A flexible nested sodium and proton coil array with wideband matching for knee cartilage MRI at 3T

Purpose: We describe a 2 × 6 channel sodium/proton array for knee MRI at 3T. Multielement coil arrays are desirable because of well-known signal-to-noise ratio advantages over volume and single-element coils. However, low tissue–coil coupling that is characteristic of coils operating at low frequency can make the potential gains from a phased array difficult to realize. Methods: The issue of low tissue–coil coupling in the developed six-channel sodium receive array was addressed by implementing 1) a mechanically flexible former to minimize the coil-to-tissue distance and reduce the overall diameter of the array and 2) a wideband matching scheme that counteracts preamplifier noise degradation caused by coil coupling and a high-quality factor. The sodium array was complemented with a nested proton array to enable standard MRI. Results: The wideband matching scheme and tight-fitting mechanical design contributed to >30% central signal-to-noise ratio gain on the sodium module over a mononuclear sodium birdcage coil, and the performance of the proton module was sufficient for clinical imaging. Conclusion: We expect the strategies presented in this study to be generally relevant in high-density receive arrays, particularly in x-nuclei or small animal applications. Magn Reson Med 76:1325–1334, 2016.