BIOMAGNETISM: Stimulating and Recording Muscle Activity

Abstract

<p>Biomagnetism is a two-sided discipline encompassing the magnetic stimulation of excitable tissue and the recording of magnetic fields generated by the action currents in those tissues. The present work explores new avenues in developing potential research and clinical tools for magnetically stimulating and recording muscle activity. This dissertation is presented within a cohesive experimental framework of stimulation, recording and processing. We sought to improve the magnetic stimulation of mammalian nerve, with a view towards niche applications where a minimally invasive procedure may be justified. Increased stimulus repetition rates and excellent spatial focus were achieved using small implantable "magnodes" in dog and rat hindlimbs, respectively. Generally, a three-fold increase in stimulus response was obtained, thus enabling a greater sustained stimulus rate at a given power level. An insensitivity to coil positioning was also observed in the rat. We believe two distinct mechanisms are responsible for the observed improvements in response. A toroidal coil magnetometer was developed, intended as a non-invasive tool for rapidly assessing overall activity in intact muscle. Design of the complete recording system included a low noice amplifier and a novel low frequncy magnetic shield composed of amorphous alloy. Employing non-physiological test sources, the unshielded system could resolve a multipole configuration as an equivalent dipole source. Unfortunately, the shielded could not be completed and the magnetometer's sensitivity remains inadequate for physiological work at this time. A signal processing method was developed for the purpose of filtering individual toroid recordings, and hence preclude the need for multiple stimuli. Due to the shielding problems the method was applied to data from another magnetometer. The filtering strategy offers the speed of fixed filtering with the quality of signal estimates typically found with adaptive techniques. This research has demonstrated the efficient stimulation of mammalian nerve using magnetic fields; the detection of magnetic fields from multipole current sources using non-superconducting technology; the design of lightweight and robust low frequency magnetic shields; and the efficient filtering of individual muscle response records. Continuation of the present work requires the design of a high speed magnetic stimulator and the completion of the magnetic shield.</p>