Measurement and advanced data post-processing of proton resonance frequency shift in 7 T MRI to obtain local temperature in a tissue-mimicking phantom
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Maíra M. Garcia
Maíra M. Garcia, M. Sc. is a Ph.D student in Electrical Engineering with focus on Medical Technology at the University Duisburg-Essen, Germany, and Westphalian University, Germany. She received her M.Sc. degree in Biomedical Engineering (2018) from the Federal University of ABC, Brazil, and her B.Sc. degree in Engineering Physics (2014) from the Federal University of São Carlos, Brazil. Her research interests include theoretical and applied bioelectromagnetics, MR safety, MR thermometry, medical imaging post-processing, among other medical technology applications.
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Tiago R. Oliveira
Prof. Johannes Bernarding, Ph.D., MD, is a full professor at the Institute for Biometry and Medical Informatics of Otto-von-Guericke University Magdeburg, Germany. He studied physics and medicine in Berlin, Germany (Technical University, and Campus Benjamin Franklin, Charité). Research interests are new NMR/MRI-techniques from ultra-low (nT) to ultra-high fields (7T), developing new coil architectures, performing real-time functional MRI including Virtual Reality environments, and developing 19F bio-compatible hyperpolarization imaging. He authored and co-authored numerous peer-reviewed publications.
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Daniel Erni
Abstract
The radio-frequency (RF) transmit power deposition in tissue during magnetic resonance imaging (MRI) at ultra-high fields, i.e. B0 ≥ 7 T, is a major challenge for pulse sequences requesting large flip angles. The absorbed RF energy can pose safety risk to patients as it is rising temperature in the tissue. The temperature can be accessed using MRI itself via the proton-resonance frequency (PRF) shift technique, which at low B0 has been shown a valid MR thermometry method. In this paper, we explore the applicability of the PRF method to the assessment of local temperature in 7 T MRI procedures. To this end, we built a phantom filled with a material presenting electrical conductivity and permittivity close to muscle tissue. Tubes filled with oil were placed nearby the phantom to observe the time dependent B0 drift. MRI phase images were acquired by gradient-echo (GRE) sequences at time points between spin-echo sequences with large flip angle allowing for a continuous assessment of the temperature during a 114 min RF-heating experiment. All acquired phase images were post-processed with attention to the time dependent instability of B0, and, in addition, to potential spatial and temporal phase discontinuities, known as wraps. In this paper, we present a strategy to analyze and to unfold these phase wraps for large measurement fields and long acquisition times. It is shown that the PRF shift method is beneficial for the assessment of temperature at 7 T MRI. The temperature maps for axial and coronal planes display a temperature increase of approximately 3.5 °C during the time of the RF-heating experiment. Overall it is shown that B0-drift correction and, importantly, the spatio-temporal unwrapping are an indispensable part of post-processing.
Zusammenfassung
Die Deposition von Hochfrequenz (HF)-Sendeleistung im Gewebe während der Magnetresonanztomographie (MRT) bei ultrahohen Feldern, d. h. B0 ≥ 7 T, ist die Hauptherausforderung für Pulssequenzen mit großen Flip-Winkeln. Die absorbierte HF-Energie kann für die Patienten ein Sicherheitsrisiko darstellen, da sie die Temperatur im Gewebe erhöht. Die Temperatur kann mit der MRT über die Protonen-Resonanzfrequenz-Verschiebung-Methode (PRF) bestimmt werden, die sich bei niedrigem B0 bereits als valide MR-Thermometrie Methode erwiesen hat. In diesem Beitrag untersuchen wir die Anwendbarkeit der PRF-Methode für die Bewertung der lokalen Temperatur in 7 T-MRT-Untersuchungen. Zu diesem Zweck haben wir ein Phantom gebaut, das mit einem Material gefüllt ist, dessen elektrische Leitfähigkeit und Permittivität den Werten des Muskelgewebes entspricht. Mit Öl gefüllte Zylinder wurden in der Nähe des Phantoms platziert, um die zeitabhängige B0-Drift zu beobachten. MRT-Phasenbilder wurden mit Gradientenecho (GRE) Sequenzen zu Zeitpunkten zwischen Spinecho (SPE) Sequenzen mit großem Flipwinkel aufgenommen, um die Temperatur während eines 114-minütigen HF-Erwärmungsexperiments kontinuierlich bewerten zu können. Alle Phasenbilder wurden analysiert, wobei besonderes Augenmerk auf die zeitabhängige Instabilität von B0 und die dadurch hervorgerufenen räumlichen und zeitlichen Phasendiskontinuitäten, sogenannte Wraps, gelegt wurde. In diesem Beitrag stellen wir eine Strategie zur Analyse und Aufhebung dieser Phasendiskontinuitäten bei großen Messfeldern und langen Aufnahmezeiten vor, sog. Phasen-Unwraping. Es wird gezeigt, dass die PRF-Verschiebungsmethode für die Bewertung der Temperatur bei der 7 T-MRT von Vorteil ist. Die Temperaturkarten für axiale und koronale Ebenen zeigen einen Temperaturanstieg von etwa 3,5 °C während der Zeit des HF-Erwärmungsexperiments. Insgesamt zeigt sich, dass die B0-Driftkorrektur und vor allem die räumlich-zeitliche Aufhebung der Phasendiskontinuitäten ein unverzichtbarer Bestandteil der Nachbearbeitung sind.
Funding source: Westphalian University of Applied Sciences
Funding source: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Award Identifier / Grant number: 88881.173609/2018-01
About the authors
Maíra M. Garcia, M. Sc. is a Ph.D student in Electrical Engineering with focus on Medical Technology at the University Duisburg-Essen, Germany, and Westphalian University, Germany. She received her M.Sc. degree in Biomedical Engineering (2018) from the Federal University of ABC, Brazil, and her B.Sc. degree in Engineering Physics (2014) from the Federal University of São Carlos, Brazil. Her research interests include theoretical and applied bioelectromagnetics, MR safety, MR thermometry, medical imaging post-processing, among other medical technology applications.
