I. Introduction
Sodium (23Na) MRI allows access to valuable cellular and metabolic information [1], [2] and the use of 23Na imaging has risen in recent years with the increasing availability of ultra-high field MRI systems [3], [4]. Although 23Na is the most abundant charged ion in the body [5], its intrinsic MR sensitivity is considerably lower and relaxation times are faster compared to those of the proton. This makes it difficult to measure MR-detectable sodium signals and it is therefore important to maintain the signal-to-noise ratio (SNR) of the sodium signal as high as possible [6]. Conducting a 23Na MR experiment requires an additional RF system, and this draws increasing attention to the technical development of efficient double-tuned RF coils. Double-tuned coils are often assembled with a 1H coil, as the 1H coil provides anatomical scout images and enables static magnet shimming capability, which is essential for 23Na acquisition. The design of these double-tuned coils has to address the coupling issue between the coils [7], [8] and the way in which coupling is minimised is one of the key factors that determines the quality of a double-tuned coil.