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.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, ϵ_r=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].

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⁺.

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.