A review of metamaterial and metasurface applications in magnetic resonance imaging

This review examines 3D metamaterials and 2D metasurface technology within the domain of magnetic resonance imaging (MRI). The discussion covers operating mechanisms, chronological development, and various empirical demonstrations in both phantom and in vivo studies. Metasurfaces and metamaterials have been applied at 0.3 T, 0.5 T, 1.5 T, 3 T, and 7 T field strengths to improve transmit field homogeneity and receive sensitivity. While various form factors exist, they generally fall into one of four broad devices classes: individual inductively coupled unit cells, arrays of subwavelength fraction parallel wires, secondary resonators, or conductive impedance surfaces. Many research groups incorporate high-permittivity dielectric materials as substrates, producing hybrid metamaterials that combine the benefits of conductive metasurface structures with bulk dielectric materials. As this technology has advanced in recent years, these designs have migrated from bulky rigid 3D metamaterials towards more flexible 2D metasurfaces that can better conform to the human body while still improving image enhancement performance. This review also focuses on applications that extend beyond conventional imaging into specialized MR uses, such as spectroscopic techniques (MRS & CEST), RF safety with implanted devices, multinuclear approaches, low-field strength imaging, and interventional MR. As further development of metamaterials and metasurfaces continues, the current primary challenges include the need for better safety monitoring and assessment as well as more focused demonstrations in clinical scenarios.