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 . 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.
Magnetic Particle Imaging (MPI) is a new tomographic imaging technique based on magnetization of ferromagnetic nano-particles. Magnetic fields of different strengths and frequencies generate and move a field free point (FFP) over the field of view, inducing a signal of the magnetic particles, if present. The magnetic fields induce current densities of high amplitude in the patients body and deposit an amount of power that might lead to painful warming in the patients periphery. Based on the specifications of the MPI system, an optimized coil configuration is suggested here, reducing high peak values of current densities and specific absorption rate (SAR), by running the field generating coils of different radius with optimized currents. The results presented here are based on numerical field calculations with a simple cylindrical model, used for the optimization procedure, and the Visible Man data-set, for evaluating the optimization results.
Numerous studies about the effects in human body of high frequency magnetic fields on the one hand and extremely low frequency fields on the other hand have been carried out. This is not the case for the mid frequency range around 100 kHz. When applying external magnetic fields to the human body in this frequency range both electric stimulation and thermal heating effects have to be considered. Magnetic Particle Imaging (MPI), a new imaging technique, and Hyperthermia, a tumor treatment therapy, both apply magnetic fields in a frequency range around 100 kHz. In MPI thermal heating of the body has to be prevented, whereas in Hyperthermia a temperature increase of about 4 K in the target region is desirable. Induced currents may lead to muscle stimulation which is not acceptable above a certain threshold. This paper presents the results of induced current densities and SAR in a numeric field calculation simulation. For the model of the human body the torso of the Visible Man Dataset has been employed, along with the dielectric properties of biological tissues investigated by Gabriel & Gabriel. The model has been exposed to a sinusoidal magnetic field with an amplitude of 10 mT. The results of the induced current densities and SAR values have been compared with the currently valid official guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). It turns out that limits of induced current densities are reached by applying a magnetic flux density of 10 mT and the SAR limit even is exceeded.