Loop T/R Coil for 7T MRI/MRS with Two Transmit/Receive channels

Introduction A loop T/R coil is simple to build and versatile to apply at different anatomies for various proton imaging or spectroscopy purposes. It has been widely used in both 1.5T and 3T MRI systems and provides reasonable B1 field uniformity in the coil coverage area for those field strengths. However, at ultra-high fields such as 7T, B1 field from a single loop coil is strongly disturbed and asymmetrical due to the increased tissue dielectric effect. It has a B1 weak band in the middle of coil coverage area, thus produces relatively poor image quality. Here we propose a new coil structure topologically similar to the single loop coil but with distinctly different operational characteristics. It has two concentric flat rings which can be tuned to two orthogonal resonant modes at the same frequency. Combining with two independent transmit/receive (T/R) channels for B1 shimming, a more uniform B1-field coverage in the sensitive region of the coil is achieved. The proposed coil can easily be used for various imaging purposes at different anatomies such as head, torso and extremities at 7T. Methods The proposed coil structure has two concentric circular flat rings placed on one side of a 1cm-thick dielectric substrate. The mean diameters of the inner and outer rings are 13cm and 17cm. The two rings share the same RF shield placed on opposite side of the dielectric substrate (see Fig. 1). To tune the inner ring to a resonant mode at 298MHz, eight capacitors are used along the ring. Capacitor values are adjusted such that a uniform RF current distribution is established along the loop with B1-field perpendicular to the ring plane (Y-direction). To tune the outer ring to a resonant mode at the same frequency, eight inductors are used evenly distributed along the ring. The inductor value is adjusted such that the current distribution along the ring is sinusoidal, which produces a B1-field parallel to the ring plane (X-direction). The FDTD numerical method is used to model this coil structure and calculate B1 (XFDTD software package, Remcom, Inc., State College, PA) [1,2]. Figure 1 shows the coil model placed 1cm above a uniform cylindrical phantom model (diameter = 30cm, height = 20cm, σ = 0.8S/m, εr = 80). Two RF excited sources are used to simulate two independent transmit channels, one source in each ring. B-field and E-field of steady-state solutions for each source are recorded separately. The combined |B1|-field (B1-field in the rotating frame) for two transmit channels is then calculated using the formula in [3], with the amplitude and phase for each source varied. |B1|-field in the coil coverage area from the inner loop only, outer loop only, and from a 2-channel optimal transmit case are compared.