Measurement and advanced data post-processing of proton resonance frequency shift in 7 T MRI to obtain local temperature in a tissue-mimicking phantom

Author:

Garcia Maíra M.12ORCID,Oliveira Tiago R.3ORCID,Chaim Khallil T.4ORCID,Bruns Christian5ORCID,Otaduy Maria C. G.4ORCID,Svejda Jan T.2ORCID,Bernarding Johannes5ORCID,Erni Daniel2ORCID,Zylka Waldemar1ORCID

Affiliation:

1. Department of Physical Engineering, Faculty of Electrical Engineering and Applied Natural Sciences , Westphalian University , D-45897 Gelsenkirchen , Germany

2. General and Theoretical Electrical Engineering (ATE), and CENIDE – Center of Nanointegration Duisburg-Essen , University of Duisburg-Essen , D-47048 Duisburg , Germany

3. Department of Biomedical Engineering, Modeling and Applied Social Sciences Center , Federal University of ABC, 09606-045 São Bernardo do Campo , SP , Brazil

4. Laboratory of Magnetic Resonance in Neuroradiology (LIM44), Hospital das Clinicas HCFMUSP, Faculdade de Medicina , Universidade de São Paulo, São Paulo , SP , Brazil

5. Institute for Biometry and Medical Informatics, Medical Faculty, Otto-Von-Guericke University Magdeburg, D-39120 Magdeburg , Germany

Abstract

Abstract The radio-frequency (RF) transmit power deposition in tissue during magnetic resonance imaging (MRI) at ultra-high fields, i.e. B 0 ≥ 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 B 0 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 B 0 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 B 0, 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 B 0-drift correction and, importantly, the spatio-temporal unwrapping are an indispensable part of post-processing.

Funder

Westphalian University of Applied Sciences

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Publisher

Walter de Gruyter GmbH

Reference32 articles.

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