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Abstract
Purpose
In presence of respiratory motion, temperature mapping is altered by in-plane and through-plane displacements between successive acquisitions together with periodic phase variations. Fast 2D Echo Planar Imaging (EPI) sequence can accommodate intra-scan motion, but limited volume coverage and inter-scan motion remain a challenge during free-breathing acquisition since position offsets can arise between the different slices.
Method
To address this limitation, we evaluated a 2D simultaneous multi-slice EPI sequence with multiband (MB) acceleration during radiofrequency ablation on a mobile gel and in the liver of a volunteer (no heating). The sequence was evaluated in terms of resulting inter-scan motion, temperature uncertainty and elevation, potential false-positive heating and repeatability. Lastly, to account for potential through-plane motion, a 3D motion compensation pipeline was implemented and evaluated.
Results
In-plane motion was compensated whatever the MB factor and temperature distribution was found in agreement during both the heating and cooling periods. No obvious false-positive temperature was observed under the conditions being investigated. Repeatability of measurements results in a 95% uncertainty below 2 °C for MB1 and MB2. Uncertainty up to 4.5 °C was reported with MB3 together with the presence of aliasing artifacts. Lastly, fast simultaneous multi-slice EPI combined with 3D motion compensation reduce residual out-of-plane motion.
Conclusion
Volumetric temperature imaging (12 slices/700 ms) could be performed with 2 °C accuracy or less, and offer tradeoffs in acquisition time or volume coverage. Such a strategy is expected to increase procedure safety by monitoring large volumes more rapidly for MR-guided thermotherapy on mobile organs.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Data availability statement
The data that support the findings of this study are available at this link (https://doi.org/10.5281/zenodo.7643565). To ease use of the SMS-EPI in such a context, support our findings and promote the reproducibility of the results, an open-source reconstruction code (https://github.com/LIRYC-IHU/gadgetron-sms) was developed to ensure an optimal phase reconstruction for temperature reconstruction. Compatibility with the two widely available SMS sequences on Siemens scanners (the commercial product and the CMRR C2P) were set up. In-line reconstruction presented in the study is therefore accessible to all major fMRI sites where CMRR sequences are usually available. After a minor modification on the scanner, the proposed reconstruction can be used with a limited latency (the code being not yet optimized for real-time reconstruction). Compatibility with in-plane parallel imaging and partial Fourier has only been tested for multiband acceleration of 2 and 3 which currently limits the use of the reconstruction for other applications.