Radiofrequency (RF) coils are a fundamental component in MRI technology and directly influence the imaging signal-to-noise ratio (SNR) and performance. Since the introduction of the surface coil for in vivo MRS [1], the circular loop resonator has been widely used for X-nuclear (non-proton) magnetic resonance spectroscopy imaging (MRSI) applications such as 2H, 17O and 31P MRSI for studying the human brain metabolism [[2], [3], [4], [5], [6]]. However, X-nuclear MRSI is largely limited by low temporal and spatial resolution due to the low natural abundance of X nuclei (e.g., 2H, 17O and 13C) or low metabolites concentration and low intrinsic detection sensitivity compared to proton, and limited RF coil sensitivity. The circular solenoid or planar spiral coil design has been used to improve the RF coil sensitivity and imaging SNR as the use of many loop turns increases the overall RF coil magnetic field (B1) strength compared to a single-turn surface loop coil [[7], [8], [9]]. In addition, the spiral coil integrated with dielectric materials such as the Meta-metallic coil [10] and multi-turn split-conductor transmission-line resonators [11,12] further increases the RF coil B1 efficiency. However, as the number of the coil turns increases, the length of conductor trace of the multi-turn coil also increases, resulting in higher ohmic resistance of the multi-turn coil, thus, higher imaging noise and limiting the SNR gain [13]. Thus, the number of turns and the spacing between adjacent conductor traces must be carefully designed to optimize the coil layout in order to improve the overall multi-turn coil sensitivity and imaging SNR [14]. There has been limited exploration of simpler surface coil designs that avoid introducing complicated coil geometries and structures while improving overall MRI or MRSI performance.

The planar concentric-loops coil design, consisting of an inner loop and an outer loop, is a simpler modification to the conventional single loop surface coil design as compared to the multi-turn loop or spiral coil design. The planar concentric-loops coil design has been widely used in MRS/MRI applications, such as serving as an inductive matching probe to couple to the main RF resonating coil(s) in order to match it to 50 Ω of the coaxial cable [15,16]. Additionally, the concentric-loops coil design is used for dual-tuned X-nucleus/proton imaging, such as the nested loop designs [17,18], where the center loop is used for X-nuclei imaging and the outer loop is used for proton imaging. Moreover, multiple concentrically placed loop-coil elements have also been used as a concentric array for parallel imaging [19]. Surprisingly, little attention has been given to examine how the self-resonance frequency of the inner or outer loop can be controlled and adjusted to provide optimal performance for the overall concentric-loops design for the single-frequency imaging application, and to rigorously compare its performance with that of a traditional single-loop (control) coil.

In this work, we optimized the performance of a concentric-loops coil design consisting of an inner primary loop as the driving coil and a co-planar outer secondary loop as a passive resonator without wire connection between them. Electromagnetic (EM) simulations and phantom MRS/MRI tests at 10.5 T 17O (60.6 MHz), 7 T 31P (120.7 MHz), and 1.5 T 1H (63.8 MHz) operating frequencies show that the optimized concentric-loops coil can significantly reduce imaging noise and increase the overall B1 strength and coil sensitivity, thus, largely enhancing imaging SNR compared to a single-loop control coil.