Optically pumped magnetometers have an edge over even some superconducting devices for precision magnetic field detection since they don’t need cryogenic cooling. QuSpin, a company focused on developing atomic devices, has come out with its second-generation sensor for integration into compact aerial and underwater drones. Meanwhile, Cerca Magnetics, a medtech startup is applying QuSpin’s OPM technology to open new capabilities in neurologic imaging.
“Our second-generation QTFM sensor is now ready,” reports Jeff Orton, senior engineer for QuSpin. “The G2 has been redesigned from the ground up to enable advanced magnetometry applications requiring uncompromising sensitivity in a rugged, small, low-power package. The design choices for G2 focused on magnetometer integration on compact unmanned aerial and underwater platforms.”
The G2 is a rubidium optically pumped atomic magnetometer based on free induction decay. It represents a significant upgrade over the company’s G1 sensor launched in 2017. The new device is significantly more robust and ruggedized for use in challenging geophysical applications. There is no slew rate limitation, it can tolerate an arbitrarily noisy, fast-changing magnetic field environment without unlocking. Deadzone is negligible. While the G1 had a large planar deadzone, G2 only has a negligible axial deadzone so it can be used at any latitude without orientation concerns. While in many high-performance magnetic anomaly detection applications, external vector magnetometers are needed to remove internal and external heading errors, in the G2 a small triaxial attachment over the sensor head converts the unit from a pure scalar magnetometer to a hybrid scalar+3-axis vector magnetometer for hybrid performance.
Basically, OPMs are passive magnetic field sensors comprised of three main components: (1) a laser, (2) a glass vapor cell containing ‘sensing’ atoms in a gaseous state, and (3) a photodetector. QuSpin makes both zero-field OPMs and total-field OPMs.
Zero-field OPMs exhibit extreme sensitivity when the magnetic background is small. When the field is nearly zero, the atoms in the vapor cell become mostly transparent allowing maximum light onto the photodetector. Any change in the field induces a change in the transparency of the atoms. The resulting change in the photocurrent gives a measure of the magnetic field signal.
Total-field OPMs can operate in Earth’s field with high accuracy. The atoms in the OPM vapor cell have a well-defined precession frequency that is directly proportional to the magnitude of the background field. In QTFM Gen-2, the precession frequency is directly measured with a high resolution frequency counter to obtain the value of background magnetic field.
Meanwhile, medtech startup Cerca Magnetics has been making progress in its efforts to commercialize a wearable brain imaging research system that uses optically pumped magnetometers from QuSpin. The system was developed by researchers at the University of Nottingham, who currently are working on development of a 128-channel wearable system. The company is a joint venture between the university and Magnetic Shields Ltd which provides specialized electromagnetic interference shielding for the system.
Magnetoencephalography with optically pumped magnetometers (OPM-MEG) is a new way to non-invasively assess brain function. Like conventional MEG, it measures magnetic fields generated by current flow in neural assemblies, and thus provides assessment of brain function in health and disease. However, unlike conventional systems it doesn’t require cryogenically cooled sensors.
Cerca was named best medtech startup of 2021 in the prestigious OBN Awards held in the UK for innovative achievement in the life sciences in recognition for developing and bringing to market the first commercial, wearable MEG device. Since the company was formed in September 2020, its systems have already been installed at the Hospital for Sick Children in Toronto for research into autism and in a special diagnostic suite at the Young Epilepsy facility in Surrey, UK where it is being used to improve how the epilepsy is diagnosed and treated.
Traditionally, explains Cerca, the only way to detect the extremely small magnetic fields generated by the brain (a billion times smaller than the Earth’s magnetic field) was to exploit superconducting sensors known as SQUIDs. SQUIDs must be kept at cryogenic temperatures (-269 °C), and this means that sensors in a conventional MEG system must be immersed in liquid helium. This, in turn, makes scanners heavy, cumbersome and very expensive. This has proven a significant barrier to widespread uptake of MEG.
Recent breakthroughs in physics have enabled the development and fabrication of OPMs that exploit the quantum properties of alkali atoms to measure very small magnetic fields. OPM sensitivity rivals that of superconducting devices, but OPMs do not require cryogenic cooling. Moreover, their size is no larger than a Lego brick. OPMs thus provide a perfect building block for their integrated wearable MEG system, explains Cerca.
The Cerca OPM-MEG System uses a unique helmet design to place 64 sensors as close as possible to the scalp surface, and an advanced array design which ensures effective cancellation of magnetic fields of no interest arising from interference sources.
Because the magnetic fields measured by MEG are so small, the scanner needs to be sited in a well-controlled, low magnetic field environment. Conventional MEG systems are housed inside a magnetically shielded room comprised of multiple layers of electromagnetic shielding material as well as a single layer of highly conductive material such as copper. This acts to reduce magnetic interference, however, in conventionally designed rooms the presence of the metal means that there is a residual static magnetic field. If OPMs are to be used and participants allowed to move freely, this field must be removed.
Recent developments have enabled the construction of new types of magnetically shielded rooms with built-in degaussing to remove residual magnetization and reduce the static magnetic field, notes Cerca. Parallel work has enabled the design and construction of electromagnetic coils which allow high fidelity control of residual fields and their variation over space inside the room. All of this combined has led to an exceptional magnetic environment with fields below 1 nT, a shielding factor of over 50,000 which facilitates the perfect environment for an OPM-MEG system.
“It’s incredible to see how far the technology has come since our initial experiments in the lab five years ago,” commented Dr Elena Boto when the award was announced. She is chief scientist for Cerca and a senior research fellow at University of Nottingham. “Seeing the system commercialised and installed to help children with neurological problems like autism and epilepsy is amazing and we are looking forward to developing and rolling the system out even further.”