P 210. An automatic, computer-controlled TMS-coil calibrator

Abstract

The effect of transcranial magnetic stimulation (TMS) on the brain depends on the focality of the induced electric field (E-field). However, with commercial TMS coils, it is typically not known precisely how the E-field behaves as a function of distance from the coil. Our aim was to develop an automatic, computer-controlled calibrator for measuring the E-field induced by TMS coils. Such an instrument would allow comparing different coils and help in ensuring that an investigator knows the properties of the E-field. In the spherical head model, the triangle construction, where two radial wires extending from the origin are connected with a short tangential path, allows straightforward computation or measurement of the E-field induced by an external coil. Thus, we constructed two orthogonal triangular loops (radial edges 70 mm, tangential path 5 mm, i.e., 4.1 °) using 0.15-mm-thick copper wire. To reach sufficient precision, we used additive manufacturing for making a coil former for the wires. The orientation of the triangles, which defines the measurement point, is controlled by two servo motors, which rotate the triangles about the origin. This allows for measuring the induced E-field on the surface of a hemisphere: the voltage induced in the triangular loops by a TMS coil is proportional to the tangential components of the E-field that would be induced at the position of the tangential edges in a spherically symmetric conductor. The calibrator is shown in the figure. The measurement, which is operated by a LabView program, proceeds as follows: We connect a TMS coil in series with a signal generator and apply a sinusoidal current time-locked to the measurement to produce an alternating magnetic field. The time-varying magnetic field induces a voltage in the triangles which is amplified and fed to a National Instruments data-acquisition (DAQ) system connected to a laptop. The DAQ system is also used to control the movement of the servo motors so that the measurement points cover the surface of a hemisphere with uniform density (typically, 1000 points). To minimize the time needed for the servo movement, we traverse through the measurement points in an order that is an approximate solution to a travelling salesman problem. After the measurement completes, we analyze the acquired data with a Mathematica program: by fitting a sinusoid to the data measured at each point, we can extract the amplitude and direction of the induced E-field. Finally, we visualize the field and extract some measures, e.g., the maximum amplitude or the focality of the field. We have used the calibrator to measure the E-field produced by a home-made coil. The measurement results agreed well with the calculated field values. We will use the device for characterizing several commercially available TMS coils. In summary, the developed calibrator gives a simple means to measure the E-field produced by any TMS coil. Furthermore, by changing the triangle module, the same instrument can measure the E-field at different depths. The measurement of the spatial profile of the induced E-field of TMS coils helps in ensuring that an investigator knows the properties of the E-field.