Synopsis

“In the light of the moon a little egg lays on a leaf”[7]

Management of cabling connecting coil arrays to system remains the nightmare of RF-engineers. Coupling within arrays to cabling reduces performance, and coupling to transmit field causes high shield currents. Consequently, traps are placed on conductors. To provide high blocking, traps are rigid, large and often heavy, hindering flexibility. Instead of a few high blocking traps, we propose a distributed system of small, elastic traps. We leverage self-shielded resonant toroids forming a caterpillar-like structure. We show that this design can attenuate shield currents while being robust to flexing.

Introduction

“On Sunday.. out of the egg comes a tiny and very hungry caterpillar”[7]
Common mode currents occur on conducting wires within the bore of the scanner, for example, on the shield of coaxial cables between coils and main system. These currents effect coil performance, result in B1 inhomogeneities and pose serious safety hazards such as RF burns[1]. To mitigate shield currents, resonant cable traps are placed at certain intervals along cable (Figure 1a). While many designs exist[2], the basic principle is similar -- creating a high impedance through a resonance LC tank. These can either leverage the conductor itself, or inductively couple, i.e. a “floating” trap[3]. Floating traps can be easily fitted and positioned but don’t allow cable to be flexed and are only effective if placed in regions of highest shield current. These structures couple, thus, cannot be place closely, and need to have a length larger than radius in order to not interact with the B1 field and perform efficiently[4]. For this reason, a few large, high performance structures are used. Here, we propose a new, distributed design of traps where smaller self-shielded substructures are placed along the full length of cable (Figure 1b). These self-shielded structures do not couple to main field or each other thus requiring no shielding. With this distributed design, any bending is unlikely to impact overall structure allowing the cable to still be flexible while providing sufficient attenuation (Figure 1c).

Methods

The substructures were chosen to be resonant toroids as they are mostly self-shielded, especially with increased number of windings. Toroids have previously been used in traps but as the inductor for large, inductively coupling structures[5]. Our smaller traps can be laid out, along the entirety of the cable in a caterpillar-like structure without impeding its flexibility. The traps are tuned using four capacitors, one of which is variable, so that electrical field due to capacitors is uniformly distributed around structure.

Simulations
Initially, simulations were conducted with air-core toroids to determine optimal parameters of the overall structure for a fixed wire and toroid thickness($$$t$$$) including diameter of toroids($$$d$$$), separation between toroids($$$s$$$) and number of windings in each toroid($$$N$$$) as shown in Figure 2. Simulation results normalized by total length($$$l$$$) of the trap show that a 24-turn toroid outperforms a 12-turn toroid while slight coupling is observed between toroids with increased toroid diameter as seen from the reduced performance of the 12mm diameter toroid with $$$s=2$$$mm compared to $$$s=5$$$mm. From simulations, optimal parameters were determined to be $$$d=9$$$mm, $$$s=5$$$mm and $$$N=24$$$ and the structure built accordingly.

Fabrication
The design of a substructure is shown in Figure 3a. To allow overall flexibility, the structure is manufactured on a low-loss Polydimethylsiloxan(PDMS) elastic substrate. A mold is designed (Figure 3b) with a slit for the wire and slots for the four capacitors. The mold is 3D printed and used for injection molding. Toroids are hand wound with magnet wire and the capacitors soldered on resulting in an elastic structure (Figure 3c).

Benchtop
Tests were conducted with setup in Figure 4a. Toroids were fitted on a 6.80mm diameter cable. The performance of the structure with different numbers of toroids was measured and compared to simulations and theoretical ideal calculations, that assume low loss wires, vacuum medium, and no interactions between toroids.

Scans
B1 distortion experiments were performed on an GE MR750WGE (Waukesha, Wisconsin). Two cables were cut to the length of a resonant dipole for 127.7MHz. Three B1-mapping measurements[6], with a flip angle of 30^o, were performed in a non-loading phantom. The first mapped the B1 field produced by the body transmitter alone. In the second, the resonant dipole was placed on the phantom, while measuring the B1 field in a slice 1.65 cm below wire. In the 3rd, a resonant dipole equipped with our caterpillar traps was placed in the same way and measured. To evaluate the performance while flexing the wire, we repeated the measurements, this time with a flip angle of 15^o, by bending wires and placing them on the phantom as shown in Figure 5.

Results and Discussion

A comparison between ideal, simulations and benchtop measurements show that both simulations and benchtop measurements were inferior to the ideal providing 3.64dB and 4.96dB reduced attenuation for single toroid respectively (Figure 4b). The benchtop measurements are also slightly inferior to simulations due to decreased permeability of PDMS compared to air. Benchtop measurements show that a single toroid is able to obtain -5.11dB attenuation at a resonant frequency of 127.7 MHz attaining a Q-factor of 106, whereas a 7 toroid structure is able to obtain -22.82dB attenuation attaining a Q-factor exceeding 350 (Figure 4c). Figure 5 displays the results of the B1 distortion experiments. The B1 field maps are displayed, in degrees. Attenuation of the shield current by caterpillar traps are evident in both configurations.

Conclusions

“Now he isn’t a little caterpillar. He is a big fat caterpillar”[7]
Our distributed design splits the cable trap into smaller, self-shielded subcomponents. This study shows that our caterpillar traps can provide sufficient attenuation to shield currents while also maintaining flexibility, enabling the entire structure from coil to system to be light, flexible and convenient.

Acknowledgements

No acknowledgement found.