Karen M. Mason

Language laterality determined by MEG mapping with MR-FOCUSS

Magnetoencephalography recordings were made on 27 patients with localization related epilepsy during two different language tasks involving semantic and phonological processing (verb generation and picture naming). These patients underwent the semi-invasive intracarotid amobarbital procedure (IAP), also referred to as the Wada test, to determine the language-dominant hemisphere. Magnetoencephalography (MEG) data were analyzed by MR-FOCUSS, a current density imaging technique. A laterality index (LI) was calculated from this solution to determine which hemisphere had more neural activation during these language tasks. The LIs for three separate latencies, within each language task, were calculated to determine the latency that correlated best with each patients IAP result. The LI for all language processing was calculated for the interval 150-550 ms, the second LI was calculated for the interval 230-290 ms (Wernickes activation), and the third LI was calculated for the interval 396-460 ms (Brocas activation). In 23 of 24 epilepsy patients with a successful IAP, the LIs for Brocas activation, during the picture naming task, were in agreement with the results of the IAP (96% agreement). One of three patients who had an undetermined or bilateral IAP had an LI calculated for Brocas activation (396-460 ms) that agreed with intracranial mapping and clinical testing. These results indicate an 89% agreement rate (24 of 27) for magnetoencephalographic LI determination of the hemisphere of language dominance.

Cortical hyperexcitability in migraine patients before and after sodium valproate treatment.

DC-magnetoencephalography (DC-MEG)waveforms arising during migraine aura were used to determine the effectiveness of prophylactic medication therapy on neuronal hyperexcitability. Nine patients were prescribed valproate (Depakote) for migraine prophylaxis. MEG scans were recorded during visual stimulation before commencing medication and again after 30 days of daily use of valproate. Cortical brain activity was recorded during stimulation with a black-and-white circular checkerboard pattern alternating at 8 Hz and were analyzed with MR-FOCUSS. Large-amplitude DC-MEG signals, imaged to extended areas of occipital cortex, were seen before therapy. After 30 days of prophylactic treatment, reduced DC-MEG shifts in the occipital cortex and reduced incidence of migraine attacks were observed. Using visual stimulation, the authors demonstrated the hyperexcitability of widespread regions throughout occipital cortex in migraine patients, explaining the susceptibility for triggering spreading cortical depression and migraine aura. This study confirms that MEG can noninvasively determine the status of neuronal excitability before and after therapy. This finding may be helpful in determining which prophylactic medications will be most effective in reducing hyperexcitability in particular patients.

MEG localization of language-specific cortex utilizing MR-FOCUSS

Objective:To demonstrate noninvasive localization of cognitive cortical areas involved in language processing with magnetoencephalography (MEG) interpreted by multiresolution FOCUSS (MR-FOCUSS), a current density imaging technique. Method:MEG data were collected during verb-generation and picture-naming tasks from 18 right-handed control subjects and 24 right-handed patients with epilepsy. Results: The averaged epic data from the verb-generation task, analyzed by MR-FOCUSS, showed initial activation in the left supramarginal gyrus, superior temporal gyrus, and angular gyrus at 239 ± 31 ms in all subjects, consistent with other language mapping studies. Average amplitude of underlying cortical sources was ~452 pAm. The averaged epic data from the picture-naming task, analyzed by MR-FOCUSS, showed activation in the left inferior frontal gyrus (IFG) area starting at 436 ± 40 ms in all subjects. Average amplitudes of underlying cortical sources were ~380 pAm. Conclusion:The time course of neuronal language processing can be imaged noninvasively with millisecond resolution by magnetoencephalography using the multiresolution FOCUSS technique.To the Editor: Bowyer et al.1 present a novel mathematical approach for determining patterns of brain activity associated with performance of language tasks. The software developed by the authors has two main advantages, which could, in theory, ensure a level of confidence sufficient for presurgical planning applications. First, it takes into account the anatomy of the brain, constraining potential “sources” of magnetic activity based on physiologic and physical considerations. Second, and perhaps most important, the software is designed to operate unsupervised, significantly reducing the impact of subjective input from the user, a limitation characteristic of the “standard” method of the iterative application of the single equivalent current dipole (ECD) that the Bowyer et al. method claims to surpass in efficiency. Incidentally, the ECD method remains the “gold standard” in magnetoencephalography (MEG) studies on the cerebral mechanisms of basic sensory functions and of language, because it is the only one that has been validated against invasive brain mapping techniques and postoperative outcome,2–5 yet Bowyer et al. do not acknowledge this. As with previous attempts to outline brain activity profiles using alternative mathematical approaches, the results reported by Bowyer et al. are both interesting and promising. Clinical applications of MEG will be greatly facilitated by the use of userindependent analysis techniques, provided, however, that the key requirement of external validation of activation maps that applies to every functional imaging method is fully met. External validation is particularly crucial for techniques that model activity in terms of spatially extensive sources (like the Bowyer et al. technique does), given the inherent uncertainty surrounding threshold selection for displaying activation images. Adopting different image thresholding criteria may modify the extent of cortical regions that appear active, thereby seriously affecting the utility of the technique for presurgical mapping applications. Hopefully, the method that Bowyer et al. advocate will soon meet the validation requirement and emerge as a “more sensitive and useful technique” than the “standard” one which has been validated. Yet until this transpires, we believe it is advisable not to confound hope with fact. Incidentally, we also believe it is advisable not to confound literary genres (i.e. epic) with segments or durations of time-series (i.e. epochs) which the authors repeatedly do in their text.

