Measurements of potential differences in human subjects induced by motion in a superconducting magnetic field.

Abstract

We have attempted to measure the electromotive forces (emfs) induced in human beings moving at a constant speed in a highly dense magnetic field. Experiments were initially conducted on a set of models, and then directly on human subjects. The models consisted of single circular loops of Tygon tubing (I.D., 0.635 cm; O.D., 0.9525 cm) filled with normal saline solution, with circumferences of 20, 40, 60, 80, and 100 cm. The models were connected to an amplifier via silver/silver-chloride electrodes. Each saline loop was mounted on a movable platform, with the plane of the loop perpendicular to the platform's axis; the platform was enabled to move at known constant speeds into and out of the bore of a 1.89-T magnet. The human subjects were then substituted for the saline loops, with the long axis parallel to the direction of motion, and with standard EKG electrodes placed at 180 degrees successively on the ankle, calf, lower thigh, upper thigh, chest, and head. In all cases, for human subjects and models, the peak induced voltage was directly proportional to the speed of movement and the square of the circumference of the bounded cross-sectional areas. Thus, for the saline loops, the correlation coefficient between induced voltage and circumference was .998, and for human subjects, .947. Under the loose assumption that for equal circumferences the bounded areas in human subjects were equal to those in the circular loops, the induced emfs in human subjects were consistently about 13% greater than those in the loops. At a mean speed of 1.18 m/s, the chest had a peak induced voltage of 260 mV, while the voltage at the ankle had a peak of 19.8 mV. The experimental data were used to estimate the corresponding induced-current density at the pericardium, 17 mA/m2. We conclude for a human subject moving at constant speed along the body's long axis into a magnetic field that Faraday's law is closely followed for various cross-sections of the body. Further, in those cases in which the magnetic field and its gradient are not well-established, one can use saline-filled loops to estimate approximate values of voltages induced in human subjects.