Abstract:

Abstract Magnetic particle imaging (MPI) is a new imaging modality using oscillating magnetic fields in the frequency range of 10 kHz to 100 kHz. The duration of data acquisition becomes smaller, and signal-to-noise ratio improves if the amplitude of these fields is increased - technically amplitudes of up to 100 mT might be feasible for human-sized systems. On the other hand, with increasing field strength, adverse health effects must be expected: oscillating magnetic fields can stimulate nerves and muscle and heat up tissue. Thresholds for stimulation with magnetic fields in this frequency range are not precisely known, neither is the local temperature rise following exposure. The ICNIRP guidelines define reference levels for magnetic field exposure for the general public that contain large safety factors - for medical diagnostics, they might be exceeded for a short time. In this article, research and guidelines in this field are briefly reviewed, and new results are presented in order to contribute to a future definition of safety limits for oscillating magnetic fields in MPI.

Abstract:

Ablation strategies to prevent episodes of paroxysmal atrial fibrillation (AF) have been subject to many clinical studies. The issues mainly concern pattern and transmurality of the lesions. This paper investigates ten different ablation strategies on a multilayered 3-D anatomical model of the atria with respect to 23 different setups of AF initiation in a biophysical computer model. There were 495 simulations carried out showing that circumferential lesions around the pulmonary veins (PVs) yield the highest success rate if at least two additional linear lesions are carried out. The findings compare with clinical studies as well as with other computer simulations. The anatomy and the setup of ectopic beats play an important role in the initiation and maintenance of AF as well as the resulting therapy. The computer model presented in this paper is a suitable tool to investigate different ablation strategies. By including individual patient anatomy and electrophysiological measurement, the model could be parameterized to yield an effective tool for future investigation of tailored ablation strategies and their effects on atrial fibrillation.

Abstract:

BackgroundMultiple wavelets and rotors are accused of maintaining atrial fibrillation (AF). However, snake-like excitation patterns have recently been observed in AF. So far, computer models have investigated AF in a simplified anatomical model. In this work, pulmonary vein firing is simulated to investigate the initiation and maintenance of AF in a realistic anatomical model.Methods and ResultsThirty-five ectopic foci situated around all pulmonary veins were simulated by a unidirectional conduction block. The excitation propagation was simulated by an adaptive cellular automaton on a realistic 3-dimensional atrial anatomy. Atrial fibrillation was initiated in 65.7% of the simulations. Stable excitation patterns were broken up in anatomically heterogeneous regions, creating a streak-like excitation pattern similar to snakes. Multiple wavelets and rotors could be observed in anatomically smooth areas at the atria's roofs.ConclusionsThe influence of macroscopic anatomical structures on the course of AF seems to play an important role in the excitation propagation in AF. The computer simulations indicate that multiple mechanisms contribute to the maintenance of AF.

Abstract:

Due to forthcoming use of MPI on humans there is an urgent need for a thorough research on possible adverse effects of this technique on patients health. However, the health impact of exposure to time-varying magnetic fields in a frequency range between 10 kHz and 100 MHz, such as the MPI drive field, are still poorly investigated.The current paper intends to give an overview on an in-silico approach to investigation of stimulating effects that could be caused by the MPI drive field. For this purpose, cell models of myocardiocyte, myocyte and neurocyte, as well as a suitable setup for the simulation of the exposure to time-varying magnetic fields have been developed. The evaluation of performed simulations was carried out on the basis of transmembrane voltage elevation and induced current densities.

Abstract:

Magnetic Particle Imaging (MPI) benefits from the non-linear magnetization curve of magnetic nano-particles. Magnetic fields applied in this new imaging modality are of fre- quencies in the kHz-range. Little research has been carried out upon absorbed power and temperature increase caused by time- varying fields in that frequency range. Presented here are tem- perature distributions in a human body model exposed to mag- netic fields of 10 kHz to 100 kHz. A numerical human model has been placed within field generating coils. The finite elements model for the field calculation and the Pennes Bio-heat equation for the temperature distribution with and without perfusion and heat transfer by convection have been used. The results indicate that the absorbed power does lead to local temperature increase but not up to a hazardous level, if certain thresholds for the mag- netic fields are considered.

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:

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.

Abstract:

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.

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.

Abstract:

Atrial fibrillation (AF) is the most common cardiac arrhythmia leading to a high rate of stroke. The underlying mechanisms of initiation and maintenance of AF are not fully understood. Several findings suggest a multitude of factors to leave the atria vulnerable to AF. In this work, a rule-based approach is taken to simulate the initiation of AF in a computer model for the purpose of generating a model with which the influence of anatomical structures, electrophysiological properties of the atria and arrhythmogenic activity can be evaluated. Pulmonary vein firing has been simulated leading to AF in 65.7 % of all simulations. The excitation pattern generated resemble chaotic excitation behavior, which is characteristic for AF as well as stable reentrant circuits responsible for atrial flutter. The findings compare well with literature. In future, the presented computer model of AF can be used in therapy planning such as ablation therapy or overdrive pacing.

Abstract:

Question: The mechanisms responsible for atrial fibrillation (AF) are not completely understood. Various conduction velocities and realistic anatomical structures of the atria are implemented into a computer model showing the influence of complex anatomical structures on the initiation and maintenance of AF.Method Used: In a computer model of the Visible Female heart (National Library of Medicine, Bethseda, Maryland, USA), the initiation of AF was simulated by pulmonary vein (PV) firing. The anatomical model had a resolution of 1,696,740 tissue voxel with 0.33 mm voxel side length. 32 foci around all pulmonary veins were set. The excitation propagation was simulated using an adaptive cellular automaton. Electrophysiological parameters depending on different tissue types can be set. In this work, only the conduction velocity was reduced compared to physiological data.Results: The initiation of AF through ectopic foci creates re-entrant circuits and quasi-chaotic excitation pattern in the computer model. 8 of 16 foci in the left superior, 3 of 4 foci in the left inferior, 5 of 8 foci in the right superior and 4 of 4 foci in the right inferior PV created AF after only 1.5 s. The excitation pattern shows stable re-entrant circuits as well as chaotic behavior. A breakup of stable re-entrant circuits was also observed when simulating the pathology for 17.5 s. The other foci caused self-terminating rotors.Conclusion: Computer models of the excitation propagation of the heart can be used to simulate AF initiated by triggers in the PV. A reduction in conduction velocity caused the establishment of re-entrant circuits and quasi-chaotic behavior. The complex model of the Visible Female heart showed the importance of anatomical structures in the maintenance of AF. Future work will include an improvement of the computer model by incorporating heterogeneities of atrial tissue and an implementation of individual patient models for therapy planning.

Abstract:

In this work, the physiological effects of time-varying magnetic fields up to 100 kHz have been investigated, namely magnetic stimulation and body warming. Simulation studies were based on numerical calculations on sophisticated cell and body models. In addition, magnetic stimulation thresholds have been determined experimentally. The project was carried out within the scope of the development of Magnetic Particle Imaging, a new imaging technology for medical diagnostics.