Non-Cryogenic 4He Sensors for Thermal Flexibility

The integration of non-cryogenic helium-4 sensors within the OPM-MEG system represents a ground-breaking shift in the system's responsiveness to magnetic changes associated with brain activity. The adoption of helium-4 technology ensures operational efficiency without the need for heating or cooling within the sensor unit, leading to negligible heat dissipation per sensor (0.02W) (Gutteling et al. 2023) and fostering a more stable and reliable measurement environment. This thermal flexibility allows strategic sensor placement directly on the scalp, boasting a cell offset of 1-3mm (Zahran et al. 2022) and enabling a larger magnitude of measured magnetic field vectors of 5 (Gutteling et al. 2023). Regardless of the patient's head geometry, the signal-to-noise ratio remains consistent on the scalp for individuals of varying age groups, ranging from children to adults with a wide range of head perimeters. The expanded dynamic range reduces the specification for shielding, making the system particularly well-suited for functional data acquisition. Its adaptability to accommodate unavoidable subject movement further positions the system as an ideal choice for studies involving children or individuals with movement disorders. 

Triaxial closed loop accuracy. 

The helium-4 based system also introduces a triaxial closed-loop system, eliminating both sensor drift and cross-axis projection error. This translates into heightened accuracy in reconstructing brain activity and ensures homogeneity in sensor accuracy within the array. The inclusion of radial and tangential measures not only contributes to a wealth of data but also reduces the specification for shielding through field nulling on the third closed-loop axis. The system's large frequency bandwidth, DC-2000Hz (Badier et al. 2023) facilitates the comprehensive recording of the full spectrum of brain activities. This bandwidth, tailored to brain electrical activity, empowers researchers to gain insights into a diverse range of brain dynamics, including conditions such as epileptic activity featuring high-frequency oscillations and fast ripple patterns. 

Unparalleled sensitivity. 

With floor level up to 27fT/rtHz, whilst on the scalp, the 4He OPM system attains heightened sensitivity, benefiting the study of subtle or localised brain activity while simultaneously improving temporal resolution. The bespoke and flexible sensor array introduces unprecedented versatility, allowing for precise modular sensor placement with no proximity limitations. This design facilitates data acquisition that precisely reflects activity in the area of interest, providing a signal-to-noise ratio advantage and enhanced repeatability. 

Portability and motion robustness. 

Portability emerges as a key advantage of the 4He OPM system, enabling seamless relocation between sites. This feature broadens the scope of research by offering access to a diverse range of healthy or patient subjects, including infants and the elderly. The system's motion robustness ensures reliable data acquisition across various environments. The system's design minimises installation constraints, requiring only lightweight shielding and eliminating the need for a cryogenic cooling chamber. This results in reduced overall installation costs, a smaller operational space requirement, and enhanced flexibility in deployment. 

Sustainable operation. 

The 4He OPM system's low-maintenance profile, with no routine gas refills due to heating or cooling, significantly reduces the potential for sensor damage and minimises the likelihood of replacement or maintenance needs. This characteristic ensures continuous operational readiness, translating into no downtime between uses. Remarkably, the same technology deployed within the system has been operational in space for a decade without maintenance. 

Seamless integration.  

The system offers a proprietary data acquisition platform, offering functionalities such as sensor localisation, montage creation, calibration, and recording, further streamlining the data analysis process for researchers. The system ensures a universal data output format (fif) and operates on a Linux-based operating system, enhancing compatibility when exporting for open-source platforms and licensed software such as BESA. This adaptability not only facilitates flexibility and customisation in data analysis but also makes advanced data analysis tools accessible to a broader range of researchers. 

References. 

Badier, J.M., Schwartz, D. and Bénar, C.G. et al. (2023). Helium optically pumped magnetometers can detect epileptic abnormalities as well as SQUIDs as shown by intracerebral recordings. eNeuro 10(12). 

Gutteling, T.P., Bonnefond, M. and Clausner, T. et al. (2023). A new generation of OPM for high dynamic and large bandwidth MEG: The 4He OPMs – first applications in healthy volunteers. Sensors 23(5): 2801 

Zahran, S., Mahmoudzadeh, M. and Wallois, F. et al. (2022). Performance analysis of optically pumped 4He magnetometers vs conventional SQUIDs: From adult to infant head model. Sensors 22(8): 3093