Electromagnetic Simulations and RF Safety
Research Topics
- Simulation-based development of multi-channel transmitter coils for UHF-MRT
- Numerical safety estimation of RF exposure based on local SAR, tissue temperature and thermal dose during MR examinations
- Simulation-based compatibility testing of medical implants
- Development of anatomical body models
- Measurement-related validation of simulation models and results
Background
In MR systems, fields from different bands in the electromagnetic spectrum are utilized to manipulate magnetic moments of nuclei as well as to detect the MR signal. Thus, the static magnetic field B0 polarizes spin ensembles and switched magnetic field gradients (Gx, Gy, Gz) at frequencies up to 10 kHz are applied for spatial localization. Further, radio frequency (RF) transmit coils generate fields at the Larmor frequency for spin excitation, whereas RF receive coils detect the MR signal. The spatial field distributions in the different frequency ranges obey Maxwell’s equations (MWE).
Numerical simulations have become an indispensable tool for compliance testing as well as for design optimization of transmit and receive coils of MR systems. Since the entire three-dimensional field distribution can be obtained, it is possible to extract various pieces of useful information for realistic exposure scenarios that cannot be obtained from measurements in phantoms or in vivo in comparable detail. In particular, with respect to compliance testing, numerical computation of RF fields in anatomical body models is currently the only practical way to obtain realistic SAR distributions as are necessary to guarantee compliance with limits for the localized SAR.
Human body tissue is lossy and energy is absorbed by body tissue during exposure to RF fields. The absorbed power is converted into a heat input which can lead to an increased tissue temperature. The IEC standard specifies limit values that must be adhered to in order to prevent possible tissue damage. The assessment of RF exposure is generally based on the local specific absorption rate (SAR). Since SAR reflects only the absorbed power and is not directly related to possible tissue damage, new approaches use bio-heat transfer equations to evaluate the temperature and the thermal dose of the tissue in addition to the SAR.
The research area of the project group 'Electromagnetic Simulations' focuses on the evaluation of RF exposure of persons during MR examinations at static field strengths ≥ 7 Tesla (ultra-high field MRI). This includes in particular the simulation-based development and optimization of innovative multi-channel transmitter coils with the aim of reduced RF exposure with homogeneous nuclear spin excitation. Further research areas are the development of anatomical body models and the implementation of thermal regulation systems for realistic safety assessment and simulation-based compatibility tests of passive medical implants.
As part of the EU-funded project "MRExcite" we are working together with the Erwin L. Hahn Institute in Essen and the high-frequency technology group of the University of Duisburg-Essen on the development of an integrated transmission system with 32 parallel transmission channels for the whole-body MRI at 7 T.
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Bachelor and Master thesis
We are looking for motivated candidates for Bachelor and Master theses in our group. If you are interested in working in our group, we look forward to receiving your enquiry. We would be happy to discuss possible projects with you. A list of open projects can be found here.