Elastomer coils for wearable MR detection

Abstract

Funding information Nano-Tera.ch, project: WearableMRI Purpose: To explore the use of conductive elastomer for MR signal detection and the utility of this approach for wearable detector arrays. Methods: An elastomer filled with silver microparticles was used to form stretchable radiofrequency coils for MR detection. Their electrical performance in terms of the Q unloaded and Q ratio was assessed in the relaxed state and under repeated strain up to 40%. In a phantom imaging study, the signal-to-noise ratio yield of conductive elastomer coils was compared with that of a reference copper coil. Four elastomer coils were integrated with a stretchable textile substrate to form a wearable array for knee imaging. The array was employed for multiple-angle and kinematic knee imaging in vivo. Results: The elastomer coils proved highly stretchable and mechanically robust. Upon repeated stretching by 20%, a medium-sized coil element settled at Q unloaded of 42 in the relaxed state and 32 at full strain, reflecting sample-noise dominance. The signal-to-noise ratio of elastomer coils was found to be 8% to 16% lower than that achieved with a conventional copper coil. Multiple-angle and kinematic knee imaging with the wearable array yielded high-quality results indicating robustness of detection performance against stretching and warping of the array. Conclusion: Conductive elastomer is a viable material for MR detection. Coils made from this material reconcile high stretchability and adequate electrical performance with ease of manufacturing. Conductive elastomer also offers inherent restoring forces and is readily washable and sanitizable, making it an excellent basis of wearable detector front ends. K E Y W O R D S conductive elastomer, knee imaging, wearable array 1 | INTRODUCTION In the design of MRI receiver coils, one of the key goals is to maximize signal sensitivity. To this end, coils and coil arrays are traditionally made from excellent conductors, predominantly copper, which minimizes detection noise caused by coil losses. Along with superior conductivity, copper offers ease of manufacturing, favorable chemical properties, and mechanical strength. The latter is helpful in forming robust, rigid coil setups as commonly used for both clinical and research uses of MRI. However, rigidity is arguably ambivalent. Receiver setups should be applicable in a range of patients with anatomies of varying size and shape. Rigid coils will then inevitably be too large for some subjects, at the How to cite this article:

A flexible MRI coil based on a cable conductor and applied to knee imaging

“…The Q-ratio of a cable loop built from RG-223 coaxial cable with a floating center conductor was 2.7 compared to 3.0 for a reference loop based on copper clad FR4 circuit board (Cu-FR4) (Table 1 ). The Q ratios indicate that the relative efficiency of the cable loop was within 3% of the Cu-FR4 loop as defined by 5 , 16 , 17 . The relative efficiency of a RG-223 coaxial loop was 26% lower.…”

“…The cable loops also inherited the benefits of coaxial cables such as low cost, widespread availability, ease of assembly, and high durability. While coaxial cables cannot match the stretchability and elasticity of other specialized conductors 3 – 5 , 12 , it remains unclear if such features are necessary for routine knee MRI. The selected RG-223 cable has a minimum bend radius of 2.1 cm, which was sufficiently flexible to provide a tight fit to the knee.…”

“…In conclusion, the proposed cable coil utilized flexible, commercially available conductors, which showed comparable SNR as a reference Cu-FR4 loop and adequate SNR robustness against geometric variability. While cable loops do not provide elastic properties that have been demonstrated elsewhere 4 , 5 , cables are relatively inexpensive (on the order of $20 per meter) and allow straightforward coil assembly. Applied to knee imaging, the cable coil array provided promising image quality, particularly in the cartilage, which has been difficult to examine with older generation low-field systems 39 , 45 , in a clinically acceptable examination time 36 , 43 .…”

“…Silver-coated thread is attractive but involves a tradeoff between mechanical flexibility and effective conductivity. Alternatively, Port et al showed that eutectic gallium indium "liquid metal" or conductive elastomer 4 , 5 coils provide excellent electro-mechanical properties and are relatively straightforward to assemble. While these methods appear to represent a major step toward truly wearable coils, the materials remain expensive or difficult to source.…”

Recent Advances in Radio‐Frequency Coil Technologies: Flexible, Wireless, and Integrated Coil Arrays

“…Previously, coil conductors filled with mercury 20,21 or braided copper conductors 22 have been used to improve the flexibility, which allowed the coil to conform to the patient but created safety concerns or produced a suboptimal image SNR. More recently, the design and manufacturing of flexible coil conductors have evolved to include elastomer-based materials 23 and silver conductors woven into fabric 24 that are highly stretchable and mechanically robust, while maintaining adequate electrical performance and improving manufacturing ease. One promising recent direction integrates modern printing techniques to construct inkjet-printed conductors 25 and rapid-production screen-printed coil (SPC) conductors, which are created by depositing multiple layers of special conductive and dielectric inks 26 onto a highly flexible plastic film that can conform to the subject.…”