Electromagnetic Simulations

Groupleader : Prof. Dr. Andreas Bitz

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Prof. Dr. Andreas Bitz
Medical Physics in Radiology
Electromagnetic Simulations

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  • 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, the high-frequency technology group of the University of Duisburg-Essen and the research group for RF systems and concepts 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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Job Offers

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.
Contact: Prof. Dr. Andreas Bitz

Peer-Reviewed

Thomas M. Fiedler, Mark E. Ladd, Andreas K. Bitz (2017)
SAR Simulations and Safety
Review Article, NeuroImage 168 (2018) 33-58: Special Issue “Neuroimaging with Ultra-High Field MRI: Present and Future”,
DOI: 10.1016/j.neuroimage.2017.03.035

Thomas M. Fiedler, Mark E. Ladd, Andreas K. Bitz (2017)
RF safety assessment of a bilateral 4-channel transmit/receive 7 Tesla breast coil: SAR versus tissue temperature limits
Medical Physics 44 (1), January 2017, DOI: 10.1002/mp.12034

Yacine Noureddine, Oliver Kraff, Mark E. Ladd, Karsten H. Wrede, Bixia Chen, Harald H. Quick, Gregor Schaefers, Andreas K. Bitz. (2018)
In vitro and in silico assessment of RF-induced heating around intracranial aneurysm clips at 7 Tesla.
Magn Reson Med, 79:568–581 (2018), DOI: 10.1002/mrm.2665

Stephan Orzada, Mark E. Ladd, Andreas K. Bitz (2017)
A method to approximate maximum local SAR in multichannel transmit MR systems without transmit phase information.
Magn Reson Med, 78:805–811 (2017), doi: 10.1002/mrm.2639

Stephan Orzada, Andreas K. Bitz, Sören Johst, Marcel Gratz, Maximilian N. Völker, Oliver Kraff, Ashraf Abuelhaija, Thomas M. Fiedler, Klaus Solbach, Harald H. Quick and Mark E. Ladd (2017)
Analysis of an integrated 8-channel Tx/Rx body array for use as a body coil in 7-Tesla MRI
Frontiers in Physics, June 2017, Vol. 5, Article 17, DOI: 10.3389/fphy.2017.00017.

Oliver Kraff, Karsten H. Wrede, Tobias Schoemberg, Philipp Dammann, Yacine Noureddine, Stephan Orzada, Mark E. Ladd, Andreas K. Bitz (2013)
MR safety assessment of potential RF heating from cranial fixation plates at 7 T.
Med. Phys. 40 (4), April 2013, doi: 10.1118/1.4795347.

Conference Abstracts

Thomas M. Fiedler, Martina  Flöser, Stefan  Rietsch, Stephan  Orzada, Harald  Quick, Mark  Ladd, Andreas K. Bitz (2017)
Comparison of a 32‐channel remote body coil for 7 Tesla with local and remote 8‐ and 16‐channel transmit coil arrays
ISMRM 25th Annual Meeting 2017, Honolulu, USA, #4306

Thomas  Fiedler, Stephan  Orzada, Martina  Flöser, Harald  Quick, Mark  Ladd, Andreas K. Bitz (2017)
RF safety assessment of a 32‐channel integrated body coil for 7 Tesla: Thermal dose evaluation at high SAR level
ISMRM 25th Annual Meeting 2017, Honolulu, USA, #5577

Stephan Orzada, Andreas  Bitz, Marcel  Gratz, Sören  Johst, Samaneh  Shooshtary, Maximilian  Voelker, Stefan  Rietsch, Martina  Flöser, Ashraf  Abuelhaija, Mark  Oehmigen, Sascha  Brunheim, Thomas  Fiedler, Oliver  Kraff, Harald  Quick, Klaus  Solbach, Mark Ladd (2017)
A 32-Channel Transmit System Add-On for 7 Tesla Body Imaging
ISMRM 25th Annual Meeting 2017, Honolulu, USA, #1219

