Introduction

Common mode currents are a common source of problems while working with MR signals. An elegant solution to reduce such currents is the floating trap¹. It has the advantages of working on multiple coaxial cables simultaneously and not having a galvanic connection with them. Up to now, the design was limited to a single blocking frequency. In coils for X-nuclear experiments, there is usually a part of the coil resonant at the ¹H Larmor frequency in addition to the resonance at the frequency of the X-nucleus. Therefore, common mode currents need to be suppressed at both frequencies. The easiest solution would be to use two floating traps in series, however sometimes spatial restrictions render this impossible. Here, the design for a double-resonant floating cable trap that consists of two nested floating traps suppressing common mode currents at two frequencies simultaneously is presented.

Methods

The double tuned floating trap consists of two classical floating traps nested inside each other (Fig. 1). Two hollow cylinders made of dielectric material split in half along their axis are covered by conductive copper layers on the inside and outside. The inner trap shares its outer copper layer with the inner copper layer of the outer trap. On one end, all three concentric copper layers are short-circuited, while tuning capacitors are connecting the outer to the middle and the middle to the inner layer on the other side. The capacitors across the outer (inner) shell coarsely control the first (second) resonance frequency of the trap, respectively. By varying the distance between the two halves of the cylinders, the frequencies can be finely adjusted, however not independently. Therefore, to finely adjust the resonance frequency of the inner trap, threaded holes in the inner dielectric cylinder are added to accommodate copper screws. The position of these screws changes the resonance frequency of the inner trap. The design was exemplarily implemented as a ³¹P/¹H trap for 7 T (Fig. 2), with the intention to be used in a flexible cardiac coil array with limited space². The trap body was 3D-printed (Rebel 2, Petr Zahradník Computer Laboratory, Ústí nad Labem, Czech Republic) from ABS plastic material. The dimensions of the cylinders were Ø = 8 mm/14.5 mm for the inner parts, and Ø = 15 mm/22 mm for the outer parts, all with a length of 55 mm. Non-magnetic capacitors (CHB series, Exxelia, Pessac, France) with values of 5.6 pF for the inner and 100 pF for the outer part were used to tune the trap to 120 and 297 MHz. The tuning range achievable by variation of the gap size, as well as by variation of the position of the copper screws in the inner shell was investigated. The blocking efficiency and bandwidth for both frequencies was measured. Measurements were performed on a network analyzer (E5061B, Keysight Technologies, Santa Rosa, CA, USA) using the method described by Seeber et al².

Results and Discussion

The tuning range for both frequencies (simultaneously) by changing the gap size between the half-cylinders was ±10%, the tuning range for the inner trap by adjusting the copper screws was ±2%. The blocking efficiency and bandwidth was ≈ -34 dB with a bandwidth of 6 MHz at 298 MHz and ≈ -10.5 dB with a bandwidth of 3 MHz at 120 MHz (see Fig. 3). However, the blocking efficiency depends on the chosen geometry of the trap body; the larger (outer) trap results in a higher attenuation, which should be taken into account when choosing the trap geometry. Since the traps for the lower frequency could in principle be placed further apart due to the longer wavelength, a smaller attenuation per trap could be acceptable when placed as densely as the traps for the higher frequency. Therefore, the outer shell should be chosen for the higher frequency. Since high voltages can occur on cable traps, the voltage withstanding of the used capacitors must be taken into account. A solution to alleviate the constraints on a single capacitor would be to place capacitors on both sides of the trap, thus, putting them in series and splitting the voltage.

Conclusion

A design for a double-resonant floating cable trap with sufficient blocking is presented. This design is especially useful for X-nuclear coils with spatial or weight constraints, and is therefore suitable for coils that are directly placed on the patient’s body.

Acknowledgements

The work was supported by the Austrian Science Fund project “pULSE” (P28059-N36) and SGS TUL (CZ).