1. An apparatus for improving operation of an MRI machine characterized by a working frequency, by improving the signal-to-noise ratio of received signals, the apparatus comprising:an array of unit cells, the array sized to be disposed within a bore of the MRI machine along with a specimen in the bore, and capable of passive operation to increase signal-to-noise ratio of signals emitted by the specimen when the MRI machine is imaging the specimen, wherein:
each unit cell has a resonant frequency, and
the array has a resonance frequency at or near the working frequency, and
the unit cells configured such that they magnetically couple with one another.