Friday, March 5, 2010

Magnetic Measurements of Peripheral Nerve Function Using a Neuromagnetic Current Probe

Section 8.9 in the 4th Edition of Intermediate Physics for Medicine and Biology describes how a toroidal probe can be used to measure the magnetic field of a nerve.
If the signal [a weak biomagnetic field] is strong enough, it can be detected with conventional coils and signal-averaging techniques that are described in Chapter 11. Barach et al. (1985) used a small detector through which a single axon was threaded. The detector consisted of a toroidal magnetic core wound with many turns of fine wire (Fig. 8.26). Current passing through the hole in the toroid generated a magnetic field that was concentrated in the ferromagnetic material of the toroid. When the field changed, a measurable voltage was induced in the surrounding coil.
My friend Ranjith Wijesinghe, of the Department of Physics at Ball State University, recently published the definitive review of this research in the journal Experimental Biology and Medicine, titled “Magnetic Measurements of Peripheral Nerve Function Using a Neuromagnetic Current Probe” (Volume 235, Pages 159–169).
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.
Ranjith and I were both involved in this research when we were graduate students working in John Wikswo’s laboratory at Vanderbilt University. I remember the painstaking process of making those toroids; just winding the wire onto the ferrite core was a challenge. Wikswo built this marvelous contraption that held the core at one spot under a dissection microscope but at the same time allowed the core to be rotated around multiple axes (he’s very good at that sort of thing). When Russ Hobbie and I wrote about “many turns of fine wire” we were not exaggerating. The insulated copper wire was often as thin as 40-gauge (80 microns diameter), which is only slightly thicker than a human hair. With wire that thin, the slightest kink causes a break. We sometimes wound up to 100 turns on one toroid. It was best to wind the toroid when no one else was around (I preferred early morning): if someone startled you when you were winding, the result was usually a broken wire, requiring you to start over. We potted the toroid and its winding in epoxy, which itself was a job requiring several steps. We machined a mold out of Teflon, carefully positioned the toroid in the mold, soldered the ends of those tiny wires to a coaxial cable, and then injected epoxy into the mold under vacuum to avoid bubbles. If all went as planned, you ended up with a “neuromagnetic current probe” to use in your experiments. Often, all didn’t go as planned.

Ranjith’s review describes the work of several of my colleagues from graduate school days. Frans Gielen (who now works at the Medtronic Bakken Research Centre in Maastricht, the Netherlands) was a post doc who used toroids to record action currents in skeletal muscle. Ranjith studied compound action currents in the frog sciatic nerve for his PhD dissertation. My research was mostly on crayfish axons. Jan van Egeraat was the first to measure action currents in a single muscle fiber, did experiments on squid giant axons, and studied how the magnetic signal changed near a region of the nerve that was damaged. Jan obtained his PhD from Vanderbilt a few years after I did, and then tragically died of cancer just as his career was taking off. I recall that when my wife Shirley and I moved from Tennessee to Maryland to start my job at the National Institutes of Health, Jan and his wife drove the rented truck with all our belongings while we drove our car with our 1-month old baby. They got a free trip to Washington DC out of the deal, and we got a free truck driver. John Barach was a Vanderbilt professor who originally studied plasma physics, but changed to biological physics later in his career when collaborating with Wikswo. I always have admired Barach’s ability to describe complex phenomena in a very physical and intuitive way (see, for instance, Problem 13 in Chapter 8 of our textbook). Of course, we were all led by Wikswo, whose energy and drive are legendary, and whose grant writing skills kept us all in business. For his work developing the Neuromagnetic Current Probe, Wikswo earned an IR-100 Award in 1984, presented by the RandD Magazine to recognize the 100 most technologically significant new products and processes of the year.


  1. Dr. Roth,

    These coils are so sensitive and work at room temperature, so why do we not see more of them in use? What are their limitations?

  2. Deborah,

    This is a good question. We originally had high hopes for the use of the clip-on toroids in the hospital, during surgery on peripheral nerves. That application may still develop, but it has not caught on so far. I guess the advantages of the toroids do not outweigh the ease of using simple extracellular electrodes (despite the fact that usually the electrode is raised out of the body fluids for the measurement). Probably inertia has something to do with it: doctors are familiar with extracellular electrodes but not with toroids, so if the advantages of the toroids are only slight, doctors are not inclined to change.