Elastically Stretchable and Flexible RF Receive Coils for Magnetic Resonance Imaging

Abstract

Great advances have been made towards patient-centric, lightweight, and flexible coils that provide greater conformability across patient sizes and anatomies. Innovations in flexible designs have been made possible through miniaturization of electronics and ultra-flexible conductors. To accommodate a variety of anatomical structures, including joints at various degrees of flexion, stretchable coils have been demonstrated. For the coil conductor, these prototypes have employed liquid metal [1] , elastomers [2] , and coated threads [3] , [4] . Our group has prototyped stretchable and flexible RF receive arrays using stitched, conductive thread. These designs provide two main advantages over other stretchable designs. Firstly, the conductive thread may be used in an automated, professional embroidery machine, facilitating rapid and consistent manufacturing of the loops. The thread is cut-resistant, solderable, and has a 12× greater break strength than 30 AWG copper. Evaluation of single loops demonstrated greater SNR over flexible PCB loops spaced 4.2 cm above the phantom, simulating a volume coil, while exhibiting ~14% reduction in SNR with similar, unstretched placement; however, the stitched design overcomes conductor-associated challenges when stretched and wrapped around curved anatomies, resulting in SNR increases compared to solely flexible counterparts. Examples of the use of this coil for wrist and breast phantom imaging can be seen in Figure 1 . This segues to the second advantage, which is the multipurpose application facilitated by the thread durability. Unlike some liquid metal applications, the distribution of the fibers and conductive material remains consistent, meaning that the SNR of the coil is almost unchanged before and after being stretched. When fully stretched, i.e., approximately a 20% increase in loop base loop diameter of 71 mm, SNR measurements showed ~30% decrease; however, this fully stretched condition would not be expected in clinical applications, as stretching of all elements to this taut degree should not be necessary if the coil array were of adequate size. For joints at varying degrees of flexion, it would be expected that elements curved around the anatomy would be stretched, but peripheral loops would be relatively undistorted. The breathable, stretchable fabrics produced no proton signal during phantom or in vivo scans.