Floating Solenoid Cable Trap

Abstract

In the current state of magnetic resonance imaging (MRI), transmission lines are vital in communicating with the transmitting and receiving networks. However, transmission lines, such as coaxial cables, operating at the radio frequencies (RF) used in MRI can be affected by common mode cable currents. These currents diminish received signals and put the patient at risk of RF burns. A common way to combat this issue is to implement cable traps on the transmission line(s). To effectively suppress cable currents and improve the ease of applying floating cable traps, a variation of a floating cable trap was created. The trap utilizes three LC circuits to provide an attenuation of −24 decibels (dB) at 127 MHz. The trap uses a custom 3D printed former that allows for straightforward construction and application onto a single RG-58 coaxial cable.

I. Introduction

Currently, RG-58 coaxial cables are typically used to send and receive data to the transmitting (T_X) and receiving (R_X) networks in magnetic resonance imaging (MRI). Since transmission lines used in MRI research applications operate at frequencies in the range of 30MHz - 300MHz, they are susceptible to common mode cable currents [1]. The most common type of transmission line used in MRI is the coaxial cable which is susceptible to common mode cable currents that impact the integrity of the received signal and can cause radio frequency (RF) burns to patients [2]. To address this issue, cable traps are used to attenuate cable currents. There are different types of cable traps, some are designed to be connected in series with the coaxial cable and others to couple to them. For example, the floating cable trap uses mutual coupling between the inner conductor of the cable trap and the shield of the coaxial cable to effectively attenuate cable currents at a desired frequency [3]. Another recently introduced (more continuous) variation of the floating cable trap is the caterpillar cable trap that utilizes a series of spaced toroids that couple to the transmission line to attenuate cable currents [4].

Both the caterpillar cable trap and discrete floating cable trap configurations are effective tools to block cable currents. However, the caterpillar cable trap requires a complex manufacturing procedure; while the discrete floating cable trap does not easily maintain its position on the cable and requires re-tuning after disassembly.

In this work, a variation of the floating cable trap was developed to accommodate a combination of high attenuation and usability. The proposed trap focused on the design of a former that ensures simplicity in construction, tuning, and assembling, while maintaining effective cable current attenuation for ¹H (127 MHz) in a 3 Tesla (T) environment. This design is characterized by an array of three LC circuits (as shown in Figure 1), a former that allows the user to easily tune and assemble the array, and a faraday cage.

Fig. 1.

Components of the: (top left) front board, (top right) baseboard, (bottom left) trap LC circuits

II. Methods

A. Trap Design

Each trap LC circuit is made from 10 AWG magnet copper wire (TEMCo) formed into a two turn, helical folded dipole. A fixed capacitor was soldered to the ends of the wire to create a resonant circuit that can be tuned by compressing the turns. The turns are initially separated by 2mm to allow the elements to be further compressed for tuning. Current injection probes were used to determine the operating frequency of the elements. The standard resonance equation (1) was used to determine the coarse impedance required to achieve the desired operating frequency. Each circuit was tuned to a higher frequency than the intended operating frequency to allow the frequency of the array to shift to a lower frequency with compression of the windings.

$$2*\pi *f=\frac{1}{\sqrt{L_{\mathit{\text{coil}}}*C}}$$ (1)

Where f is the desired operating frequency, L_coil is the inductance of the element, and C is the capacitance of the fixed capacitor.

B. Former and Shield Design

A former was designed, and 3D printed to form and assemble the LC circuits in a stable position (Fig. 2). All components were printed with Nylon 11 powder using a FormLabs Fuse 1 Nylon 3D printer.

Fig. 2.

(left) Trap former with circuit (right) trap and faraday cage attached to an RG-58 coaxial cable.

Each LC circuit was oriented around the coaxial cable with 120 degrees of separation, creating a circular array. The LC circuits were carefully arranged to achieve proper tuning and strong attenuation at one frequency.

To prevent coupling with the T_X network, the trap was shielded with copper. Six equally spaced 0.8cm holes were placed along the shields’ center to enable access to the tuning mechanism of the trap. To secure the shield to the coax cable, a pair of half bowtie keys are used, as shown in Fig. 3.

Fig. 3.

Trap’s shield: (left) front view (right) front and top view.

The trap former shown in Fig. 2 allows the user to: (1) easily form each element of the array, (2) provide a fine-tuning mechanism for each circuit, and (3) assemble each component onto the coaxial cable with ease. The trap’s former consists of a baseboard and a front board Fig. 1. The baseboard is a 5cm × 1cm × 0.5cm rectangle with four pegs on the outer edges that are used to form the circuit. Two out of the three baseboards include a hexagonal hole in the center to host a M6 nylon nut for tuning. The other baseboard was designed to have a 0.95cm diameter hole instead of a hexagonal hole. This design simplified tuning by preventing the trap from having to be rotated to tune each circuit. and 4ing from the bottom of the baseboard is a curved rail guide that utilizes a half bowtie key and keyhole design to allow the user to easily assemble each element onto the cable, as shown in Figs. 2 and 4.

Fig. 4.