Dr. Tiago R. Oliveira is an Assistant Professor of Biomedical Engineering at Federal University of ABC, Brazil. He received a B.Sc. degree in Physics in 2005, an M.Sc. in Physics in 2008, and a Ph.D. in Applied Physics in 2014 from the University of São Paulo, Brazil. With over ten years of experience in MRI-guided therapy systems, he has coauthored over 30 publications. His research interest focuses on MRI-guided thermotherapy and developing innovative methods to deliver drugs to the brain.
Khallil T. Chaim is currently a Medical Physicist at the Autopsy Room Imaging Platform (PISA), working with the only WB 7 T in Latin America. He holds a bachelor's and master's degree in Medical Physics from the University of São Paulo (USP). He is currently a Ph.D. student on the Radiology course at the Medical School of the USP. His general research interests include nanoparticles, clinical and preclinical MRI, and post-processing methods for quantitative MRI.
Dipl.-Phys. Christian Bruns received diploma degree in physics in 2015 at Otto-von-Guericke University, Magdeburg, Germany. He is currently working toward the Ph.D. degree in physics at the Otto-von-Guericke University, Magdeburg, Germany. His general research interests include applied electromagnetics in medical application, currently focusing on magnetic resonance imaging and spectroscopy. Additionally, he is working in the field of medical informatics in the area of the German medical informatics initiative.
Maria Otaduy is the vice-coordinator of the Magnetic Resonance Laboratory (LIM44), and a collaborative professor, at the Medical School of the University of São Paulo (FMUSP), Brazil. She received her undergraduate degree in Chemistry from the Eberhard-Karls Universität Tübingen (Germany) and her PhD in Physics from the University of Kent (UK). In 2002 she completed a postdoctoral fellowship at the Radiology Department of FMUSP, where she has been working since, giving support to clinical research studies using quantitative MRI techniques.
Dr. Jan T. Svejda earned his Ph.D. in Electrical Engineering from the University of Duisburg-Essen in 2019, following a background in Communications and Information Technology. Specializing in electromagnetics, his research covers fundamental experiments, computational electromagnetics, antenna design, and near-field measurements. He is interested in biomedical engineering, material characterization, and open-source electromagnetic solvers. Dr. Svejda is dedicated to advancing knowledge in these fields and is passionate about teaching the next generation of scientists and engineers.
Prof. Johannes Bernarding, Ph.D., MD, is a full professor at the Institute for Biometry and Medical Informatics of Otto-von-Guericke University Magdeburg, Germany. He studied physics and medicine in Berlin, Germany (Technical University, and Campus Benjamin Franklin, Charité). Research interests are new NMR/MRI-techniques from ultra-low (nT) to ultra-high fields (7T), developing new coil architectures, performing real-time functional MRI including Virtual Reality environments, and developing 19F bio-compatible hyperpolarization imaging. He authored and co-authored numerous peer-reviewed publications.
Daniel Erni is a full professor for General and Theoretical Electrical Engineering at the University of Duisburg-Essen, Germany. He received two degrees in electrical engineering from the University of Applied Sciences Rapperswil (OST), and ETH Zürich in 1986 and 1990, respectively, and a Ph.D. degree in laser physics from ETH Zürich in 1996. His current research include nanophotonics, plasmonics, RF, mm-wave, THz and biomedical engineering, bioelectromagnetics, computational electromagnetics, multiscale and multiphysics modeling, topological optimization, and science and technology studies (STS).
Prof. Dr. Waldemar Zylka is a full professor in Physics and Medical Engineering at the Westphalian University, Germany. He received the Diplom-Physiker and the Doctor degree in Theoretical Physics from the Albert-Ludwigs-University Freiburg i.Br., Germany. He has authored numerous scientific publications and patents. He is serving as reviewer for international meetings, journals, and as a member of program committees. His current research focuses are system biology, multi-scale modelling, and theoretical and computational electromagnetics.
Acknowledgments
The authors thank Ewald Bonberg, Gustavo D. Maia, Lucas M. Martins, Fábio S. Otsuka, and Markus Plaumann for technical assistance.
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Research ethics: Not applicable.
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Author contributions: The authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Competing interests: The authors state no conflict of interest.
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Research funding: MMG was supported by the Brazilian agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES): process no. 88881.173609/2018-01, and by the Westphalian University.
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Data availability: The raw data can be obtained on request from the corresponding author.
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Articles in the same Issue
- Frontmatter
- Research Articles
- Measurement and advanced data post-processing of proton resonance frequency shift in 7 T MRI to obtain local temperature in a tissue-mimicking phantom
- Algorithmic assessment of drag on thermally cut sheet metal edges
- Messunsicherheit geometrischer Prüfmerkmale – Automatisiert und praxisgerecht mit Koordinatenmessgeräten ermitteln
- Measuring the frequency response of an optical microphone system with a fiber based setup
- Spatial detection and localisation of multiple laser beams in optical measuring systems
Articles in the same Issue
- Frontmatter
- Research Articles
- Measurement and advanced data post-processing of proton resonance frequency shift in 7 T MRI to obtain local temperature in a tissue-mimicking phantom
- Algorithmic assessment of drag on thermally cut sheet metal edges
- Messunsicherheit geometrischer Prüfmerkmale – Automatisiert und praxisgerecht mit Koordinatenmessgeräten ermitteln
- Measuring the frequency response of an optical microphone system with a fiber based setup
- Spatial detection and localisation of multiple laser beams in optical measuring systems
