MRI contrast using solid-state, B₁-distorting, microelectromechanical systems (MEMS) microresonant devices (MRDs)

Presently, signal generation in MRI depends on the concentration and relaxivity of protons or other MR-active nuclei, and contrast depends on local differences in signal. In this proofof-principle study, we explore the use of nonchemical, solidstate devices for generating detectable signal and/or contrast in vitro and in vivo. We introduce the concept of microresonant devices (MRDs), which are micron-sized resonators fabricated using microelectromechanical systems (MEMS) technology. Fifteen-micrometer (15-μm)-thick, coil MRDs were designed to resonate at the 3T Larmor frequency of protons (127.7 MHz) and were fabricated using tantalum (Ta) oxide thin-film capacitors and copper-plated spiral inductors. The performance of MRDs having final diameters of 300, 500, and 1000 μm were characterized in saline using a radio frequency (RF) scanning microscope and a clinical 3T MR scanner. The measured B₁, fields of 300 μm to 1000 μm MRDs ranged from 3.25 μT to 3.98 μT, and their quality factors (Q) ranged from 3.9 to 7.2. When implanted subcutaneously in the flank of a mouse, only MRDs tuned to the resonant frequency of protons generated a measurable in vivo B₁ field. This study lays the foundation for a new class of solid-state contrast agents for MRI.