Dedicated research is currently being conducted on novel thin film magnetoelectric (ME) sensor concepts for medical applications. These concepts enable a contactless magnetic signal acquisition in the presence of large interference fields such as the magnetic field of the Earth and are operational at room temperature. As more and more different ME sensor concepts are accessible to medical applications, the need for comparative quality metrics significantly arises. For a medical application, both the specification of the sensor itself and the specification of the readout scheme must be considered. Therefore, from a medical user’s perspective, a system consideration is better suited to specific quantitative measures that consider the sensor readout scheme as well. The corresponding sensor system evaluation should be performed in reproducible measurement conditions (e.g., magnetically, electrically and acoustically shielded environment). Within this contribution, an ME sensor system evaluation scheme will be described and discussed. The quantitative measures will be determined exemplarily for two ME sensors: a resonant ME sensor and an electrically modulated ME sensor. In addition, an application-related signal evaluation scheme will be introduced and exemplified for cardiovascular application. The utilized prototype signal is based on a magnetocardiogram (MCG), which was recorded with a superconducting quantum-interference device. As a potential figure of merit for a quantitative signal assessment, an application specific capacity (ASC) is introduced. In conclusion, this contribution highlights metrics for the quantitative characterization of ME sensor systems and their resulting output signals in biomagnetism. Finally, different ASC values and signal-to-noise ratios (SNRs) could be clearly presented for the resonant ME sensor (SNR: $-90$ dB, ASC: $9.8\times {10}^{-7}$ dB Hz) and also the electrically modulated ME sensor (SNR: $-11$ dB, ASC: 23 dB Hz), showing that the electrically modulated ME sensor is better suited for a possible MCG application under ideal conditions. The presented approach is transferable to other magnetic sensors and applications.

Objective: Electroneurography has been an essential method for assessing peripheral nerve disorders for decades. During this procedure, a nerve is briefly electrically excited, and nerve conduction properties are identified by indirect means from the behavior of the innervated muscle. The magnetic field of the resulting muscle response can also be recorded by novel, uncooled magnetometers, which have become very attractive for different medical applications over recent years. These highly sensitive magnetometers are called optically pumped magnetometers. Methods: We performed unaveraged and averaged magnetic signal detection of electrically evoked muscle responses using optically pumped magnetometers. We then discussed the suitability of this procedure for clinical applications in the context of diagnostic value and in direct comparison with the current electrical gold standard. Results: The magnetic detection of muscle responses is possible using optically pumped magnetometers. Our magnetic results (averaged and unaveraged) closely match those from electrical measurements. Conclusion: Optically pumped magnetometers provide an alternative, contactless technology for electrode-based motor studies, but they are currently not ready for routine clinical use. This costly technology requires additional earth magnetic shielding because this is a prerequisite for proper operation. Currently, there are no diagnostic advantages over electrical measurements. Additionally, the required measurement setup and procedure are much more complicated. Significance: In contrast to already published proof-of-principle studies for magnetomyography, we report in detail the results of the magnetic measurements of electrically evoked muscle responses in a shielded environment by applying supramaximal stimulation and finally validate our findings with electroneurography data as a reference.

In principle, electrode-based bioelectrical signal acquisition can be complemented by biomagnetic sensing and therefore requires a more detailed assessment, especially because of the availability of novel noncryogenic sensor technologies. The current development of thin film magnetoelectric (ME) sensors ensures that the ME technology is becoming a prospective candidate for biomagnetometry. The main obstacle for large-scale usage is the lack of extremely low noise floors at the final sensor system output. This article highlights the current state of ME sensor development based on a magnetocardiography pilot study involving a healthy volunteer in a magnetically shielded chamber. For assessment, an ME prototype ( converse magnetoelectric thin film sensor ) will be applied for the first time. This sensor type ensures a noise amplitude spectral density below 20 pT/√ Hz at 10 Hz by using a sophisticated magnetic layer system. The main aim of this pilot study is to evaluate the applicability of this promising sensor for the detection of a human heart signal and to evaluate the sensor output with competitive optical magnetometry technology. A magnetic equivalent of a human R wave could be successfully detected within a one-minute measurement period with the sensor presented here. Finally, the article will provide an outlook on future ME perspectives and challenges, especially for cardiovascular applications.

In the development of any type of magnetic field sensor based on magnetic films, special consideration must be given to the magnetic layer component. The presented work investigates the use of scalable flux closing magnetostrictive multilayers for inverse magnetoelectric sensors. In such a type of magnetic field sensor, highly sensitive AC and DC field detection relies on strong excitation of the incorporated magnetic layers by piezoelectrically driven cantilever oscillation at mechanical resonances. The provoked periodic flux change is influenced by the magnetic field to be measured and is picked up by a coil, which generates the measured output. The multilayered inverse magnetoelectric sensor is investigated with regard to linearity, noise behavior, and detection limit of DC and AC signals. A significant advancement for inverse magnetoelectric thin film sensors is demonstrated in this study. Using exchange bias stabilized magnetic multilayers with flux closure structures, detection limits are improved by an order of magnitude to less than 8 pT/Hz1/2 at 10 Hz and 18 pT/Hz1/2 at DC.

Ultra-low magnetic field sensing is rapidly emerging as a technology in various applications, providing a non-invasive and instantaneous method of data acquisition. The increasing availability of improved or innovative types of magnetometers promotes the need for a benchmark metric. This contribution focuses on the evaluation of single magnetometer devices that detect magnetic fields in the femtotesla range and are usually used in a magnetically shielded environment. For assessment, a Device Alignment Actuator system is presented that enables an accurate 3D alignment of the Device Under Test around a single point within a known, homogeneous magnetic test field inside the highly magnetically shielded room, called BMSR-2.1 at the Physikalisch-Technische Bundesanstalt Berlin, Germany. The defined benchmark metric includes several key parameters, such as frequency response, amplitude stability, frequency stability, and directivity, to name just a few. These parameters are introduced by using different commercial state-of-the-art optically pumped magnetometers (SERF-OPMs) and compared to the reference system based on superconducting quantum interference devices, which remain the gold standard in magnetometry. This approach ensures consistent evaluation across different magnetometer types, provided they operate at room temperature and fit within the evaluation platform. The platform can also be adapted for multi-channel magnetometer systems.

Despite obvious advantages over their electric counterparts, magnetic measurements are still performed very rarely for medical diagnosis. The high operating costs of established systems with a sufficient limit of detection, mainly based on superconducting quantum interference devices, prevent the spread of magnetic diagnostic procedures. Consequently, the demand for alternative low-cost magnetic sensor systems is large. Several suitable, uncooled sensor concepts exist, but their usability-especially, when operated outside magnetically shielded chambers-requires improvement. This paper highlights basic, real-time signal processing concepts, and implementations to improve the signal quality for different biomedical applications with a focus on cardiology in unshielded measurement environments. Several processing steps in the digital domain are described, such as noise reduction (by means of cancellation and suppression), adaptive sensor signal combination, and adaptive signal averaging. Finally, basic cardiologic feature extraction methods are performed using uncooled magnetometers in combination with the described signal enhancement stages.