1520
A Preclinical Volume Coil with Artificial Magnetic Shield for 7 Tesla MRI1Faculty of Physics and Engineering, ITMO University, Saint-Petersburg, Russian Federation, 2Department of Electronics and Nanoengineering, Aalto University, Aalto, Finland, 3Department of Radiotherapy, UMC Utrecht, Utrecht, Netherlands, 4Fresnel Institute, Aix-Marseille University, Marseille, France
Synopsis
In this work the artificial magnetic shield structure was proposed aimed to improve the performance of preclinical volume coil for 7 Tesla MRI. Particularly, transmit efficiency as well as receive sensitivity of small-animal bird-cage coil was studied in the presence of both ideal electric and magnetic screen at the first step. Next, practical artificial magnetic shield structure using the principles of operation of high-impedance corrugated surfaces was suggested. B1+ distribution of the small-animal bird-cage coil equipped with corrugated surface structure was calculated and compared with the reference case of ideal electric screen. Numerical results demonstrated improved transmit efficiency and receive sensitivity of the bird-cage coil with the artificial magnetic shield structure comparing to the reference case.
Purpose
Volume coils are traditionally used in MRI for radiofrequency (RF) pulse excitation with homogeneously distributed magnetic field over a large region of interest (ROI). The same coils are typically used in preclinical applications for imaging of the whole subject volume, for instance, a small animal. RF volume coils are usually equipped with a copper RF-shield reducing interaction with the gradient system and electronics. Сontrariwise, the presence of a copper shield limits transmit efficiency of the coil, while reducing SNR provided by the latter in receive mode. To mitigate these drawbacks, artificial magnetic shield (MS) structures were proposed recently to improve the performance of surface [1] and volume [2] MR coils.
In this work, we propose an alternative RF-shield design based on an artificial magnetic conductor (AMC). We also propose and numerically study a practical realization of the AMC based on a cylindrical miniaturized corrugated surface with high surface impedance for small-animal imaging at 7 T. Firstly, a preclinical bird-cage [3] coil was studied numerically. The case with an ideal MS with infinite surface impedance was numerically compared with the case of a conventional copper shield for the same bird-cage. Next, the coil was simulated together with the proposed practical AMC structure and compared with copper shield considering transmit efficiency and receive sensitivity.
Methods
Numerical simulations were made in CST Studio 2017 on the elliptical-cylinder homogeneous phantom (30x40x80 mm, material properties σ = 0.5 S/m, ε = 34). The simulated high-pass birdcage had 8 legs and the diameter of 72 mm. On this step, the copper and the ideal magnetic shields both had the diameter of 82 mm. The sagittal cut views of both simulation setups are depicted in Figure 1. To tune to Larmor frequency of 300 MHz the end-ring capacitance of the bird-cage was chosen as 12 and 4 pF in the presence of the copper and ideal MS respectively. The practical AMC was implemented as a rotationally-symmetric array of metal corrugations periodic in the direction of B0. Such structures refer to as hard-and-soft corrugated surfaces [4] demonstrating high surface impedance at the resonance frequency. To tune the resonance frequency of each of six corrugations, they were filled with a high-permittivity dielectric material (tan δ = 0.001 S/m, ε = 177). The corrugated surface was placed coaxially to the bird-cage coil (Figure 2). Each corrugation has the radial thickness of 22 mm and the length of 20 mm in z-direction. As a reference case, bird-cage was calculated with the copper shield having the same radius as the outer radius of the corrugated structure.Results
Figure 3 demonstrates the central transverse plane B1+ distributions created by the bird-cage coil in the presence of the copper (a) and the ideal magnetic (b) shield. These results clearly illustrate the effect of the MS manifesting itself in improvement of the filling factor of the central region of the coil. This goes along with removing the strong localization of the magnetic field between the legs and the shield, typical for the case of the conventional copper shield. This can be explained by in-phase reflection from the MS (constructive interference) instead of the out-of-phase reflection (destructive interference) from a metal shield. This beneficial behavior of the ideal MS gives the improvement of B1+/Pacc of more than two (from 3.5 uT with the copper shield to 7.5 uT with the ideal MS for Pacc=1W). Moreover, the transverse B1+ field patterns in the phantom (Figure 3) holds homogeneous when replacing the copper shield with the ideal magnetic one.
Figure 4 illustrates transverse plane B1+ distribution of the birdcage coil with the proposed corrugated structure (a) and the copper screen (b), as well as the corresponding B1+ profiles plotted along X axis (c). These maps and profiles clearly illustrate that the proposed structure has a similar effect to the transmit field in the ROI as the ideal MS. In particular, it enhances the transmit efficiency of the bird-cage coil by the factor of 1.25 keeping good homogeneity.
Conclusion
In this work the effect of an artificial MS replacing a conventional copper one was numerically studied, and the proposed practical AMC design was shown operating similarly as an ideal high-impedance boundary condition assigned to an ideal MS. For a small-animal 7T bird-cage coil it was shown by numerical calculation of B1+/Pacc on a homogeneous phantom, that in the presence of the proposed AMC, the transmit efficiency, and therefore, the receive sensitivity of the same coils are improved by the factor of 1.25 while the field pattern remains homogeneous in the phantom.Acknowledgements
This work was supported by the Russian Science Foundation (Project No. 18-19-00482). This work was supported by the European Union's Horizon 2020 research and innovation program under grant agreement No 736937.References
[1] Chen, Z., Solbach, K., Erni, D., & Rennings, A. Improving B1 Efficiency and Signal-to-Noise-Ratio of a Surface Coil by a High-Impedance-Surface RF Shield for 7-T Magnetic Resonance Imaging. IEEE Trans. on Microwave Theory and Techniques, vol. 65(3), 988-997, 2017.
[2] Van Leeuwen C., Lunenburg M., Glybovski S., Luijten P., Klomp D., Van den Berg C., Raaijmakers A. Improved performance of birdcage coils using a split-ring resonator magnetic shield. In Proceedings of Joint Annual Meeting ISMRM – ESMRMB, 2018.
[3] Hayes EC, Edelstein WA, Schenck JF et al. An efficient highly homogeneous radiofrequency coil for whole-body NMR imaging at 1.5 T. J. Magn. Reson. vol. 63:622–828, 1985
[4] Kildal P.-S. . Artificially soft and hard surfaces in electromagnetics. IEEE Trans. on Ant. and Propag., vol. 38(10), 1990.
Figures
Figure 2. Sagittal cut view of the proposed corrugated structure (a), proposed corrugated structure with the birdcage coil and the phantom (b)
