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1H-23Na Dual-Tuned Radio-Frequency volume coil design for MRI at 7 T1Physical Sciences, Earth and Environment, University of Siena, Siena, Italy, 2Pisa Division, INFN, Pisa, Italy, 3IRCCS Stella Maris, Imago7 Foundation, Pisa, Italy, 4Hyperfine Research inc, Guilford, CT, United States, 5GE Healthcare, Aurora, OH, United States, 6Life, Health & Environmental Sciences, University of L’Aquila, L'Aquila, Italy
Synopsis
We present a systematic comparison between two dual-tuned (DT) RF coil models through electromagnetic simulations. The first model (imbricated) consists of two concentrically placed birdcages, whereas the second model (four-rings) consists of two High-Pass birdcage-like structures nested over an internal Low-Pass birdcage. For both DT-RF coil models, the dimensional parameters have been varied in order to optimize the B1+ field homogeneity and the coil efficiency at the proton (298.03MHz) and sodium (78.86MHz) Larmor frequency at 7T. Results show that the longest four-rings DT-RF coil model has the best performances.
Introduction
In case of imaging with non-proton nuclei (X-nuclei) Dual-Tuned (DT) RF coils should be preferred in order to allow an optimal co-registration between the X-nucleus and the anatomically detailed proton images. The aim of the current study is the comparison between two DT-RF coil models for the sodium and proton imaging of the human head at 7T. The first model, referred as imbricated, consists of two concentrically placed birdcages1,2. The second one, referred as four-rings, consists of two High-Pass birdcage-like structures nested over an internal Low-Pass birdcage3,4.Methods
It has been demonstrated that birdcage coils allow to achieve a superior B1+ field homogeneity with respect to other coil designs, and, in case of quadrature driving, an improved efficiency5. Both DT-RF coil models we analysed are based on the birdcage design. Both have been driven in quadrature by two ports positioned 90° apart. For each model, the B1+ field homogeneity and the coil efficiency have been computed through full electromagnetic simulation, varying the model geometrical parameters. The B1+ field maps have been evaluated in the transverse plane (z=0), in the presence of a spherical phantom that mimics dimension and electrical properties of the human head (radius r=90mm, electric conductivity σ=0.6S/m, dielectric constant εr=80), by means of an electromagnetic simulator (CST MWS) using the Finite-Difference-Time-Domain (FDTD) algorithm. The B1+ field homogeneity has been evaluated as6,
[1-(max(|B1+|)-min(|B1+|)/(max(!B1+|)+min(|B1+|)]
before performing the matching in order to avoid having to restore the unavoidable asymmetries caused by the insertion of the matching network, and thus achieving more reproducible results. Instead, the coil efficiency, evaluated as ,
average(|B1+|/√Pin)
has been evaluated from the B1+ field maps computed after the matching procedure, so to neglect power reflection at ports. The two models and the related variations of the geometrical parameters are reported in Tab. 1.
Results
Results are showed in Tab. 2 a), where the B1+ field homogeneity is reported, and in Tab. 2 b), where the efficiency is reported for both the four-ring and the imbricated DT-RF coil models.Discussion
For both the four-ring and the imbricated models, the ‘longest’ configuration has the best performances and the ‘shortest’ configuration the worse ones. Concerning the B1+ field homogeneity at proton Larmor frequency, the results in Tab. 2 a) show that the lower homogeneity of the four-ring model observed for the ‘shortest’ and ‘medium-size’ configuration can be improved by properly adjusting the four-rings model length. In particular, the homogeneity of the four-ring model over performs the one of the imbricated model if the ‘longest’ configuration is chosen. At the same time, the B1+ field homogeneity of the four-rings model is always superior than that one of the imbricated one at sodium Larmor frequency. The second significant result is the higher coil efficiency observed in the four-rings model. Referring to Tab. 2 b), we can see that the coil efficiency of the four-rings model is always higher at the both Larmor frequencies.Conclusion
In conclusions, the four-rings and the imbricated DT birdcage coils designed for human head imaging at 7T have been characterized by full electromagnetic simulations. An analysis of some geometrical parameters has been conducted to optimize the coil efficiency, and the excitation field homogeneity, which is particularly hampered at the proton Larmor frequency at 7T. Useful trends have been highlighted which are for the optimisation of the RF field distributions and the efficiency of the DT-RF coil models in the presence of a uniform spherical phantom with size and dielectric properties close to the human head.Acknowledgements
This work has been partially funded by GE Healthcare, the Italian Ministry of Health, the nextMR project (INFN-CSN5, 2014-2016), and the INAIL BRIC ID39 PAR 2016-2018.References
1. J. R. Fitzsimmons, B. L. Beck, H. R. Brooker. Double resonant quadrature birdcage. Magn.Reson. Med. 1993; vol. 30, pp 107-114.
2. A. Galante, M. Fantasia, M. Alecci. Optimization study of a double-tuned nested birdcage RF coil for 1H/23Na MRI. Proc. Intl. Soc. Mag. Reson. Med. 2018; vol. 26, pp. 1719.
3. J. Murphy-Boesch, R. Srinivasan, L. Carvajal, R. R. Brown. Two configurations of the four-ring birdcage coil for 1H imaging and 1H decoupled 31P spectroscopy of the human head. J. Magn. Reson. 1994; B103, pp 103-114.
4. Maggiorelli et al. Double Tuned 1H-23Na Birdcage Coils for MRI at 7 T : Performance evaluation through electromagnetic simulations. 2018 IEEE International Symposium on Medical Measurements and Applications (MeMeA) 4. Hoult, C. Chen, V. Sank. Quadrature detection in the laboratory frame. Magn, Reson. Med. 1984; vol. 1, no. 3.
5. National Electrical Manufacturers Association. Determination of Image Uniformity in Diagnostic Magnetic Resonance Images. NEMA Standards Publication, MS 3-2008 (R2014).
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