Dedicated surface coils for MR studies in the temporal and the frontal lobes of the human brain at 7T
Jérémie Daniel Clément1, Lijing Xin2, Rolf Gruetter3,4,5, and Özlem Ipek2

1CIBM-LIFMET, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 2CIBM-AIT, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 3LIFMET, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 4Department of Radiology, University of Geneva, Geneva, Switzerland, 5Department of Radiology, University of Lausanne, Lausanne, Switzerland

Synopsis

The purpose of the study was to build dedicated surface coils for the temporal and the frontal lobes of the human brain at 7T. Their transmit field efficiency and single-voxel spectroscopy performances were compared with a birdcage coil with a dielectric pad. An increased B1+-field for the brain regions was measured with the surface coils compared to the birdcage with the pad, which allows single-voxel SPECIAL spectroscopy and anatomical image acquisitions in the peripheral temporal and frontal lobes.

Introduction

Birdcage coils demonstrate high transmit field at the center of the brain and lower transmit field close to the cortex at 7T. Adding a dielectric pad inside the coil leads to a substantial increase of the transmit field under the pad [1]. Surface coils can offer high transmit field in their vicinity. The purpose of the study was to design highly efficient surface coils for the peripheral temporal and frontal lobes of the human brain regions and compare them with the volume coil with dielectric pads in terms of transmit field efficiency and single-voxel spectroscopy performance.

Methods

Ear-to-Ear loops (ETE-loops)(110x90mm2 each side) and Frontal-Region-Operating loops (FRO-loops)(two loops with 80x60mm2 each) were built with copper and tuned/matched to 297.2 MHz (> -15dB). The decoupling was better than -15dB and common-modes on the coaxial cables were diminished with baluns. FRO-loops were driven in quadrature mode while ETE-loops can be driven as one-side only or two sides, depending on the interest. Cases (Fig.1) were drawn on Solidworks (Dassault Systèmes, France) and printed in ABS (Acrylonitrile Butadiene Styrene, CubePro DUO 3D printer,3DSystems,MA,USA). For comparison, a birdcage coil (,16 legs,32-channel receivers,Nova Medical, Inc. MA) and a dielectric pad (100x100x5mm3, ϵr=160,BaTi in deuterated water), placed on the temporal and frontal regions of the brain were used. The birdcage coil with pad (BC-pad) and without pad (BC) were studied. Finite difference time domain (FDTD) simulations were performed, for the exact model of the coils, on Sim4Life 2.0 (ZMT,Switzerland) on a Virtual Family human model [2] at 1 mm iso-gridded. In simulations, the birdcage and loop coils were tuned to 297.2 MHz and matched to input impedance (> -15 dB). In-vivo measurements were performed on Magnetom 7T MR scanner (Siemens, Erlangen, Germany). B1+ maps were acquired with SA2RAGE sequence [3]. Spectroscopy measurements were performed with a semi-adiabatic SPECIAL sequence (VOI = 20x20x20 mm3,TR/TE = 6500/16 ms,NA = 64). To position MRS voxel, anatomical images (Fig.3A-B-C) were acquired with MP2RAGE sequence [4].

Results

The experimental and simulated B1+ maps are shown for one-side of the ETE-loops and FRO-loops (Fig.2). The mean B1+ values of 12.7μT and 14.5μT at the voxel placed in the temporal (Fig.2A) and the frontal (Fig.2B) regions were measured for 500μs-long 90° hard pulse which corresponds to 11.7 μT. Simulated B1+ maps (Fig.2C-D) demonstrate similar distributions to the experimental ones (Fig.2A-B). 3D-MP2RAGE human brain images (Fig.3A-B-C) and the single-voxel proton spectra acquired with the ETE-loops (Fig.3D) and FRO-loops (Fig.3E) are shown within the SAR limits (Fig.4A-B). The loops were compared to the BC-pad. Placing the dielectric pad improves the field in the voxel placed just under the pad (Fig.5), and the mean B1+ values of 10.8μT and 9.3μT are achieved at the voxel placed in the temporal (Fig.5B) and the frontal (Fig.5B) regions, for 500μs-long 90° hard pulse. The single-voxel spectroscopy could be performed with the ETE-loops (Fig. 3D) while it has not been possible with the BC-pad due to insufficient B1+.

