The birdcage Radio Frequency (RF) coil is one of the most used configurations in Magnetic Resonance Imaging (MRI) scanners for the detection of the proton (1H) signal over a large homogeneous volume. More recently, birdcage RF coils have been successfully used also in the field of X-nuclei MRI, where the signal of a second nucleus (e.g.13C,23Na,31P, and many others) needs to be detected with high sensitivity and spatial homogeneity. To this purpose several technical solutions have been adopted to design Double Tuned (DT) volume RF coils, including the recent configuration of the nested birdcage RF coils. One of the main problems in the design of DT RF coils is the decoupling between the1H and X channels, and a number of solutions have been adopted over the years. In this work, based on numerical and workbench methods, we report the decoupling optimization of DT (1H/23Na) nested RF birdcage coils suitable for 2.35 T MRI scanners encompassing an inner Low-Pass (LP) birdcage used for X-nuclei, an outer High-Pass (HP) birdcage for1H and an external cylindrical RF shield. We show that a suitable geometrical selection of the two coaxial RF birdcage coils (relative angular orientation, diameters and lengths) and RF shield (diameter, length) allows a significant decoupling optimization. We also provide valuable information about the RF B1+field homogeneity and efficiency. Our approach was validated both with numerical simulations and workbench testing using DT nested RF coil prototypes.

In this paper, we present a novel procedure for an efficient and accurate electromagnetic simulation of microstrip circuits and printed antennas etched in layered media. The proposed approach, based on a new algorithm referred to herein as the equivalent medium approach (EMA), is applied for a rapid design of the preliminary desired circuit prototype. The illustrated technique yields reliable results and reduces the computational time in comparison with the conventional method of moments (MoM). Some examples that demonstrate the accuracy and the efficiency of the described procedure are included.