Improving access to magnetic encephalograph diagnostics

Using magnetoencephalography in diagnostics

In the magneto encephalograph, or ‘MEG’, approach, the ‘sensed’ magnetic field is the field that surrounds an electric current in the brain. To date, this has involved the use of cryogenically cooled SQUID magnetic sensors to measure extremely weak magnetic fields, such as those present at the surface of the skull due to neuronal electric currents.

Recent studies using MEG have reported successful classification of patients with multiple sclerosis, Alzheimer’s disease, schizophrenia, Sjögren’s syndrome, chronic alcoholism, facial pain and thalamocortical dysrhythmias. MEG can also be used to distinguish these patients from healthy control subjects, suggesting a future role of the tool in diagnostics.[1]

Despite the heightened sensitivity of magentoencephalography, the size, cost and complexity of the cryogenically cooled superconducting sensors involved have so far prohibited its widespread use. Because of this, most researchers continue to use EEG and tolerate the limitations in signal measurements, which in turn results in limitations to the information about neuronal currents that can be inferred. Innovation is still clearly needed to bring MEG technology to the fore.

Recent innovations in magnetoencephalography

An important recent development in this area has been the optically pumped MEG sensor (OPM), a quantum sensor which offers radical improvements to MEG systems because of its small size. This has enabled the use of large numbers of channels, increasing the detail that can be extracted in the resulting image.

These OPM sensors are low temperature (~50ºC) quantum devices that do not need cryogenic cooling to 4K. Although there is some heat production, the thermal management is far easier than with the previous generation of sensors. OPM sensors provide magnetic field sensing that has a low noise floor, giving high sensitivity and providing more information than previously possible. They also provide a dynamic response to support real time visualisation. The cost per channel of the OPM is also reduced relative to SQUID based systems, to allow magnetoencephalography systems with an increased number of channels to be constructed. With further iterations of the OPM by various manufacturers, the cost is expected to reduce further.

How this works in practice is that the OPM devices sense, sample and transmit a stream of information to a central processor. Multiple channels are combined to produce dynamic spatial imaging of the surface magnetic field, allowing electrical currents in the brain to be inferred.

Current MEG medical applications

This technology is currently being used for research purposes, however there are ongoing efforts to mature OPM based MEG into a recognised diagnostic scanning device. A number of organisations developing OPM based MEG systems for research have already been reported in open literature. These include Nottingham University in the UK (famous for having developed MRI scanning), CEA – Leti in France, which has a startup called MAG4Health which is applying its own OPM sensor, and Fieldline Inc in the US, which offers its own system based on the OPM technology provided by Qspin.

With a growing number of recognised medical applications, this is a promising start for enabling MEG diagnostics. However, it seems that maturing OPM technology is not without its challenges.

The challenges of maturing MEG diagnostics technology

From a system engineering perspective, there appear to be several key challenges for device developers in maturing OPM as a sensor system into a standard, serially manufactured system:

1) Reducing OPM costs

The costs of the OPM sensors will need reduction to allow widespread adoption of the technology. For example CEA – Leti in France has developed a device using the optically pumped MEG sensor, as has Sandia National Labs in the US[1]. Quspin Inc in the US has also developed its own devices that are commercially available, which was funded at least in part by US government investment. The Quspin device is currently listed with costs of $7,700 in quantities of 32 devices, indicating a rough cost of $770k for a 100-receiver system. Iteration of the OPM sensor is likely needed to reduce both the size and cost to allow affordable systems that can capture more detailed information about neuronal currents.

Image source: The QuSpin Zero-Field Magnetometer (QZFM) 

2) Usability challenges

The user experience from embodying a multiple sensor helmet that can fit patients of a range of sizes will present challenges. With a cable from each sensor and potentially 60-100 sensors, good design will be needed to make this scalable and to provide an acceptable user experience for comfort and weight. There is also some heat production by the sensors that need to be managed.

3) Magnetic screening requirements

A high degree of magnetic screening is required to ensure a quiet background magnetic environment, in order to get high fidelity basic signals from each OPM channel. This could involve a magnetic screened room and also background adaptive cancellation of the residual external field – a non-trivial challenge.

4) Locating sensors in 3D space

Accurate location of where the sensors are in 3D space will be needed to enable accurate measurement and imaging, and to allow for patient motion in an extended study.

5) Algorithmic technologies

Algorithmic technologies will be needed to support the accurate gathering of data, logging, cleaning of signals and the combining of these into space and time dynamic recordings for research and clinical use. This is a promising but challenging area in which to use machine learning to support clinician diagnosis as well as research needs.

6) Regulatory strategy

Finding an appropriate regulatory strategy to get medical device certification for a complex new diagnostic technology through clinical studies is likely to be a challenging part of a device development using OPM.

Enabling magentoencephalography diagnostics – the way forward

The key to developing better and more affordable magnetoencephalography diagnostics will be to demonstrate cost effective scanning to allow clinicians to diagnose and offer beneficial treatment. The optically pumped MEG solves the key challenge of size and performance, but as mentioned there are systems engineering challenges that will need time and investment to solve too. This will be a key step in getting any OPM system ready to be used in routine clinical practice.

It will be exciting to watch this new technology develop and for these challenges to be solved. As it does, it will allow us a better understanding of the deep workings of the brain and a greater ability to help diagnose and treat disease.

[1] Four-channel optically pumped atomic magnetometer for magnetoencephalography

Anthony P. Colombo, Tony R. Carter, Amir Borna, Yuan-Yu Jau, Cort N. Johnson, Amber L. Dagel, and Peter D. D. Schwindt

Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185, USA