A numerical method is shown for calculating the noise correlation coefficient in arrays of magnetic resonance imaging (MRI) coils loaded with capacitively-loaded ring metamaterial lenses, and in the presence of a conducting half-space resembling a sample. This numerical method is validated by comparison with experimental results obtained in two different experimental procedures for double check: noise resistance measurements with a network analyzer and noise correlation measurements in an MRI system. It is found that, for practical array configurations such as overlapping coils or capacitively-decoupled coils, the noise correlation coefficient turns negative for coils loaded with metamaterial lenses. In particular, the analysis is carried out with metamaterial structures known as magnetoinductive lenses, which have been demonstrated in previous works to improve the signal-to-noise ratio of MRI coils. Results are also shown to demonstrate that negative noise correlations have as an effect the improvement of the g-factor in coil arrays for parallel MRI.

A coil design termed as broadside-coupled loop (BCL) coil and based on the broadside-coupled split ring resonator (BC-SRR) is proposed as an alternative to a conventional loop design at 7T. The BCL coil has an inherent uniform current which assures the rotational symmetry of the radio-frequency field around the coil axis. A comparative analysis of the signal-to-noise ratio provided by BCL coils and conventional coils has been carried out by means of numerical simulations and experiments in a 7T whole body system.

In this brief review, we present the fundamentals of bulk resonant ring metamaterial (RRM) theory. Metamaterials made of resonant rings are discussed, and some basic design rules are provided. Homogenization (including spatial dispersion) of 3-D resonant ring latices is reviewed, with emphasis in isotropic designs. Edge effects in finite size metamaterial samples are discussed. Finally, possible applications and future trends are briefly reviewed.