An Assessment of MEG Coherence Imaging in the Study of Temporal Lobe Epilepsy

Purpose:  This study examines whether magnetoencephalographic (MEG) coherence imaging is more sensitive than the standard single equivalent dipole (ECD) model in lateralizing the site of epileptogenicity in patients with drug‐resistant temporal lobe epilepsy (TLE).

Magnetoencephalographic localization of the basal temporal language area.

Magnetoencephalography (MEG) recordings were made on 25 native English-speaking patients with localization-related epilepsy during a semantic language task (verb generation). Eighteen right-handed subjects with normal reading ability had MEG scans performed during the same language task. MEG data was analyzed by MR-FOCUSS, a current density imaging technique. Detectable MEG signals arising from activation in the left fusiform gyrus, also known as the basal temporal language area (BTLA), occurred at 167 +/- 18 ms (n = 43) in all subjects. The BTLA has been associated with a variety of language production and comprehension tasks involving processing of semantic, orthographic, and phonologic information. MEG may become an important tool in efforts to further define the linguistic operations of specific regions within this language area.

A retrospective analysis of the effect of general anesthetics on the successful detection of interictal epileptiform activity in magnetoencephalography

BACKGROUND:A magnetoencephalography (MEG) study requires the patient to lie still for a prolonged period of time. In children and uncooperative adults with epilepsy, general anesthesia or sedation may be required to insure a good quality study. As general anesthetics have anticonvulsant and proconvulsant properties, we investigated whether the use of anesthesia reduced the successful detection of interictal epilepsy activity. METHODS:MEG testing was performed on 41 epilepsy patients (10 women, 31 men; 1–48 yr) while anesthetized. To determine the impact of anesthesia on the identification of epileptiform activity, the anesthesia group of patients was compared with all other patients with epilepsy who were recorded in our laboratory without anesthesia, as well as with a subgroup of children with epilepsy who were able to be recorded without the need for anesthesia. RESULTS:Propofol was used in 38 patients, etomidate in two, and one received sevoflurane. Twenty-nine (71%) were found to have interictal epileptiform activity in their MEG results. The percentage of MEG studies with a positive yield for interictal epileptiform activity is comparable with the percentage (63%) found in the patients with epilepsy undergoing MEG without anesthesia. In the 38 children younger than 18 yr, 28 (74%) had interictal epileptiform activity compared with 80% done without anesthesia. CONCLUSION:We conclude that levels of anesthesia needed to provide unconsciousness and immobility during MEG studies do not significantly alter the likelihood of recording interictal epileptiform spike activity with MEG.

Retrospective review of MEG visual evoked hemifield responses prior to resection of temporo-parieto-occipital lesions

SummaryVisual evoked cortical magnetic field (VEF) waveforms were recorded from both hemifields in 21 patients with temporo-parieto-occipital mass lesions to identify preserved visual pathways. Fifteen patients had visual symptoms pre-operatively. Magnetoencephalographic (MEG) VEF responses were detected, using single equivalent current dipole (ECD), in 17/21 patients studied. Displaced or abnormal responses were seen in 15 patients with disruption of pathway in one patient. Three of 21 patients had alterations in the surgical approach or the planned resection based on the MEG findings. The surgical outcome for these three patients suggests that the MEG study may have played a useful role in pre-surgical planning.

