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Epilepsy: a new magnetoencephalography technique

Brussels, Belgium – The Erasmus University Hospital (ULB) is testing a new magnetoencephalography technique which overcomes the limitations associated with a helmet adapted to liquid helium, cooled to minus 269 degrees. The icing on the cake, the technique is more suitable for children, is much more precise and costs much less.

The team from the Laboratory of Translational Neuroimaging and Neuroanatomy at the Erasme University Hospital (Free University of Brussels) has just taken a new step in the use of magnetoencephalography to detect and localize epileptic activity. She publishes a study in Radiology which demonstrates the major interest of new magnetic field sensors, “optical pumping magnetometers” [1]. This new technology allows the sensors to be placed directly on the patient’s scalp, which significantly increases the sensitivity of detecting epileptic activity. Explanations of Professor Xavier De Tiègewho leads this team.

What is the decisive step that this new technology allows you to take in the detection of epilepsy?

Professor Xavier De Tiège : Magnetoencephalography (MEG) is an innovative technique for measuring the magnetic fields induced by the electrical activity of brain neurons. As we know it today, it involves the use of a rigid helmet which has only one size suitable for adults. Some adults’ cranial perimeters do not “fit” into this rigid helmet, we have had a few patients whose heads have never been able to fit into the MEG. On the other hand, we have many patients with a smaller head and this is typically the case for children. MEG records the magnetic fields generated by the electrical activity of the brain. These fields decrease with distance: the further the sensors are from the brain, the lower the signal amplitude will be. However, the more space there is in the helmet, the more you can move. In children and some adults, the distance from the sensors to the brain is increased because the head is smaller than the helmet. Moreover, even if you have a head that adapts completely to the helmet, as conventional so-called “cryogenic” sensors must be immersed in liquid helium at minus 269 degrees, it is necessary to place a layer of insulation, a air space that separates the interior of the MEG with the sensors and the helium and the exterior of the helmet. The sensors are therefore immediately placed approximately 3 centimeters from the surface of the helmet. By possibly adding the distance caused by a head smaller than the unique size of the helmet, we can find ourselves with a distance of several centimeters, and therefore magnetic fields detected from further away and therefore at lower amplitude.

What is the consequence?

Professor Xavier De Tiège : As a result, the system is less sensitive in detecting brain activity, there is more movement and we may miss things that we could pick up if we were closer. Besides that, MEG systems that use cryogenic sensors are very expensive, since installing a complete system costs between 2 and 3 million. In addition, this technology which requires liquid helium has limitations. And so researchers have tried to develop new magnetic field sensors or new non-cryogenic technologies that would allow the sensors to be placed directly on the scalp.

How did they do it?

Professor Xavier De Tiège : To be able to do this, it is necessary to get rid of liquid helium, and therefore of the superconducting quantum interference device (SQUID) technology, which the sensors used in current cryogenic MEGs use. One of the technologies developed consists in using optically pumped sensors, which do not need to be cooled and can operate at room temperature. Therefore, they can be placed directly on the scalp, thus being much closer to the brain. An increase in signal amplitude is obtained, and if the noise level of optically pumped sensors is similar to that of SQUIDs, a better signal-to-noise ratio and therefore better quality data. And as the sensors move with the head, we overcome the problems of locating the head in the helmet during recordings.

How many of you use these sensors?

Professor Xavier De Tiège : There are a few centers around the world that are studying the use of these new optically pumped sensors to study brain activity. As we are a clinical centre, we are interested in the use of these sensors, in particular for epilepsy, which is the internationally recognized clinical indication for MEG. The aim of this study was to be able to demonstrate the interest of these sensors in epileptic children.

What you are tracking is hyper-activity of the neurons when the epileptic seizure is triggered?

Professor Xavier De Tiège : In epileptic patients, there are indeed epileptic seizures as such which are generally infrequent and occur randomly. This is called ictal activity, but between these, there are inter-ictal epileptic discharges, therefore between seizures, which are discharges of abnormal cerebral activity, generally lasting of less than 100 milliseconds and which are not accompanied by observable clinical observation. But that’s the signature of seizure activity. MEG makes it possible to record this inter-ictal activity. In the majority of patients, the region of the brain that generates this interictal activity is the same as that which generates the seizures. Patients were taken who had frequent interictal activity, as demonstrated on previous electroencephalograms (EEG). Cerebral activity was recorded with the optical pumping magnetometers placed on a bonnet and epileptic activity was then recorded with conventional MEG. The idea was to compare the type of signals that were recorded with these two devices. We were able to demonstrate, as expected, that having these sensors placed on the scalp increases the amplitude of epileptic activity, but also the signal-to-noise ratio. This suggests that one could be much more sensitive in detecting epileptic activity using optically pumped MEG, and that one could possibly discover new things about this epileptic activity.

It is recalled that MEG is not curative as such, but constitutes an aid to a better diagnosis…

Professor Xavier De Tiège : Cryogenic MEG systems as we currently have them are mainly used in the pre-surgical development of epilepsy. Thirty percent of patients with epilepsy will not respond to drug treatment and will continue to have seizures despite antiepileptic drugs. So, to try to eliminate seizures in these patients, the only curative treatment is surgery. For this, it is necessary to be able to precisely locate the area of ​​the brain that generates epileptic seizures, if possible without having to open the skull and in a non-invasive way. It must also be ensured that it is an area of ​​the brain that is not particularly important, such as the regions that control motor functions or language functions. If we operate on the patient in these areas, we risk paralyzing him or developing a language disorder.

How do you know if you have good candidates for surgery?

Professor Xavier De Tiège : So there is a long focus before any operation to make sure. We make EEG-video recordings to record epileptic seizures, we do a structural MRI to see if there is a lesion, then we do a pet-scan with 18F-FDG (glucose and fluorine-18) to see whether we can find regions of the brain that are malfunctioning, whether or not there is damage. Fluorodeoxyglucose (18F) is a radiopharmaceutical analogue of glucose in which the hydroxyl of carbon 2 of glucose is replaced by fluorine-18, a radioisotope of fluorine. It is used as a tracer in medical imaging by positron emission tomography (PET). It is in fact primarily metabolized by cells that consume a lot of glucose, particularly in the brain and in the liver, as well as by cancer cells. MEG is one of those examinations where epileptic activity can be localized in a totally non-invasive way. Then, we gather all of these examinations and we determine if the patient is operable.

In the long term, could MEG with optically pumped sensors replace the cryogenic MEG currently used?

Professor Xavier De Tiège : Indeed. It will cost less, it will be more adapted to patients and will have the advantage of providing more information since we are closer to the brain. The other advantage of going further is to use this system in clinical routine for the diagnosis of epilepsy. We do it less with the classic MEG, because it’s too expensive. We can now consider recording hours of activity to capture seizures, which is not done in MEG at the moment. It’s hard to imagine staying half a day, sitting or lying down, with your head locked in a helmet and unable to do anything else. With a hat, it is more possible. We can move a little, we are more free.

Financially, this technique is already five times cheaper for a better result. And what is certain is that like any technology, its price will drop rapidly when its production becomes more industrial.

The article was originally published on Mediquality.

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