Measurement of current density distribution induced in vivo during magnetic stimulation
Magnetic stimulation is clinically used for the excitation of neural tissues. However, the regions of excitation are not well defined. This study was designed to determine regions of stimulation by using the loaded probe to measure induced current densities and resistivities in saline and in living cats. Various magnetic signal and coil size were used to induced currents in a saline tank. The current density values measured in the tank were compared to an analytical closed-form solution and were found to correlate. Current density was parallel and opposite to the direction of the coil current. It was maximum when near the edge of the coil and decreased with distance. Results demonstrate that the loaded probe can measure magnetically induced currents. Current distribution patterns were also found to remain the same irrespective of excitation signal. Additional saline studies conducted introduced material boundary in the tank to interrupt current pathways. The measured current distribution pattern was greatly altered. The loaded probe technique was then used to measure currents induced in vivo in cats' brains. The first study (15 cats) measured currents at a fixed location as the coil was moved in lateral direction and in anterior-posterior direction. Results were similar to saline studies in that induced currents were parallel and opposite to coil current and maximum induced current density occurred when the coil's edge was closest to the probe. The current density distribution followed the closed-form solution, but the amplitude was half the expected value. This is due to the head of the cat limiting the number of flux through the brain. In the second study (10 cats), the coil was fixed and measurements were obtained at twenty-seven different sites in the brain. The measured resistivities for the white matter and the gray matter were implemented in a finite element model (FEM). Measured results showed that current density generally decreases with distance from coil's edge, but will vary depending upon medium resistivity. Regions of lower resistivity tend to have higher current densities. The FEM model was also found to induced currents in similar fashion. This understanding of the induced current characteristics in the brain during transcranial stimulation provides information to the clinicians concerning the possible regions of excitation. However, other factors such as the duration and amplitude of the stimulus, the direction of induced currents with respect to the nerve fiber, and the diameter and length of the nerve fiber also have to be considered.
Grace So Tay,
"Measurement of current density distribution induced in vivo during magnetic stimulation"
(January 1, 1992).
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