Wearable Receive Arrays
Andreas Port1
1Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland

Synopsis

This talk will give an overview of flexible and stretchable electrical conductor concepts, from which textile-embedded coil elements and fully wearable receive arrays are formed. It will also touch on the electronic interface particular to wearable coil arrays in terms of tuning, matching, signal digitization and transmission.

Why wearable receive arrays?

Receive arrays used in clinical practice are commonly rigid and of fixed size. This can impair sensitivity for some patients and comfort for others. Wearable arrays promise to overcome this tradeoff by high adaptability to the anatomy being imaged. Wearable arrays also add to the capabilities of MRI by enabling variable angle and kinematic imaging, which promises interesting functional insights. Resembling ordinary pieces of clothing, wearable arrays intuitively enable patients to put on the array themselves even outside of the scanner room, which can improve workflow.
In achieving high degree of adaptability of a wearable coil, the major challenge is to find a suitable coil conductor, which can provide high flexibility or stretchability. Which types of coil conductors can provide these characteristics? And how are these conductors textile-integrated to form a wearable array?

From rigid to flexible to stretchable

Most commonly employed coil arrays used in today’s clinical practice employ coil elements made from rigid copper. Rigid copper offers very high electrical conductivity, which is advantageous in terms of SNR. However, copper is not flexible nor stretchable, which would be needed for forming a wearable coil array.
Several flexible coil conductor concepts have been proposed1-12. One approach uses conductive paste screen-printed onto a flexible substrate and integrated into a textile fabric5,10. Another flexible coil array is formed by textile-integrating flexible and highly decoupled conductor loops6,9. A glove array was demonstrated for dedicated hand imaging making use of flexible coaxial cable attached to a textile carrier7.
For even greater adaptability, coil conductors can be made stretchable relying on one of three principles of forming an electrical path under strain. The first way to create a stretchable conductor is by the principle of length reserve. In one work, copper braid was sewn onto a medical bandage13. Copper braid is an assembly of interwoven copper wires, which, when sewn onto a textile with suitable restoring forces, can be elongated and will then be retracted by the substrate. Making use of the same principle of length reserve, meandering copper14 and conductive thread15,16 can also be used to form stretchable coil elements. A second principle of stretchable conductor formation is liquid displacement, where liquid metal is employed as a coil conductor. In one implementation, eutectic Gallium Indium liquid metal was stencil printed onto neoprene foam17, forming a stretchable coil element. Another approach contains the Gallium Indium liquid metal in stretchable silicone tubes18,19. Four of the so formed coil elements were embedded in a textile, forming an array tailored for MR imaging of the human knee. A third class of stretchable MR coil conductors are conductive elastomers, which rely on the principle of particle rearrangement to form conductive pathways. Coil elements made from this kind of material have been textile integrated, forming a wearable receive array for knee imaging20.

Interfacing wearable coils

Stretchable coil elements undergo size variations when adapting to individual anatomies of patients. Resulting resonance frequency shifts and variable loading can be addressed by π-matching and the use of preamplifiers robust to changes in source impedance21.
Wearable receive arrays are commonly interfaced to the MR scanner acquisition hardware by means of analog cables. With increasing channel counts, however, these cables can compromise handling, safety and imaging performance of the array. On-coil digitization22–24 and optical signal transmission22–28 may overcome these limitations. Ultimately, wireless MR detection29–37 may replace cables after all.

A more patient-centered MR experience

A fully adaptable, wearable receive array allows the patient to undergo an MR examination with more comfort. The ability to perform imaging of the flexion of joints can provide patient specific functional assessment. In the future, wearable arrays may be complemented with optical or ultimately wireless MR data transmission. Altogether, wearable coil array technology can take us one step forward towards MR systems that adapt to the patients and not the other way around.

Acknowledgements


References

1. Malko, J. A., McClees, E. C., Braun, I. F., Davis, P. C. & Hoffman, J. C. A flexible mercury-filled surface coil for MR imaging. Am. J. Neuroradiol. 7, 246–247 (1986).

2. Rousseau, J., Lecouffe, P. & Marchandise, X. A new, fully versatile surface coil for MRI. Magn. Reson. Imaging 8, 517–523 (1990).

3. Mager, D. et al. An MRI receiver coil produced by inkjet printing directly on to a flexible substrate. IEEE Trans. Med. Imaging 29, 482–487 (2010).

4. Jia, F. et al. Knee MRI under varying flexion angles utilizing a flexible flat cable antenna. NMR Biomed. 28, 460–467 (2015).

5. Corea, J. R. et al. Screen-printed flexible MRI receive coils. Nat. Commun. 7, 10839 (2016).

6. Vasanawala, S. S. et al. Development and Clinical Implementation of Very Light Weight and Highly Flexible AIR Technology Arrays. in Proc. Intl. Soc. Mag. Reson. Med. 24 (2016).

7. Zhang, B., Sodickson, D. K. & Cloos, M. A. A high-impedance detector-array glove for magnetic resonance imaging of the hand. Nat. Biomed. Eng. 2, 570–577 (2018).

8. Frass-Kriegl, R. et al. Flexible 23-channel coil array for high-resolution magnetic resonance imaging at 3 Tesla. PLoS One 13, e0206963 (2018).

9. McGee, K. P. et al. Characterization and evaluation of a flexible MRI receive coil array for radiation therapy MR treatment planning using highly decoupled RF circuits. Phys. Med. Biol. 63, (2018).

10. Winkler, S. A. et al. First clinical pilot study using screen-printed flexible MRI receive coils for pediatric applications. Proc. Intl. Soc. Mag. Reson. Med. 26 (2018).

