Toroidal Pick-up Coil Research Articles

Detection of Magnetic Fields Created by Biological Tissues

The recording of magnetic fields associated with cellular current flow in nerve and muscle fibers was first reported nearly three and a half decades ago. These magnetic fields are detected using a room-temperature toroidal pickup coil called the neuromagnetic current probe. These probes can be employed to measure action currents of biological tissues while they are resting in their own natural settings or in saline baths, thereby reducing the risk associated with elevating and drying the tissues in the air during experiments. These magnetic fields are relatively insensitive to muscle movement since these probes are not directly connected to the tissues, and distortions of the recorded data due to changes in the electrochemical interface between the probes and the tissues are minimal. In this paper, the magnetic fields of single nerve axons, muscle fibers and nerve bundles are discussed. Furthermore, one of the possible applications of the neuromagnetic current probe to the intraoperative assessment of damaged nerve bundles is discussed.

A Non-Destructive Beam Monitoring System Based on an LTS-SQUID

Monitoring of beam currents in particle accelerators without affecting the beam guiding elements, interrupting the beam or influencing its profile is a major challenge in accelerator technology. A solution to this problem is the detection of the magnetic field generated by the moving charged particles. We present a non-destructive beam monitoring system for particle beams in accelerators based on the Cryogenic Current Comparator (CCC) principle. The CCC consists of a high-performance low-temperature DC superconducting quantum interference device (LTS DC-SQUID) system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. This device allows the measurement of continuous as well as pulsed beam currents in the nA-range. The resolution and the frequency response of the detector strongly depend on the toroidal pick-up coil and its embedded ferromagnetic core. Investigations of both the temperature and frequency dependence of the relative permeability and the noise contribution of several nanocrystalline ferromagnetic core materials are crucial to optimize the CCC with respect to an improved signal-to-noise ratio and extended transfer bandwidth.

Magnetic measurements of peripheral nerve function using a neuromagnetic current probe

The progress made during the last three decades in mathematical modeling and technology development for the recording of magnetic fields associated with cellular current flow in biological tissues has provided a means of examining action currents more accurately than that of using traditional electrical recordings. It is well known to the biomedical research community that the room-temperature miniature toroidal pickup coil called the neuromagnetic current probe can be employed to measure biologically generated magnetic fields in nerve and muscle fibers. In contrast to the magnetic resonance imaging technique, which relies on the interaction between an externally applied magnetic field and the magnetic properties of individual atomic nuclei, this device, along with its room-temperature, low-noise amplifier, can detect currents in the nano-Ampere range. The recorded magnetic signals using neuromagnetic current probes are relatively insensitive to muscle movement since these probes are not directly connected to the tissue, and distortions of the recorded data due to changes in the electrochemical interface between the probes and the tissue are minimal. Contrary to the methods used in electric recordings, these probes can be employed to measure action currents of tissues while they are lying in their own natural settings or in saline baths, thereby reducing the risk associated with elevating and drying the tissue in the air during experiments. This review primarily describes the investigations performed on peripheral nerves using the neuromagnetic current probe. Since there are relatively few publications on these topics, a comprehensive review of the field is given. First, magnetic field measurements of isolated nerve axons and muscle fibers are described. One of the important applications of the neuromagnetic current probe to the intraoperative assessment of damaged and reconstructed nerve bundles is summarized. The magnetic signals of crushed nerve axons and the determination of the conduction velocity distribution of nerve bundles are also reviewed. Finally, the capabilities and limitations of the probe and the magnetic recordings are discussed.

Application of LTS-SQUIDs in Nuclear Measurement Techniques

Low temperature superconducting quantum interference devices (LTS SQUIDs) are used to make precision measurements of electromagnetic fields in applications ranging from biomedicine to high energy physics. We have previously described an LTS SQUID-based device for nuclear physics which employs the Cryogenic Current Comparator principle (CCC). The CCC consists of a high-performance LTS DC SQUID system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. Theoretical investigations show that as external noise decreases, improvements in performance depend on the properties of the ferromagnetic core material embedded in the pick-up coil. Here we present the temperature- and frequency-dependence of several candidate ferromagnetic and nanocrystalline materials. We discuss these results in light of the optimization of the CCC sensor performance.

