Publications

Abstract:

Magnetic resonance imaging (MRI) is a valuable diagnostic method for many cardiovascular diseases. To date, patients with pacemakers are contra-indicated for cardiac MRI exams due to several effects that can occur during the MRI procedure: a) heating of the lead-tip, and b) less hazardous sensing errors and device malfunctions. Almost all measurements on MRI pacemaker compatibility have been conducted on classic 1.5 or 3T cylindrical whole-body MRI systems. In contrast, this study focused on the use of a high field open MRI (HFO) system due to its advantageous properties of RF fields which are commonly made responsible for the induction of lead heating.

Abstract:

As of today, the use of MRI procedures on patients with implanted cardiac pacemakers is prohibited due to safety issues. The implants can interact with the RF fields of the MRI device. The most hazardous effect is heating at the tip of the lead, less dangerous are sensing errors and malfunctions of the devices, because they disappear completely after the procedure. The majority of the previous studies used classic cylindrical whole-body MRI systems. The influence of different alignments of the pacemaker/lead system and the RF fields were evaluated by comparing temperature changes occurring in a cylindrical device with the effects induced in a high field open MRI (HFO) system.

Abstract:

Magnetic Resonance Imaging (MRI) is a widely used means of imaging and becoming increasingly popular for cardiac applications as well. But for patients with im- planted pacemakers, the use of MRI is not allowed in Eu- rope and the United States due to potentially hazardous interactions of the RF pulses with the pacemaker-electrode system. Here heating at the tip of the electrode is regarded as the most important one.In this simulation study, the occurring current densities and E-fields should be determined by employing numerical field calculation. Computer models of metallic objects like straight wires, simplified pacemakers and a replication a commercial bipolar electrode were placed in a plexiglas box positioned inside a birdcage coil. The results con- firmed findings of previous in-vitro studies regarding the influence of size and position of the exposed objects and thereby proved the validity of the presented approach.

Abstract:

The number of implanted cardiac pacemakers and defibrillators is constantly increasing. At the same time, more and more of those patients use wireless mobile communication devices.Aim of this work was the development of a pacemaker-electrode model and its “implantation” into a detailed anatomical correct voxel model. Additionally generic body models were examined. It consists of several layers with varying thickness and conductivity/permittivity values corresponding to different tissue types. This approach was chosen to avoid numerical errors at tilted boundaries. The excitation sources were modeled as generic dipoles and as plane waves operating at the frequency range normally used by cellular phones and wireless networks (900 to 2450 MHz). The dipoles were designed to provide maximum radiation efficiency at the frequencies of interest. Finally numerical calculation of induced fields by external signal sources were conducted. The results were then evaluated regarding the compliance to the guidelines of ICNIRP and a draft by DIN/VDE.For the Visible Man model, the computed specific absorption rate (SAR) values were well below the thresholds both for single and multi-antenna setups and for all frequencies of interest if the power did not exceed the regulatory specifications. The same results were obtained for the electrical field values determined at commonly used implantation sites for pacemakers. For some tissue configurations in the generic model, higher SAR values than allowed by regulations could be observed.

Abstract:

The development of new devices used for defibrillation and cardioversion is often supported by numerical simulations of the induced electric potentials and current distributions. The commonly used tools incorporate isotropic models of the tissue properties present in the human torso. A comparative study was conducted to characterize the influence of anisotropic compared to isotropic tissue modeling. Defibrillation shocks with amplitudes of 1000 V and 2000 V were simulated and a set of varying conductivity values and anisotropy ratios was examined. The inclusion of tissue anisotropy produced significantly smaller values for current density compared to isotropic calculations especially in the myocardial tissue.

Abstract:

For patients with implanted cardiac pacemakers, Magnetic Resonance Imaging (MRI) is still contra-indicated. Coupling of RF-fields into metallic leads of pacemakers can induce currents, that warm up the tissue near the electrode. Estima- tions of this heating effect are commonly obtained either by experimental setups or numerical simulations with continous RF-excitation based on average SAR values. As during real MRI RF-fields are applied in pulsed sequences, high radiation powers occur for short timespans, followed by pauses that could act as cooling time. To determine deviations of thermal heating subject to pulsed and continous excitation, a series of numerical simulations with a saline filled phantom inside a birdcage coil was performed. All simulation parameters were set according to a real 1.5 T MRI system with 64 MHz. This study revealed that by modelling pulsed excitation patterns and a corresponding downscaled continous excitation, a good estimation of heating during real MRI sequences can be achieved. Using a fine mesh, peak temperature rise could be localized more precisely and showed to be highly concentrated.

Abstract:

Anatomical voxel models are used for example for dosimetric assessment and numerical field calculations. For best matching between models and patients properties an accurate model should be created by Magnetic Resonance Imaging or Computer Tomography images for every patient. Due to complexity, time and costs this is not always possible. In contrast 3D laser scanning is a fast and easy method to gain information about a patients body surface. In this work generic organ models are developed to fit into the laser scan envelope of any patient. One model set was processed as to fit to the volume reference values of the International Commission on Radiological Protections of six different ages. To accomplish a more realistic shape, orthogonal scaling factors are used to simulate growth in three different directions.