Adaptable, wearable, and stretchable coils: A review

Abstract

Over the last four decades, there have been various evolutions in the design and development of coils, from volume coils to the recent introduction of wireless receive arrays. A recent aim has been to develop coils that can closely conform to the anatomy of interest to increase the acquired signal. This goal has given rise to designs ranging from adaptable transmit coils to on-body stretchable receive arrays made using fabric or elastomer substrates. This review covers the design, fabrication details, experimental setup, and MRI results of adaptable, wearable, and stretchable MRI coils. The active and passive automatic tuning and matching strategies are examined with respect to mitigating signal-to-noise ratio reduction when the coil form is altered. A brief discussion of wireless MRI coils, which provide a solution to overcome the cabling issues associated with MRI coil development, is also included. The adaptable, wearable, and stretchable coils and various coil tuning techniques represent innovative radiofrequency coil solutions that pave the way for next-generation MRI hardware development.

Keywords: RF coils; adaptable; automatic tuning; stretchable; wearable; wireless MRI.

FIGURE 1

(A) Schematic of the receive coil system used in the adaptable eight‐channel wrist array. (B) The double hump resonance obtained by adjusting the π‐matching network in (A).

FIGURE 2

The longitudinally stretchable two‐channel receive array for knee imaging at 1.5 T (top) and the rigid copper‐based array for performance comparison (bottom), as demonstrated by Gruber et al. The arrays were placed on a body phantom using a frame for the stretchable array and a flat base for the rigid array. The geometric decoupling of the array beneath the receive electronics is not shown in the illustration.

FIGURE 3

(A) The pressure control system as demonstrated by Lopez Rios et al. Negative pressure is created using the vacuum hand pump, which is maintained using a check valve, and atmospheric pressure is established using an air valve. The pump and valves are connected to the expandable bellows through a network of tubes. The negative pressure due to the vacuum pump compresses the bellows, resulting in coil movement away from the head, whereas the reverse displacement occurs when atmospheric pressure is restored. (B) The 32‐channel anterior array of the 64‐channel 3T pediatric body coil as described by Zhang et al. The rigid posterior array is not shown in the illustration. The hinges between the anterior array columns make the coil setup semiflexible over the abdomen.

FIGURE 4

The modular 16‐channel anterior array as described in Graessl et al. The bottom row of each 2 × 2 array is shifted by approximately one diameter (d) to better decouple the coils at the edges (Elements 1 and 4). Adjacent elements share a trace with a variable capacitor (red arrows) for capacitive decoupling. Each 2 × 2 array is shifted by 3 cm for optimal intermodule decoupling. The 16‐channel flat posterior array is not shown.

FIGURE 5

(A) Top layer of the conductive trace printed on one side of the flexible substrate, and the second half of the coil printed on the other side of the substrate. The composition of the dielectric substrate between two conductive layers is like a parallel plate capacitor. This method can be adopted for both inkjet printing and screen printing of MRI coils. (B) Alternate method for manufacturing tuning capacitors using the screen‐printing method. The complete coil loop is formed first by printing a conductive layer on top of a flexible substrate. The initial metal layer is covered with a dielectric resin on top of which another short trace of conductive layer is deposited to form the tuning capacitors.

FIGURE 6

The twisted pair transceive coil demonstrated in Vliem et al. (A) When a counterclockwise current is excited on the red (signal) wire, a clockwise current is introduced in the gray (shield) wire. The high potential difference at the gap in the gray wire induces a current on the red wire flowing in the opposite direction as the original current. The induced current causes partial cancellation of the current in the red wire, thus making the gray wire the primary current carrier (clockwise current in the combined wires figure). (B) The twisted pair cable. (C) The fabrication of the twisted pair transceive coil. A gap is introduced on the gray (shield) wire at the top and a gap is introduced on the red (signal) wire in the bottom to act as the feed point. Images are adapted with permission from Vliem et al.

FIGURE 7

The transmission line resonator (TLR) coils shown in Kriegl et al. (A) The single‐gap (left) and dual‐gap (right) design of TLR coils. The single channel coils are shown at the top, and the two‐channel array configurations with decoupling annexes are shown at the bottom. (B) The four‐channel TLR array used on a flat phantom (left) and on a cylindrical phantom (right), which demonstrates the flexibility of the TLR array. Images are adapted with permission from Kriegl et al.

FIGURE 8

(A) The conductive thread on fabric technique in which copper‐based conductive threads are embroidered on the fabric using normal threads. (B) Stretchable coil developed using liquid metal in a silicone tube. The copper contacts are used to connect the coil to the external circuit, which are pushed into the silicone tube to remove any air cavities at the junction. Glue is then applied at the interface followed by the application of heat shrink tubing for a permanent connection. (C) A stretchable coil with interdigital capacitor (IDC) developed using microfluidic channels containing liquid metal on an elastomer substrate. The coil setup uses copper contacts or copper wires to interface with external circuits.