Gregory L. Barkley

MEG and EEG in epilepsy.

Summary: Both EEG and magnetoencephalogram (MEG), with a time resolution of 1 ms or less, provide unique neurophysiologic data not obtainable by other neuroimaging techniques. MEG has now emerged as a mature clinical technology. While both EEG and MEG can be performed with more than 100 channels, MEG recordings with 100 to 300 channels are more easily done because of the time needed to apply a large number of EEG electrodes. EEG has the advantage of the long‐term video EEG recordings, which facilitates extensive temporal sampling across all periods of the sleep/wake cycle. MEG and EEG seem to complement each other for the detection of interictal epileptiform discharges, because some spikes can be recorded only on MEG but not on EEG and vice versa. Most studies indicate that MEG seems to be more sensitive for neocortical spike sources. Both EEG and MEG source localizations show excellent agreement with invasive electrical recordings, clarify the spatial relationship between the irritative zone and structural lesions, and finally, attribute epileptic activity to lobar subcompartments in temporal lobe and to a lesser extent in extratemporal epilepsies. In temporal lobe epilepsy, EEG and MEG can differentiate between patients with mesial, lateral, and diffuse seizure onsets. MEG selectively detects tangential sources. EEG measures both radial and tangential activity, although the radial components dominate the EEG signals at the scalp. Thus, while EEG provides more comprehensive information, it is more complicated to model due to considerable influences of the shape and conductivity of the volume conductor. Dipole localization techniques favor MEG due to the higher accuracy of MEG source localization compared to EEG when using the standard spherical head shape model. However, if special care is taken to address the above issues and enhance the EEG, the localization accuracy of EEG and MEG actually are comparable, although these surface EEG analytic techniques are not typically approved for clinical use in the United States. MEG dipole analysis is approved for clinical use and thus gives information that otherwise usually requires invasive intracranial EEG monitoring. There are only a few dozen whole head MEG units in operation in the world. While EEG is available in every hospital, specialized EEG laboratories capable of source localization techniques are nearly as scarce as MEG facilities. The combined use of whole‐head MEG systems and multichannel EEG in conjunction with advanced source modeling techniques is an area of active development and will allow a better noninvasive characterization of the irritative zone in presurgical epilepsy evaluation. Finally, additional information on epilepsy may be gathered by either MEG or EEG analysis of data beyond the usual bandwidths used in clinical practice, namely by analysis of activity at high frequencies and near‐DC activity.

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.

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

Purpose:u2002 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.

Techniques for DC Magnetoencephalography

DC shifts are known to occur in association with a number of physiologic phenomena including spreading depression, hypoxia, epilepsy, and hypercapnia and possibly in migraine, closed head injury, and ischemia. Magnetoencephalography (MEG) makes it possible to record these shifts by prolonged DC monitoring of brain activity and offers several advantages over DC EEG and DC electrocorticography. Among the advantages of MEG is its non-invasive nature and the lack of impedance changes at the electrode-tissue interface that produce baseline shifts in DC EEG. In DC MEG measurements, great care must be taken in dealing with a variety of artifactual signals. Environmental noise can be reduced by magnetic shielding and recognized by use of reference magnetometers. Patient-generated artifacts are numerous and can be recognized and limited by a variety of methods.

Turning a new page in clinical magnetoencephalography: Practicing according to the first clinical practice guidelines

Magnetoencephalography (MEG) has been in existence for four decades (Cohen, 1968, 1972), and now, a large body of literature exists (Bagi c et al., 2009), including well-designed studies demonstrating its clinical value (Knowlton et al., 2008a, 2008b, 2009; Sutherling et al, 2008). Clearly, MEG is no longer a “new technology,” and it is a propitious time to promulgate guidelines for MEG evaluations and to practice according to them. The main reasons, of course, are the usual ones: a crying need to ensure that MEG laboratories are adhering to good practice, a desire for systematic comparison across laboratories and in multicenter studies that demand consistent practices, and some minimal standards that both laboratory directors and payers can point to. It also is in keeping with the tradition of the American Clinical Neurophysiology Society, which for the past several decades has formulated and revised Clinical Practice Guidelines (CPGs) on a variety of neurophysiologic diagnostic tests (see http://www.acns.org/guidelines.cfm for a listing). Other bodies will dictate what good practice is if we do not. Society and regulatory bodies want to ensure competency, and medical practitioners expect leadership toward quality (Clavien et al., 2005; Nahrwold, 2010). With health care reform high on the list of federal priorities and no money to spend on it, there will certainly be added scrutiny focused on new and expensive procedures. The very existence of voluntarily produced and expertly reviewed guidelines demonstrates a level of professionalism and maturity that establishes a baseline of clinical credibility. Clinical Practice Guidelines have been a reality in the medical profession for decades (e.g., Schorow and Carpenter, 1971; Talley et al., 1990; Wiebe, 2010; http://www.acns.org/guidelines.cfm). Yet, actual penetration of these guidelines into clinical practice varies (Haneef et al., 2010; Wiebe, 2010). To move toward excellence in MEG, as in all areas of clinical medicine, we must first obtain a clear picture of the current practices and the roles of the people practicing. Hence, the process of establishing the American Clinical Magnetoencephalography Society’s (ACMEGS) first CPGs started with an assessment of the state of clinical MEG in the United States (Bagi c, 2011). This survey was conducted in 2008 and included 90% of MEG centers providing clinical services at that time. Of course, not all individuals practicing clinical MEG from each participating center responded, and the field has dramatically grown even further in the past three years. Despite these and other limitations, this survey is the first systematic attempt to recount the prevailing clinical MEG practice in the United States, and it provides several important points to consider (Bagi c, 2011). The survey revealed a diversity of organizational structures and a large variability in daily practice. In more than a quarter of the surveyed centers, clinical reports of epilepsy MEG studies are signed by nonneurologists, two of whom were nonphysicians. Another remarkable finding was that the turnaround time from test to report ranged from 0.5 to 30 days, and this reporting time variability was not related to volume. These results demonstrate not only numerical variability but also suggest fundamental differences in practice and raise important questions. Should those of us struggling to complete our analysis and reports within even several days or a week be embarrassed that we cannot complete them within a day? Should we attempt to massively streamline our practice? And on the flip side, is there any reason why reports should take up to 30 days to send out in any clinical MEG center? Integration into the overall clinical neurophysiology community is crucially important. Considering the complementary nature of MEG and EEG techniques (Barkley and Baumgartner, 2003, Ebersole and Ebersole 2010), it was reassuring to find that all centers claimed to be using EEG collected simultaneously with MEG in some way, but it remains concerning that EEG is used variably in study processing and interpretation. Some centers use EEG only to define the time slice of the MEG signal for dipole modeling, while rare centers also engage in EEG source localization. Although only a small point, the fact that the number of averaged responses used for mapping a particular modality ranged across centers by a factor of 19 is further illustrative of a wide variability in practicedor is a high number of averages an indication of fundamentally low signal quality?

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.