Effects of time varying currents and magnetic fields in the frequency range of 1 kHz to 1 MHz to the human body - a simulation study

Abstract

Exposure to time-varying magnetic fields evokes two effects in biological tissue: Firstly, an electric field is induced that generates eddy currents in conductive tissues, and, secondly, power deposit might increase local temperatures. Field effects of frequencies up to 1 kHz and above 1 MHz are well known. The intermediate frequency range lacks intensive research. Only little attention has been paid so far. Yet due to recent innovations in medical diagnostics and therapies like Magnetic Particle Imaging or RF-Hyperthermia, the need arises to investigate the frequency range from 1kHz to 1 MHz. This work presents results of numerical field calculations of a human body model placed within simple coil configurations. Induced current densities, generated by alternating coil currents, are simulated. The effect of current densities are demonstrated and evaluated on schematic cell models of excitable tissue. In order to generate an action potential at the cell membrane, a difference in electric potential from intra- to extracellular space must be present. It can be shown that in case of sufficient field strength, stimulation of nerves and muscles is possible up to a frequency of 100 kHz. The aim of this paper is to transfer simulation results from the macroscopic model to the microscopic model in order to estimate field effects of big field generating coils.

Magnetostimulation Limits in Magnetic Particle Imaging

“…8. Percent overestimation caused by the approximate FLM given in (11) when compared to the exact relation in (19), plotted for various values. For , the error caused by the simplified relation is 9.8% at 1 KHz, decreasing very rapidly to below 1% at 6 kHz.…”

“…It can be shown that for the E-field to be larger than , the derivative of the magnetic field has to be greater than (14) The time points that do not satisfy this criterion should not be included while calculating the thresholds, as they will otherwise result in an overestimation of the limits. Hence, for , the time points that contribute to stimulation have to satisfy (15) For time points between (i.e., in one period of the pulse), this corresponds to (16) The B-field at time point can be calculated as (17) Using (9) and , the B-field excursion from time point to will result in a stimulation if (18) Combining (16) - (18), the exact relation for FLM can be written as (19) Any field greater than a peak-to-peak amplitude of will induce a PNS sensation in the subjects. Because there is no closed form solution for , it must be computed numerically.…”

“…The exact relation for the threshold versus frequency curve is given in the Appendix, in (19). We refitted and based on this exact relation, but did not find any significant differences ( , paired Wilcoxon sign-rank test) when compared to the parameters calculated using the approximate relation.…”

“…It is important to note that PNS is also the dominant safety 0278-0062 © 2013 IEEE constraint for fast imaging in MRI, where the field gradients cannot exceed a dB/dt limit of 20 T/s [17]. Other work has explored the SAR limits in MPI [18], or the PNS thresholds in human arm using an inhomogeneous figure-eight butterfly coil [19]. Our aim in this work is to study the PNS safety limits for homogeneous coils used for the drive field in MPI.…”

Magnetic particle imaging scanner with 10-kHz drive-field frequency

“…However, the signal-to-noise ratio for a 10-kHz drivefield frequency is decreased owing to the lower dM/dt compared with the 25-kHz MPI systems. The influence of drive-field frequency and its signal strength on the generation of action potentials in nerves is currently under discussion [2,20].…”