seem to be able to increase the sensitivity of the method.     

Depression of motor cortex excitability by low-frequency transcranial magnetic stimulation

We studied the effects of low-frequency transcranial magnetic stimulation (TMS) on motor cortex excitability in humans. TMS at 0.1 Hz for 1 hour did not change cortical excitability. Stimulation at 0.9 Hz for 15 minutes (810 pulses), similar to the parameters used to induce long-term depression (LTD) in cortical slice preparations and in vivo animal studies, led to a mean decrease in motor evoked potential (MEP) amplitude of 19.5%. The decrease in cortical excitability lasted for at least 15 minutes after the end of the 0.9 Hz stimulation. The mechanism underlying this decrease in excitability may be similar to LTD. TMS-induced reduction of cortical excitability has potential clinical applications in diseases such as epilepsy and myoclonus. Spread of excitation, which may be a warning sign for seizures, occurred in one subject and was not accompanied by increased MEP amplitude, suggesting that spread of excitation and amplitude changes are different phenomena and also indicating the need for adequate monitoring even with stimulations at low frequencies.     

Right hippocampal contribution to visual memory: a presurgical and postsurgical study in patients with temporal lobe epilepsy

Crowding of Culex sitiens Wiedemann larvae was investigated to determine whether pupation was delayed by the presence of chemical factors in the water or by mechanical interference. As the larval density increased from 0.05 to 1.0 larvae per milliliter of water, the duration of larval development increased from 7.5 to 23.0 d and larvae mortality increased from 1.7 to 99.6%. At high larval densities (2 larvae per milliliter), the larvae did not pupate, but remained alive as 4th instars for at least 50 d, even after their abundance subsequently declined due to mortality. When not crowded, 80% of the larvae fed at the surface; the remainder fed mainly on the bottom (even at a depth of 35 cm). Removal of chemical retardants by changing the water daily increased pupation to 65% (compared with 4% in controls, in which the water was not changed), but this was less than the effect of reducing mechanical interference by increasing the surface area, which increased pupation to 98%. Therefore, the main density-dependent factor was mechanical interference during feeding, but prolonged development to 19 d showed that chemical retardants also were important. The chemical retardant was not stable, so that 2nd generation larvae reared in the same water were unaffected, but its effect was irreversible, because most crowded larvae moved to clean water and a lower density did not pupate.     

Altering the brain circuits for reading through intervention: A magnetic source imaging study.

Intervention-related changes in spatiotemporal profiles of regional brain activation were examined by whole-head magnetoencephalography in 15 children with severe reading difficulties who had failed to show adequate progress to quality reading instruction during Grade 1. Intensive intervention initially focused on phonological decoding skills (for 8 weeks) and, during the subsequent 8 weeks, on rapid word recognition ability. Clinically significant improvement in reading skills was noted in 8 children who showed "normalizing" changes in their spatiotemporal profiles of regional brain activity (increased duration of activity in the left temporoparietal region and a shift in the relative timing of activity in temporoparietal and inferior frontal regions). Seven children who demonstrated "compensatory" changes in brain activity (increased duration of activity in the right temporoparietal region and frontal areas, bilaterally) did not show adequate response to intervention. Nonimpaired readers did not show systematic changes in brain activity across visits. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Magnetic resonance imaging in lactic acidosis

Mitochondrial defects, defects in gluconeogenesis, and biotin-responsive multiple carboxylase deficiency are disorders characterized by primary lactic acidosis. In this review, characteristic findings in magnetic resonance imaging (MRI) of the brain, as related to histopathological abnormalities, are described for the different disorders and the diagnostic value of the MRI findings is discussed. Inborn errors of metabolism with primary lactic acidosis should be considered in particular when MRI shows lesions similar to or reminiscent of effects of focal or generalized hypoxia-ischaemia, or when MRI shows signs of chronic neurodegeneration, but rarely in cases with predominantly white-matter changes.     

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.