  Magnetic resonance imaging (MRI) is the cornerstone technique for diagnostic
medicine, biology, and neuroscience. This imaging method is highly innovative,
noninvasive and its impact continues to grow. It can be used for measuring
changes in the brain after enhanced neural activity, detecting early cancerous
cells in tissue, as well as for imaging nanoscale biological structures, and
controlling fluid dynamics, and it can be beneficial for cardiovascular
imaging. The MRI performance is characterized by a signal-to-noise ratio,
however the spatial resolution and image contrast depend strongly on the
scanner design. Here, we reveal how to exploit effectively the unique
properties of metasurfaces for the substantial improvement of MRI efficiency.
We employ a metasurface created by an array of wires placed inside the MRI
scanner under an object, and demonstrate a giant enhancement of the magnetic
field by means of subwavelength near-field manipulation with the metasurface,
thus strongly increasing the scanner sensitivity, signal-to-noise ratio, and
image resolution. We demonstrate experimentally this effect for a commercially
available MRI scanner and a biological tissue sample. Our results are
corroborated by measured and simulated characteristics of the metasurface
resonator, and our approach can enhance dramatically functionalities of widely
available low-field MRI devices.
