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Magnetic nerve stimulation has proven to be an effective, non-invasive technique to excite peripheral and central nervous systems. In this technique, the excitement of the neural tissue depends on exposure to a transient magnetic field generated by passing a high pulse of current through a coil. By positioning the coil in a specific orientation over the targete tissue, the transient magnetic field will induce an electric field in the conductive milieu of the body. If this field reaches a certain threshold within a specific time period, neural depolarization is then evident. Recently, transcranial magnetic stimulation showed promise as a novel treatment for mood disorder and other psychiatric illnesses. Considering that these sicknesses are currently one of the larges causes of disability (with a cost sometimes exceeding that of diabetes, heart disease, and hypertension) it is evident that pursuing this therapeutic approach is beneficial. However, as magnetic stimulation is relatively new, many limitations and obstacle are to be addressed before this innovative technology is approved for clinical applications. The main limitations in applying magnetic nerve stimulation are poor field focality and inadequate strength in deep tissues (targeted area). This, the excitation of these regions requires very high coil currents to achieve a strong field that is capable of penetrating deep tissues. However, these fields might activate adjacent tissues as well as the targeted area. Further, the high currents will result in coil heating, especially for a high rate of repetition. Another limitation, especially in transcranial stimulation, is the lack of knowledge of precisely which area of neural tissue is being affected since no immediate behavioral effects of current reversal in the coils and the virtual cathode of the coil suggests conflicting results. The primary objective of this thesis is the development and testing of new coil designs that can focus the magnetic field more effectively. Two such coils have been built. The first coil has an air core, while the other has a magnetic core. The magnetic fields of these coils, applied to the human upper limb, have been determined theoretically, and the results compared to the field generated by the most generated by the most common commercial coil, the Figure-8 coil. To design these coils and to test them experimentally, a current pulse generator has been designed and built. Further, a novel meaurement system using surface mount inductances and a computer based data acquisition system has been designed and built. The experimental results confirm the theoretical findings, that the air core coil is slightly better than the Figure-8, as far as field strength and focality are concerned. In addition, the experimental results, prove that the coil with the ferromagnetic core, is superior. The second objective is to investigate the effect of stimulus waveforms theoretically, experimentally, and through in vivo study. The goals of the study are to establish a quantitative relationship among various waveforms and to investigate the effect of these waveforms in determining the site of stimulation. Accordingly, a multi subject trail was conducted: a Figure-8 coil was applied to the median nerve of ten subjects at the upper limb. The motor responses of the thenar muscle were then recorded. The study results show that the biphasic stimulating pulse is more effective than the monophasic pulse, as has been concluded by other researchers. However, the effective stimulating point, or virtual cathode of the coil, was found to be not simply 3 to 4 cm from the coil center as had been reported. In fact, the study shows that the site of the virtual cathode is affected by the current amplitude and the degree of inhomogeneity of the tissues surrounding the nerve. Furthermore, reversing the coil current direction results in a different level of stimulation but does not affect the virtual cathode position. In summary, the research presented in this thesis covers theoretical concepts, experimental aspects, and human studies related to neuro magnetic stimulation. The results of the experiment and the study are consistent with the theoretical analysis. The proposed coil design is novel and offers promise for a better coil system for magnetic nerve stimulation Doctor of Philosophy (PhD) |