Slow brain activity (ISA/DC) detected by MEG

Summary: Infraslow activity (ISA), direct coupled (DC), and direct current (DC) are the terms used to describe brain activity that occurs in frequencies below 0.1 Hz. Infraslow activity amplitude increase is also associated with epilepsy, traumatic brain injuries, strokes, tumors, and migraines and has been studied since the early 90s at the Henry Ford Hospital MEG Laboratory. We have used a DC-based magnetoencephalography (MEG) system to validate and characterize the ISA from animal models of cortical spreading depression thought to be the underlying mechanism of migraine and other cortical spreading depression–like events seen during ischemia, anoxia, and epilepsy. Magnetoencephalography characterizes these slow shifts easier than electroencephalography because there is no attenuation of these signals by the skull. In the current study, we report on ISA MEG signals of 12 patients with epilepsy in the preictal and postictal states. In the minutes just before the onset of a seizure, large-amplitude ISA MEG waveforms were detected, signaling the onset of the seizure. It is suggested that MEG assessment of ISA, in addition to activity in the conventional frequency band, can at times be useful in the lateralization of epileptic seizures.

Periodic Discharges: Insight from Magnetoencephalography

Purpose: This study used magnetoencephalography (MEG) dipole localization and coherence measurement to evaluate the magnetic fields associated with periodic discharges. The primary goal of the study was to evaluate whether MEG could consistently localize quasiperiodic discharges that were observed on the EEG portion of the recording. The secondary objective was to evaluate whether coherence measurements would correlate with topographic maxima of epileptiform activity. Methods: A total of 13 inpatients, whose electrographic records demonstrated lateralized periodic discharges (LPDs), were recruited from Henry Ford Hospital neurology and intensive care units. Nine patients were found clinically to be in status epilepticus before the EEG determination of LPDs. Spontaneous cortical brain activity was recorded with 148-channel MEG for 10 minutes. Data were sampled at 508 Hz and DC-100 Hz and filtered from 1 Hz to 40 Hz. Interictal events were imaged with single equivalent current dipole localization. Magnetoencephalography coherence source imaging analysis was performed and compared with the cortical topography of LPD patterns and with the focal lesions seen on the MRI (9 patients) or computed tomography (5 patients) imaging modalities. Results: The morphology of periodic waveforms was similar between EEG and MEG portions of the study. In patients with substrate positivity on imaging studies, coherence analysis revealed a tendency for LPDs to arise from the interface between the lesion and the surrounding, uncompromised cortex rather than from the lesion itself. In nonlesional patients with recent status epilepticus, the localization of maximal coherence was in the temporal lobes. Conclusions: This study demonstrated that MEG is able to detect and localize LPDs arising from damaged and adjacent cortex. The MEG coherence source imaging measurements also suggest the presence of epileptogenic networks perilesionally in cases with focal lesions on imaging. In patients without acute anatomic abnormality, the MEG coherence identified the epileptogenic networks in temporal lobe structures. Magnetoencephalography coherence source imaging may provide physicians with markers for differentiating between LPDs arising from acute injury currents versus LPDs arising from prolonged status epilepticus.

What you need to know to become a MEG technologist

ABSTRACT. Magnetoencephcilography (MEG) is a way to non-invasively localize sources of electrical activity within the human brain, by measuring the very weak magnetic fields just outside ofthe head. This paper is an introduction to MEG for technologists who are interested in performing MEG studies. We have organized the paper into a brief overview of what MEG measures and how it does it, as well as a short history of the MEG manufacturers. There is a discussion of the dijferences in coils/sensors used to detect the magnetic fields, followed by a detailed description of what an average MEG technologist does to perform a MEG study. Some MEG centers may require more dutiesfrom the MEG technologist than are listed here and others may require fewer duties. We fnish the paper with the contraindications for a MEG study, a job description for the MEG technologist, and a MEG procedure checklist to help keep the tasks organized.