Detection of magnetic fields generated by deep brain stimulation (DBS) using optically pumped magnetometers (OPMs) operating in the spin-exchange relaxation-free (SERF) regime holds significant promise. High bandwidth DBS signals are particularly valuable for understanding current OPM limitations and developing predictive strategies to overcome them. This primary study investigates the response of commercially available SERF-OPMs (QZFM Gen-2, QuSpin Inc.) with their limited 135 Hz bandwidth and open-loop operation when exposed to magnetic DBS signals emitted by a commercially available DBS lead and generated by a DBS implantable pulse generator (IPG), which are characterized by short-duration rectangular pulses. Such magnetic fields emitted from a real DBS lead were measured using OPMs across a range of DBS pulse repetition frequencies from 2 to 255 Hz. A versatile 304-channel superconducting quantum interference device (SQUID) vector magnetometer system was employed in the Berlin Magnetically Shielded Room-2.1 for reference measurements. Our findings identified peaks at several spectral frequencies in addition to those relevant to the DBS signal, including the known DBS repetition frequency and its spectral harmonics peaks. These additional peaks are attributed to the interaction between the lock-in detection-based OPM's 923 Hz modulating field, supplied by the OPM's on-sensor coils, and the DBS repetition frequency. These peaks were successfully reconstructed by developing an OPM lock-in model and applying this model to signal characteristics provided by SQUID reference measurements. This letter identifies and clarifies the origin of various spectral peaks observed when using bandlimited OPMs for high bandwidth signal detection. This understanding allows magnetometer users of this particular sensor technology to differentiate between artificially introduced signal components and the DBS signal components. These findings are essential for improving the application utility of OPM-based detection for DBS and contribute to the advancement and understanding of OPM technology and address its current limitations.

Accurate calibration is key for any reliable sensor system. Magnetoelectric (ME) sensors, in particular, are influenced in their operating point by external parameters such as the Earth’s magnetic field or the ambient temperature. In this paper, we introduce a new planar coil design for the generation of a magnetic test field within the plane of the ME sensor. Furthermore, we implemented a method for measuring the sensor behaviour using a short-term magnetic noise signal. The combination of the printed circuit board (PCB) coil and the accelerated sensor characterization method allows the sensor system to be calibrated at the measurement site (in situ) without the need for laboratory equipment. We can show that the presented method for calibration achieves high-quality results in 10 seconds for a sensor affected by external interference fields.

Reconstructing an Amplitude-modulated (AM) signal with a modulation index below 0.01 is a significant challenge for conventional and established direct and digital demodulation methods. This work examines the importance of appropriate demodulation techniques and methods for analyzing and evaluating their performance using the sensing principle of electrically modulated Magnetoelectric (ME) sensors. These magnetic field sensors produce a double-sideband amplitude-modulation with carrier (DSB-AM-WC) signal at the sensor output, and their ultra-low modulation index often limits an accurate recovery of magnetic signal information. Addressing this limitation requires optimized demodulation strategies to realize the full potential of this ME sensor system for use in biomedical applications. This study assesses signal integrity during frequency conversion and defines detection limits for an analog demodulator designed for DSB-AM-WC signals with an ultra-low modulation index, which is explicitly designed for this sensor concept. The proposed low-cost demodulation system converts the 500 kHz (Radio Frequency, RF) DSB-AM-WC sensor output to an Intermediate Frequency (IF), allowing subsequent analog-to-digital for digitization. This work analyzes the analog signal path of electrically modulated ME Sensors and develops a sensor emulator setup for controlled testing of analog demodulation schemes. The emulator can generate AM signals (500 kHz carrier, Sideband ranging from 10 Hz–12 kHz, modulation index from 0.00001–1) while primarily considering electrical noise contributions from the hardware at room temperature, thus bridging the gap between theoretical analysis and practical implementation. Experimental results demonstrate that the proposed analog demodulation system, featuring a double-balanced frequency mixer (SRA-8+, Mini-Circuits, Brooklyn, NY), achieves an ultra-low noise input-referred noise of 10 nV/ √ Hz , enabling the recovery of modulation indices as low as 0.000249, delivers an signal-to-noise ratio (SNR) of 20 dB, and provides a stable dynamic range of 64 dB at output. The results confirm the effectiveness of the developed low-cost demodulation system while highlighting limitations, including the effect of input power and the phase shift available in diode-based mixers on maintaining signal integrity for low modulation index AM signals. The results identified the first possible guidelines for optimizing analog front-end systems for ME sensors. Future work will focus on investigating the signal output of real ME sensor prototypes, benchmarking the proposed system against state-of-the-art technologies, and considering additional environmental factors influencing performance.

Clinical motion analysis plays an important role in the diagnosis and treatment of mobility-limiting diseases. Within this assessment, relative (point-to-point) tracking of extremities could benefit from increased accuracy. Given the limitations of current wearable sensor technology, supplementary spatial data such as distance estimates could provide added value. Therefore, we propose a distributed magnetic tracking system based on early-stage demonstrators of novel magnetoelectric (ME) sensors. The system consists of two body-worn magnetic actuators and four ME sensor arrays (body-worn and fixed). It is enabled by a comprehensive signal processing framework with sensor-specific signal enhancement and a gradient descent-based system calibration. As a pilot study, we evaluated the technical feasibility of the described system for motion tracking in general (Scenario A) and for operation during treadmill walking (Scenario B). At distances of up to 60 cm, we achieved a mean absolute distance error of 0.4 cm during gait experiments. Our results show that the modular system is capable of centimeter-level motion tracking of the lower extremities during treadmill walking and should therefore be investigated for clinical gait parameter assessment.

In order to integrate highly sensitive magnetometers based on surface acoustic wave (SAW) sensors into clinical applications, some fundamental challenges regarding their readout must first be overcome. The present paper proposes a readout concept that is based on a phase-locked loop (PLL) to demodulate a frequency-modulated (FM) signal. The presented demodulator is designed to meet three fundamental requirements: it is able to tolerate the frequency drift of the sensor system, it is able to demodulate low frequency as well as low amplitude and broadband signals, and it is sufficiently inexpensive and simple to be scaled for arrays. Preliminary measurements demonstrate that the chosen topology meets these requirements to a satisfactory extent, indicating its potential for broader implementation in clinical settings.