Thomas M. Fiedler, Sascha Brunheim, Martina Flöser, Mark E. Ladd, Stephan Orzada, Harald H. Quick, Andreas K. Bitz
RF safety assessment of a 32-channel whole-body transmit array at 7 Tesla
10th Anniversary of the Erwin L. Hahn Lecture 2016, Essen, Germany

Thomas M. Fiedler, Mark E. Ladd, Andreas K. Bitz
Comparison of SAR, Tissue Temperature and Thermal Dose in Local and Remote 8‐ch Body Arrays ISMRM Workshop on: Ultra High Field MRI: Technological Advances & Clinical Applications
06 - 09 March 2016, Heidelberg, Germany

Stephan Orzada, Andreas K. Bitz, Oliver  Kraff, Mark  Oehmigen, Marcel  Gratz, Sören  Johst, Maximilian  Völker, Stefan  Rietsch, Martina  Flöser, Thomas M. Fiedler, Samaneh  Shooshtary, Klaus  Solbach, Harald  Quick, Mark E. Ladd
A 32-Channel Integrated Body Coil for 7 Tesla Whole-Body Imaging
ISMRM 24rd Annual Meeting 2016, Singapore

Thomas M. Fiedler, Mark E. Ladd, Andreas K. Bitz
RF safety assessment of a 7 Tesla breast coil: SAR versus tissue temperature limits
ISMRM 24rd Annual Meeting 2016, Singapore, #3666

Thomas M. Fiedler, Mark E. Ladd, Andreas K. Bitz
Local SAR increase in the human head induced by high-permittivity pads at the sodium (23Na) resonance frequency at 7 Tesla
ISMRM 24rd Annual Meeting 2016, Singapore, #3663

Thomas M. Fiedler, Mark E. Ladd, Andreas K. Bitz
Local SAR elevations in the human head induced by high-permittivity pads at 7 Tesla
ISMRM 23rd Annual Meeting 2015, Toronto, Canada, 30.05-05.06.2015 #3213
ISMRM Merit Award: Magna cum laude

Thomas M. Fiedler, Aaron S. Kujawa, Frank Resmer, Patrick Stein, Titus Lanz, Mark E. Ladd, Andreas K. Bitz (2015)
RF safety assessment of a bilateral 4-channel Tx/Rx 7T breast coil
ISMRM 23rd Annual Meeting 2015, Toronto, Canada, 30.05-05.06.2015, #3233

Andreas K. Bitz, Oliver Kraff, Stephan Orzada, Tim Herrmann, Johannes Mallow, Johannes Bernarding, and Mark E. Ladd (2014)
RF safety evaluation of different configurations of high-permittivity pads used to improve imaging of the cerebellum at 7 Tesla
Proc. Intl. Soc. Mag. Reson. Med. 22 (2014), # 4892

Andreas K. Bitz, Rene Gumbrecht, Stephan Orzada, Hans-Peter Fautz, Mark E. Ladd (2013)
Evaluation of Virtual Observation Points for Local SAR Monitoring of Multi-Channel Transmit RF Coils at 7 Tesla
Proc. Intl. Soc. Mag. Reson. Med. 21 (2013), #4414

Andreas K. Bitz, Irina Brote, Stephan Orzada, Oliver Kraff, Stefan Maderwald, Harald H. Quick,
Klaus Solbach, Achim Bahr, Hans-Peter Fautz, Franz Schmitt, Mark E. Ladd
Comparison of simulation-based and measurement-based RF shimming for whole-body MRI at 7 Tesla
Proc. Intl. Soc. Mag. Reson. Med. 18 (2010), # 4720

Andreas K. Bitz, Irina Brote, Stephan Orzada, Oliver Kraff, Stefan Maderwald, Harald H. Quick, P. Yazdanbakhsh, Klaus Solbach, A. Bahr, T. Bolz, K. Wicklow, Franz Schmitt, and Mark E. Ladd
An 8-channel add-on RF shimming system for whole-body 7 Tesla MRI including real-time SAR monitoring

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