Anatomically Adaptive Coils for MRI—A 6-Channel Array for Knee Imaging at 1.5 Tesla

Abstract

Purpose: Many of today's MR coils are still somehow rigid and inflexible in their size and shape as they are intentionally designed to image a specific anatomical region and to fit a wide range of patients. Adaptive coils on the other hand, are intended to follow a one-size-fits-all approach, by fitting different shapes, and sizes. Such coils improve the SNR for a wide range of subjects by an optimal fit to the anatomical region of interest, and in addition allow an increased handling and patient comfort as one MRI receive-coil is maintained instead of multiple. Material and Methods: To overcome the SNR losses by non-fitting and thus poorly loaded RF coils, we propose a stretchable antenna design. Each loop has the ability to reversibly stretch up to 100% of its original size, to be anatomically adaptive to different shapes and sizes, and therefore make the coil usable for a wide patient population. Besides the mechanical challenge to find a robust but flexible conductive material, various other problems like frequency and matching shifts affect the SNR. Through bench measurements and MR Imaging at 1.5 T, we investigated different stretchable conductor materials, that fit the defined requirements. Finally, a rigid reference coil and an adaptive 6-channel array for knee imaging at 1.5 Tesla were developed to investigate the potential improvement in SNR. Results: The material tests identified two potentially useful materials: Highly ductile copper and a silver-plated stranded copper wire. Although, the adaptivity causes a frequency shift of the resonance frequency, which entails in variations of the impedance that each coil presents to its connected pre-amplifier, there are strategies to mitigate these effects. The adaptive array prototype made of partly-stretchable loops, showed an improved SNR of up to 100% in 20 mm depth from the phantom surface, and therefore demonstrates the effectiveness of adaptive coils.

Flexible Multi-Turn Multi-Gap Coaxial RF Coils: Design Concept and Implementation for Magnetic Resonance Imaging at 3 and 7 Tesla

“…Screen-printed MRI coils [17] developed by Corea et al have shown to be a reliable alternative to bulky conventional coils with comparable SNR, suitable especially for pediatric imaging [18]. Stretchable Rx coils produced with copper braids [19], [20], meandered conductors [21], liquid metal printed into neoprene support material [22], liquid metal filled tubes [23], [24] or conductive elastomers [25] mostly target wrist or knee imaging and offer a truly wearable solution.…”

Characterization of a Low-Profile, Flexible, and Acoustically Transparent Receive-Only MRI Coil Array for High Sensitivity MR-Guided Focused Ultrasound

“…In this paper, we aim to improve both imaging sensitivity and acoustic transparency in one apparatus by presenting a very thin, low-profile, receive-only 8-channel head coil (FUS-Flex) operating at 3T. The design is inspired by stretchable [ 34 ], [ 35 ], flexible [ 36 ]–[ 40 ] and lightweight [ 41 ] coil technologies, offering a coil array with full conformity to the head. The novelty of our work lies in the use of very thin (~1 mm) RF elements (providing low interaction with the acoustic field), and the use of higher channel count than currently available in the literature, increasing the available imaging SNR, the sensitivity of the coil and improving/enabling parallel imaging.…”

How thin can you go? Performance of thin copper and aluminum RF coil conductors

“…For example, lightweight, flexible coils are more comfortable 4 and can provide better SNR than rigid setups due to closer proximity to the imaging region. Examples include screen‐printing to create RF coils that are incorporated into infant blankets, 4 as well as flexible, meandering wires that allow the coil conductor’s path to change 5,6 . Conductive textiles can be stretched up to a 100 times their size, 7 and a flexible coil made from a conductive elastomer based on silver microparticles achieves an SNR just 14% lower than a reference copper coil 8 .…”

“…Examples include screen-printing to create RF coils that are incorporated into infant blankets, 4 as well as flexible, meandering wires that allow the coil conductor's path to change. 5,6 Conductive textiles can be stretched up to a 100 times their size, 7 and a flexible coil made from a conductive elastomer based on silver microparticles achieves an SNR just 14% lower than a reference copper coil. 8 Liquid metal 9,10 and braided conductors 11 also allow for stretchable coils that closely fit moving and flexing body parts.…”