11. Nohava, L. et al. Flexible multi-turn multi-gap coaxial RF coils ( MTMG-CCs ): design concept and bench validation. in Proc. Intl. Soc. Mag. Reson. Med. 27 (2019).

12. Obermann, M. et al. Ultra-flexible and light-weight 3-channel coaxial transmission line resonator receive-only coil array for 3T. in Proc. Intl. Soc. Mag. Reson. Med. 27 (2019).

13. Nordmeyer-Massner, J. A., De Zanche, N. & Pruessmann, K. P. Stretchable coil arrays: Application to knee imaging under varying flexion angles. Magn. Reson. Med. 67, 872–879 (2012).

14. Gruber, B. & Zink, S. Anatomically adaptive local coils for MRI Imaging – Evaluation of stretchable antennas at 1.5T. in Proc. Intl. Soc. Mag. Reson. Med. 24 (2016).

15. Vincent, J. M. & Rispoli, J. V. Conductive thread-based stretchable and flexible radiofrequency coils for magnetic resonance imaging. IEEE Trans. Biomed. Eng. (2019). doi:10.1109/TBME.2019.2956682

16. Kahraman Agir, B., Bayrambas, B., Yegin, K. & Ozturk Isik, E. Wearable and stretchable surface breast coil. in Proc. Intl. Soc. Mag. Reson. Med. 27 (2019).

17. Varga, M. et al. Adsorbed eutectic GaIn structures on a neoprene foam for stretchable MRI coils. Adv. Mater. 29, 1703744 (2017).

18. Port, A. et al. Liquid metal in stretchable tubes: A wearable 4-channel knee array. in Proc. Intl. Soc. Mag. Reson. Med. 27 (2019).

19. Port, A., Luechinger, R., Brunner, D. O. & Pruessmann, K. P. Towards kinematic knee imaging with a liquid metal array. in Proc. Intl. Soc. Mag. Reson. Med. 28 (2020).

20. Port, A., Luechinger, R., Brunner, D. O. & Pruessmann, K. P. Conductive Elastomer for Wearable RF Coils. in Proc. Intl. Soc. Mag. Reson. Med. 28 (2020).

21. Nordmeyer-Massner, J. A., De Zanche, N. & Pruessmann, K. P. Mechanically adjustable coil array for wrist MRI. Magn. Reson. Med. 61, 429–438 (2009).

22. Possanzini, C. et al. Scalability and channel independency of the digital broadband dStream architecture. in Proc. Intl. Soc. Mag. Reson. Med. 19 (2011).

23. Sporrer, B. et al. A fully integrated dual-channel on-coil CMOS receiver for array coils in 1.5-10.5 T MRI. IEEE Trans. Biomed. Circuits Syst. 11, 1245–1255 (2017).

24. Port, A. et al. Towards wearable MR detection: A stretchable wrist array with on-body digitization. in Proc. Intl. Soc. Mag. Reson. Med. 26 (2018).

25. Memis, O. G., Eryaman, Y., Aytur, O. & Atalar, E. Miniaturized fiber-optic transmission system for MRI signals. Magn. Reson. Med. 59, 165–173 (2008).

26. Yuan, J., Wei, J. & Shen, G. X. A 4-channel coil array interconnection by analog direct modulation optical link for 1.5-T MRI. IEEE Trans. Med. Imaging 27, 1432–1438 (2008).

27. Fandrey, S., Weiss, S. & Müller, J. A novel active MR probe using a miniaturized optical link for a 1.5-T MRI scanner. Magn. Reson. Med. 67, 148–155 (2012).

28. Reber, J. et al. An in-bore receiver for magnetic resonance imaging. IEEE Trans. Med. Imaging (2019). doi:10.1109/tmi.2019.2939090

29. Wei, J. et al. A realization of digital wireless transmission for MRI signals based on 802.11b. J. Magn. Reson. 186, 358–363 (2007).

30. Lu, J. Y., Robb, F., Pauly, J. & Scott, G. Wireless Q-spoiling of Receive Coils at 1.5T MRI. in Proc. Intl. Soc. Mag. Reson. Med 25 (2017).

31. Aggarwal, K. et al. A Millimeter-Wave Digital Link for Wireless MRI. IEEE Trans. Med. Imaging 36, 574–583 (2017).

32. Scott, G., Vasanawala, S., Robb, F., Stang, P. & Pauly, J. Pilot Tone Software Synchronization for Wireless MRI Receivers. in Proc. Intl. Soc. Mag. Reson. Med. 26 (2018).

33. Reykowski, A. et al. High Precision Wireless Clock Recovery for On-Coil MRI Receivers Using Round- Trip Carrier Phase Tracking. in Proc. Intl. Soc. Mag. Reson. Med. 26 (2018).

34. Byron, K. et al. An MRI Compatible RF MEMs Controlled Wireless Power Transfer System. IEEE Trans. Microw. Theory Tech. 67, 1717–1726 (2019).

35. Vassos, C., Robb, F., Vasanawala, S., Pauly, J. & Scott, G. Characterization of In-Bore 802.11ac Wi-Fi Performance. in Proc. Intl. Soc. Mag. Reson. Med. 27 (2019).

36. Ko, Y., Bi, W., Felder, J. & Shah, N. J. Wireless Digital Data Transfer based on WiGig/IEEE 802.11ad with Self-Shielded Antenna Gain Enhancement for MRI. in Proc. Intl. Soc. Mag. Reson. Med. 27 (2019).

37. Nohava, L., Ginefri, J., Willoquet, G., Laistler, E. & Frass-Kriegl, R. Perspectives in Wireless Radio Frequency Coil Development for Magnetic Resonance Imaging. Front. Phys. 8, 11 (2020).


Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)