Abstract:

In Magnetic Particle Imaging (MPI) the human body is exposed to magnetic fields of the lower kHz-range. The effects of those fields to biological tissue are yet to be determined. According to Faraday's Law, time-varying magnetic fields induce rotating electric fields. In body tissues, the induced electric field may cause an electric current, the formation of electric dipoles or the reorientation of present dipoles, depending on the strength of the magnetic field, the frequency and the properties of the body tissue, i.e. electrical conductivity and permittivity. Both electrical conductivity and permittivity vary with the frequency of the applied field. In case of a circular loop, the induced current density is proportional to the radius, the rate of change of the magnetic flux density and the conductivity of the tissue. Low frequency electric currents are able to stimulate skeletal muscles. Because of the capacitive characteristic of the cell membrane, the stimulating effect decreases with rising frequency. Furthermore, very short impulses (t ≪ 1 ms) cannot open the Na-ion channels involved in nerve stimulation as effectively as longer ones. At higher frequencies, energy absorption becomes an issue. Exposure to electro-magnetic fields can lead to local temperature increase. The amount of absorbed energy per tissue weight is expressed by the specific absorption rate (SAR). The aim of this work is to calculate and evaluate the effects of the magnetic fields applied in MPI and propose ways to minimize them. At the same time, the risk of painful muscle stimulation should be minimized, even if MPI satisfies the requirements related to patient safety. In order to fulfill this task, simulation studies as well as experiments are being carried out. The following sections give an overview about the projects that are running or will be started in the near future.

Abstract:

Since the idea of Magnetic Particle Imaging (MPI) has been initially published in 2005, a lot of effort has been invested to improve temporal and spatial resolution. Most recently, first in vivo 3D real-time MPI scans were presented revealing details of a beating mouse heart [1]. In MPI, besides a strong time-constant field gradient, an alternating magnetic field of about 25 kHz frequency is applied to the patient. For the development of a new imaging technique, it is important to investigate the effects of the induced fields with respect to current densities and specific absorption rates (SAR) to ensure safe operation. This work presents simulations of the fields induced by a typical MPI laboratory setup and an MPI system scaled up to whole body dimensions.

Abstract:

Alternating magnetic fields induce AC currents in conductive materials. These currents can depolarize the cell-membrane, causing an action potential. In current literature stimulation thresholds for alternating magnetic fields were published up to a frequency of 1 kHz only. Beyond 1 kHz there are thresholds for electrode stimulation only. To convey these thresholds for magnetic fields the current densities caused by the alternating magnetic fields in an anatomical model were calculated. With these current densities the necessary flux densities were reevaluated. The intention of this paper was the development of a system for inductive stimulation of peripheral muscles to validate the simulation in the frequency range from 1 kHz to 50kHz.