Tom Swiontek, Ph.D.

B.S.

Electrical Engineering

Marquette University

1969

Ph.D.

Electrical Engineering

Marquette University

1975

Biomedical Engr. Scholastic Achievement Award

Graduate Cum Laude

Oscar Werwath Distinguished Teacher Award, MSOE

1998

Bacon Fellowship

NSF Traineeship

Undergraduate Design Project to Aid the Disabled

NSF Grant

Principal Investigator

Measurement of current density induced in cat brain during transcranial magnetic Stimulation

Chilbert, M., Tay, G., Battocletti, J., Sances, A., Swiontek, T., Maiman, S., Suh J. U.

1992-06-01

This study measures in vivo current densities induced in the brains of fifteen cats during transcranial magnetic stimulation. This wasessential for determining regions of excitation. Three magnetic stimulators were used in this study. Two of the stimulators were constructedin-house; one used a 5-cm diameter coil that was driven with a damped sinusoidal signal; the other used the same coil and was drivenwith an undamped sinusoidal signal. The damped system was operated at a frequency of 620 Hz and damping time constant of 0.912 ms.It generated a peak magnetic field of 140 gauss at the center of the coil. The undamped system was operated at a frequency of 3 KHz andgenerated a peak magnetic field of 28 gauss. The third system is a Cadwell MES-10 stimulator. Results showed that current distributionpatterns of the laboratory-made stimulators were similar. Thus, a coil driven with a damped sinusoidal input induces the same currentdistribution as when it is driven with an undamped sinusoid. Also, current density induced in the tissue was found to be maximum near theedge of the coil and decreased as the edge of the coil moved away from the region of interest. This corresponds to mathematical modelspreviously developed (Guidi, 1989; Ferguson, 1989; Roth, 1990; Reuter, 1988). Inducing maximum current at a region, however, does notnecessarily imply tissue stimulation in that particular region. Neurophysiologic factors, such as nerve fiber diameter and orientation,infuence neural excitability. Thus, it is necessary to consider these parameters during magnetic stimulation.

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A Probe for Measuring Current Density During Magnetic Stimulation

Tay, G., Chilbert, M.A., Battocletti, J., Sances, J.A. and Swiontek, T.

1991-06-03

Time-varying magnetic fields induce currents in conductive media, and when the induced current is large enough in excitable tissue, stimulation occurs. This phenomenon has been applied to the human brain and peripheral nerves for diagnostic evaluation of the neural system. One important aspect that is presently unknown is the current level necessary in tissue for stimulation induced by magnetic fields. This study presents a method of measuring the induced current density from pulsed magnetic fields in vitro and in vivo. The current-density probe was inserted into three concentrations of saline and into the brains of ten anesthetized cats. Two stimulation systems with coils 9 cm and 5 cm in diameter were used. The two systems provided sinusoidal and pulsatile coil currents. Measurements made in saline were compared with those calculated theoretically for a semi-infinite medium. The measured values were within 5% of the calculated values. Measurements made in the cat brain showed a 67% decrease compared with the theoretic model. This variance is attributed to the finite bounds of the skull. The results indicate that direct measurement of current density is possible. Subsequent measurements will aid in the design of improved magnetic stimulation systems.

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Electrical Field Distribution within the Injured Cat Spinal Cord: Injury Potentials and Field Distribution

Khan, T., Myklebust, J., Swiontek, T., Sayers, S., Dauzvardis, M.

1994-06-01

This study investigated the spontaneous injury potentials measured after contusion or transection injury to the cat spinal cord. In addition, the distribution of electrical field potentials on the surface and within the spinal cord were measured following applied electrical fields after transection and contusion injuries. After transection of the spinal cord, the injury potentials were −19.8 ± 2.6 mV; after contusion of the spinal cord, the injury potentials were −9.5 ± 2.2 mV. These potentials returned to control values within 2.5–4 h after injury. The electrical field distribution measured on the dorsal surface, as well as within the spinal cord, after the application of a 10 μA current, showed little difference between contusion and transection injuries. Scalar potential fields were measured using two configurations of stimulating electrodes: dorsal to dorsal (D-D), in which both electrodes were placed epidurally on the dorsal surface of the spinal cord, and ventral to dorsal (V-D), in which one electrode was placed dorsally and one ventrally. As reported in normal uninjured cats, the total current in the midsagittal plane for the D-D configuration was largely confined to the dorsal portion of the spinal cord; with the V-D configuration, the current distribution was uniform throughout the spinal cord. In the injured spinal cord, the equipotential lines midway between the stimulating electrodes have a wider separation than in the uninjured spinal cord. Because the magnitude of the electrical field E is equal to the current density J multiplied by the resistivity r, this suggests that either the current density is reduced or that the resistivity is reduced.

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