Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain

MRI is proving to be a very useful tool for sodium quantification in animal models of stroke, ischemia, and cancer. In this work, we present the practical design of a dual-frequency RF surface coil that provides H-1 and Na-23 images of the rat head at 4 T. The dual-frequency RF surface coil comprised of a large loop tuned to the H-1 frequency and a smaller co-planar loop tuned to the Na-23 frequency. The mutual coupling between the two loops was eliminated by the use of a trap circuit inserted in the smaller coil. This independent-loop design was versatile since it enabled a separate optimisation of the sensitivity and RF field distributions of the two coils. To allow for an easy extension of this simple double-tuned coil design to other frequencies (nuclei) and dimensions, we describe in detail the practical aspects of the workbench design and MRI testing using a phantom that mimics in vivo conditions. A comparison between our independent-loop, double-tuned coil and a single-tuned Na-23 coil of equal size obtained with a phantom matching in vivo conditions, showed a reduction of the Na-23 sensitivity (about 28%) because of signal losses in the trap inductance. Typical congruent H-1 and Na-23 rat brain images showing good SNR (Na-23: brain 7, ventricular cerebrospinal fluid 11) and spatial resolution (Na-23: 1.25 x 1.25 x 5 mm(3)) are also reported. The in vivo SNR values obtained with this coil were comparable to, if not better than, other contemporary designs in the literature. (c) 2006 Elsevier Inc. All rights reserved.