Status of the halo current sensor project for JET-EP

Abstract Halo currents (HCs), flowing through the plasma and the vacuum vessel during plasma disruptive events, induce severe mechanical stresses in the plasma facing components and in the vessel, and are a major concern for present and future fusion experiments like ITER. The new system of halo current sensors (HCS) designed for JET-EP (enhanced performance, EP) should help to evaluate HC density distribution, localization and rotation as well as toroidal and poloidal current asymmetries. The HCS system will include four identical mechanical structures each including eight Rogowski coils and two toroidal pick-up coils. The Rogowski coils will measure directly the current flowing through some of the tiles of the upper dump plate. The toroidal field pick-up coils will estimate the total poloidal HC. The coil assemblies are installed at the top of the vessel close to secondary X-point in four octants equally spaced along the toroidal coordinate. In the paper, the design and the manufacturing of the HCS system are presented in detail.

New current transformer device based on non-magnetostrictive amorphous ribbons

The design of a sensitive current transformer is presented in this work. Such a device is based in the detection of an induced voltage in a toroidal pick-up coil wound around a high permeability nucleus, when an ac electrical current (frequency: 50 Hz) flows through a conductor wire coaxial with its axis. In the present device, the use of a nearly zero magnetostrictive amorphous ribbon (Co 71.3Fe 3.7Si 15B 10) as the ferromagnetic nucleus, is analyzed and compared with the sensor response employing a Fe-based highly magnetostrictive amorphous ribbon (Fe 73.5Si 13.5B 9Cu 1Nb 3). The performed analysis has allowed us to develop a final prototype with similar accuracy than the commercial Hall clamp meter devices.

A mathematical model for calculating the vector magnetic field of a single muscle fiber

A mathematical model is described for calculating the volume-conducted magnetic field from active muscle fibers in an anisotropic bundle. With earlier models, the azimuthal magnetic field of a nerve bundle was calculated and the results were compared with the fields measured by toroidal pickup coils. The present model is capable of evaluating all three of the magnetic field components and is thus applicable for analyzing SQUID magnetometer recordings of fields from a muscle bundle. The component of the magnetic field parallel to the fiber axis is more than an order of magnitude smaller than either of the other two components. The amplitude of the magnetic signal is strongly dependent upon the anisotropy of the muscle bundle, the intracellular conductivity, the radius of the muscle fiber, the radius of the muscle bundle, and the location of the fiber in the muscle bundle. The peak-to-peak amplitude of the single-muscle-fiber action field increases linearly with increasing intracellular conductivity, as the square of the radius of the muscle fiber, and exponentially with the distance between the location of the fiber and the center of the bundle.

Magnetic field of a single muscle fiber. First measurements and a core conductor model

We present the first measurements of the magnetic field from a single muscle fiber of the frog gastrocnemius, obtained by using a toroidal pickup coil coupled to a room-temperature, low-noise amplifier. The axial currents associated with the magnetic fields of single fibers were biphasic and had peak-to-peak amplitudes ranging between 50 and 100 nA, depending primarily on the fiber radius. With an intracellular microelectrode, we measured the action potential of the same fiber, which allowed us to determine that the intracellular conductivity of the muscle fiber in the core conductor approximation was 0.20 +/- 0.09 S/m. Similarly, we found that the effective membrane capacitance was 0.030 +/- 0.011 F/m2. These results were not significantly affected by the anisotropic conductivity of the muscle bundle. We demonstrate how our magnetic technique can be used to determine the transmembrane action potential without penetrating the membrane with a microelectrode, thereby offering a reliable, stable, and atraumatic method for studying contracting muscle fibers.

Magnetic measurements of action currents in an isolated lobster axon

We present the first measurements of the magnetic field produced by action currents propagating along a single giant nerve axon. These data were obtained with a toroidal pick-up coil and a room temperature semiconductor amplifier.

Recent developments in the measurement of magnetic fields from isolated nerves and muscles (invited)

We have developed a Superconducting Quantum Interference Device (SQUID) magnetometer with a miniature, room-temperature toroidal pick-up coil that has sufficient sensitivity and frequency response to measure the magnetic field associated with the action potential of isolated nerves and muscles. A magnetically shielded SQUID operates in a liquid helium storage dewar so that the measurements of the weak magnetic fields can be made in a typical laboratory. The nerve or muscle fiber is immersed by Ringer’s solution and is threaded through the ferrite core of the toroidal pick-up coil so that electrical current within the fiber will thread the toroid but the return current external to the fiber will not. The time-dependent 100 pT magnetic field produced by the cellular action current induces currents in the pick-up coil that are detected by the SQUID with signal-to-noise ratios of 10 to 1 in a 2 kHz bandwidth. In the case of a single large nerve axon, this technique will allow quantitative measurement of the intracellular and transmembrane currents, the effective intracellular axial resistivity, and the time dependence of the transmembrane conductance without puncturing the nerve membrane with a microelectrode. This technique is being extended to study current flow in cardiac muscle.