A flexible metasurface to improve knee MRI

Abstract

Magnetic resonance imaging (MRI), a non-invasive and safe imaging method, is largely used for the assessment of multiple organs including knee. Due to the complexity of the knee joint, better images quality are required. Over the last decades, a variety of coil solutions has been proposed to improve MR image quality. Interestingly, dielectric or metamaterial structures have been used as additional devices for their ability to tailor electromagnetic field at a given scale. However, the use of these devices is often limited by their complexity and bulkiness. The present study aimed at improving the B1 transmit field for knee imaging at 3T through the design and manufacturing of a convenient and comfortable passive metasurface. A cylindrical array of conductive stripes was used to redistribute by inductive coupling the radiofrequency field generated by the body coil of the MRI scanner. This design takes no more space than a thin sheet placed around the leg of the patient. We have shown in simulation and experimentally the accuracy of this solution. For a given flip angle during signal acquisition, the improved transmit field allowed a reduction of the necessary input power. In addition to that, the structure had a negligible influence on the electric field inside the tissue and so did not significantly increase the specific absorption rate (SAR).