Discussion and conclusion

The dedicated surface coils present notable advantages compared to the birdcage coil with the dielectric pad. ETE-loops and FRO-loops offer head-conformal placement of the coils to yield high transmit field efficiency in their vicinity. Although adding the dielectric pad inside the birdcage coil increases the $$$(B_1^+)^2/SAR_{10g,max}$$$ ratio by 33% in the frontal voxel and 27% in the temporal voxel, performing B1+-demanding spin-echo single-voxel spectroscopy is not always possible. Furthermore the $$$(B_1^+)^2/SAR_{10g,max}$$$ ratio achieved for the BC-pad in the frontal lobe is 20% higher than that in the temporal lobe, suggesting that an increased transmit efficiency under the pad is dependent on the region of the brain and head-shape. Similar study [5] was performed by using a dielectric pad placed in a birdcage coil and two-times SNR increase was achieved compared with and without pad in the medial-temporal lobe with less-SAR sensitive STEAM spectroscopy sequence. We concluded that ETE-loops and FRO-loops offer higher transmit field compared to the BC-pad, which make them more suitable for the application in edges of the brain with the drop of B1+ field.

Acknowledgements

This study was supported by Centre d’Imagerie BioMédicale (CIBM) of the UNIL, UNIGE, HUG, CHUV, EPFL and the Leenaards and Jeantet Foundations.

References

[1] W. Teeuwisse, W. Brink and A. Webb. Quantitative Assessment of the Effects of High-Permittivity Pads in 7 Tesla MRI of the Brain. Magnetic Resonance in Medicine, vol. 67, pp.1285-1293 (2012)

[2] M-C Gosselin et al. Development of a new generation of high-resolution anatomical models for medical device evaluation: the Virtual Population 3.0. Physics in Medicine and Biology, vol. 59, n.18 (2014)

[3] F. Eggenschwiler et al. SA2RAGE: A New Sequence for Fast B1+-Mapping. Magnetic Resonance in Medicine, vol. 67, pp. 1609–1619 (2012)

[4] JP Marques et al. MP2RAGE, a self bias-field corrected sequence for improved segmentation and T1-mapping at high field. Neuroimage, vol. 15, 49 (2010)

[5] J.E.M Snaar et al. Improvements in high-field localized MRS of the medial temporal lobe in humans using new deformable high-dielectric materials. NMR in Biomedicine, vol. 28 (2010)

Figures

Figure 1. The cases and placement on the patient bed A) for ETE-loops and C) FRO-loops. In A) ETE-loops are placed on each side of the head. B) One side of the ETE-loops and D) FRO-loops are shown.

Figure 2. Experimental (A,B) and simulated (C,D) B1+ maps for A) one-side ETE-loops (dashed line), for 450W B) FRO-loops (solid line), for 200W. The white square represents the voxel-of-interest where the spectroscopy data is acquired.

Figure 3. 3D MP2RAGE transverse images for A) One side ETE-loops B) Two sides ETE-loops, C) FRO-loops. H-spectrum acquired with a semi-adiabatic single voxel SPECIAL sequence for D) one-side ETE-loops and E) FRO-loops.

Figure 4. Simulated 10g SAR, normalized to 1W absorbed power for A) One-side ETE-loops, B) FRO-loops, C) and D) BC-pad in lateral and frontal regions, respectively, in the slice where the maximal local SAR10g was occurred. The maximal value on the colorbars presents the SAR10g,max for 1W absorbed power.

Figure 5. Experimental and simulated B1+ maps for A) BC without the dielectric pad and for B) BC-pad placed on the frontal and the temporal lobe regions. The white square represents the voxel-of-interest where the spectroscopy data is acquired.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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