OPM MEG in Pre-surgical Mapping for Patients With Epilepsy

1. BACKGROUND Magnetoencephalography (MEG) is a non-invasive brain imaging technique which offers unique, non-redundant clinical information in the planning of surgery for patients with epilepsy. The MEG data are used to localise the inter-ictal activity that in turn can help formulate hypotheses on the localisation of the seizure onset zone. Better outcomes are associated with earlier surgery (i.e., when a child is younger), and with greater preservation of functional tissue. Aston IHN has been offering a MEG service to children and adults awaiting neurosurgery for two decades. MEG works by measuring the magnetic fields that indicate the electrical signatures of neural populations acting in synchrony. The established MEG technology relies on cryogenic cooling of the sensors to obtain very high sensitivity, and therefore traditional MEG devices are bulky with a rigid helmet, and also expensive to run. The patient is required to remain very still during the recording, which can be uncomfortable especially for children. Furthermore, the majority of MEG systems worldwide are designed for adult-sized heads and provide poor signal to noise ratios in younger patients. There is a new generation of MEG systems called OPM-MEG systems. With a different sensor design, these "wearable" devices operate at or near room temperature and make use of a lightweight helmet or hat that can allow the patient to move during recordings and can be sized appropriately for any age. In theory, OPM-MEG promises better data quality, because sensors are closer to the head (increasing signal), and may improve the efficacy of MEG as a pre-surgical diagnostic tool by increasing spatial resolution and inter ictal discharge detection. The aim of this research is to enable the direct comparison of OPM-MEG with traditional (cryogenic) MEG, so that it can be determined whether the new OPM-MEG technology offers the same benefits to patients. Specifically the aim is to record data on both devices, for a small series of 20 cases, and to develop parallel analysis routines to compare the accuracy of the results obtained from each device.
2. RATIONALE OPM-MEG devices employ different sensor technology from traditional cryogenic MEG systems; also, the sensors are also placed closer to the scalp, and in a less dense configuration. There are additional variations in sensor properties such as signal to noise ratio which when coupled with variations in sensor placement can alter the detectability of brain signals. Furthermore, interpretation of MEG data relies on analysis software pipelines, the vast majority of which have been developed for cryogenic MEG.

Before OPM-MEG can be fully validated for clinical work with patients, it is necessary to establish the equivalent data recording and data analysis pipelines, that enable direct comparison between the outputs of each measurement technique. Specifically, for pre-surgical MEG measurements in patients with epilepsy, it is important to know whether OPM-MEG can replicate or improve upon the spatial accuracy and signal sensitivity of cryogenic MEG in identifying inter-ictal abnormalities in brain data that can help identify the epileptogenic source.

Theoretical studies do suggest that OPM-MEG should offer sufficient sensitivity, but there is a lack of available patient data to enable the development of realistic data processing pipelines for a systematic comparison between the two kinds of system. This study therefore aims to collect a case-series of datasets where the same patient has participated in a MEG recording on both a traditional cryogenic MEG system and a novel OPM MEG system. These data will be used to refine data processing pipelines, to explore the patient experience, and to perform an initial comparative study between the two systems.

This work is worthwhile to patients, because if OPM-MEG is found to be at least equivalent to traditional cryogenic MEG it offers, firstly a much more ergonomic, comfortable recording environment where patients can move instead of being in a restricted position. Secondly, the helmet housing the sensors can easily be adjusted to fit children of all ages, meaning that MEG can contribute to pre-surgical evaluations in the youngest patients who currently don't fit in a traditional system. Finally, OPM-MEG systems, not requiring cryogenic cooling, are much cheaper to run than the traditional MEG systems and therefore potentially more cost-effective. In summary, OPM-MEG has the potential to benefit much greater numbers of patients than traditional cryogenic MEG.