3D model of assembled trap components formers with arrows to describe the movement of the components: (1) front board that moves towards or away from the baseboard (2) baseboard, the black cylinder simulates an RG-58 coaxial cable.

C. Trap Tuning

To fine tune the LC circuits, a 5cm × 1.35cm × 0.25cm front board was used. The front board consists of four peg shaped holes that match the peg position on the baseboard, and a 0.95cm hole in the center for a M6×15 nylon screw to be inserted for tuning. By tightening or loosening the nylon screw, the circuit can be tuned by the compression of the baseboard and front board, Fig. 4.

D. Trap Attenuation Testing

Before tuning the trap, one M6 nylon nut was super glued into the hexagonal hole of two of the three baseboards. After the glue cured, the 10 AWG magnet copper wire (TEMco) with a length of 26.5cm was wrapped around the four pegs of the baseboard to create the desired shape. The solenoid was then removed from the baseboard to prevent any damage to the former while soldering. The solenoid was formed so that the two ends of the wire were positioned away from the coaxial cable. The insulation at both ends of the wire was scraped off to allow for easy soldering. A 3.9 pF fixed capacitor was soldered directly onto one of the exposed wire leads. A piece of copper tape was soldered to the exposed end of the opposite lead and then to the capacitor. The copper tape provides flexibility to the circuit while being tuned. The circuit was then fitted back onto the pegs of the baseboard. The front board was then placed on top of the solenoid and a M6×15 nylon screw was fed through the center of the front board, screwing into the nylon nut in the baseboard. This step was repeated using the second baseboard. For the third baseboard, the same technique was used to create the circuit and the front board was placed on top of the element. However, a M6×15 nylon screw was inserted into the baseboard and tightened to a M6 nylon nut placed in front of the front board.

The trap was evaluated on the bench with an Agilent Technologies E5071C ENA Series Network Analyzer, and a current injection probe, consisting of two 27mm diameter ferrite hinged cores (Wurth Electronic: 7427155), a resin panel and a copper ground plane. The ferrite cores were separated by 28.5cm from their centers. The network analyzer was calibrated for a two port S21 measurement with a center frequency of 127 MHz and a span of 100 MHz. All data was recorded in decibels (dB). A 1.04-meter-long RG-58 cable was inserted into the center of the ferrite cores and positioned on top of the resin panel. The cable current on the coaxial cable was then measured and recorded. The components of the trap were then assembled onto the coaxial cable and tuned, Fig. 5. The faraday cage was applied, and attenuation of the shielded trap was recorded. The shielded trap was removed from the coaxial cable and then reassembled on the same cable without tuning. After each disassembly and reassembly, the attenuation was recorded. This process was repeated 10 times to ensure tuning remained consistent, Table 1.

Fig. 5.

Complete assembly of the trap applied to a coaxial cable: the trap without the shield (left) and the trap with the shield (right)

TABLE I.

Trap Attenuation After Assembly and Disassembly

Trails	Attenuation of Floating Solenoid Cable Trap at 127 MHz (dB)
1	−26.25
2	−22.84
3	−24.25
4	−24.89
5	−25.24
6	−24.11
7	−25.37
8	−25.50
9	−25.15
10	−26.10

III. Results

The shielded trap achieved an attenuation of −24.63 dB +/− 0.962 dB at 127 MHz, Fig. 6. The combination of the rail mechanism of the former and the half bowtie keys on the faraday cage allowed the trap to maintain its position on the cable, Fig. 2. The trap retained an effective attenuation value after being disassembled and reassembled multiple times without returning, shown in Table 1.

Fig. 6.

Trap attenuation of cable currents with the faraday cage: Shielded Trap represents the data acquired with the faraday cage, and the control represents the data acquired without the trap.

The trap attenuation was compared to the attenuation of a 5.5cm in length and 3.4cm diameter floating balun by using the same test parameters and methodology, Table 2. The floating balun was printed using PLA filament and a FormsLab 3D printer [5]. The floating balun attenuated 95.6% +/− 0.00315% of cable currents while the trap attenuated 94.32% +/− 0.657%.

TABLE II.

Attenuation Comparison of Trap to a Floating Balun

Attenuation of Floating Solenoid Cable Trap (dB)	Attenuation of Floating Balun (dB)
−24.97 +/− 0.962	−27.1 +/− 0.632

IV. Discussion

The shielded trap effectively attenuated cable currents at 127 MHz by providing an attenuation of −24.97 dB −/+ 0.962 dB. As expected, the rail mechanism made the device easy to assemble and disassemble without significantly changing the attenuation of the trap, Table 1. The half bowtie key and rail mechanism allowed the shielded trap to maintain its position on the cable without sliding. The tuning mechanism was successful as it provided an easy and reliable method to tune the device.

When comparing the proposed trap to a discrete floating balun of the same length, the trap attenuation was similar. Like many floating cable traps, the device can be used in series with other floating solenoid cable traps to effectively attenuate cable currents at one or multiple frequencies. The weight of the trap could make cable management more complicated, especially if multiple traps are in series. However, the proposed cable trap is a modular device that has the potential to become a multi-tuned device. This potential can decrease the quantity of traps needed for a cable by placing a multi-tuned floating solenoid cable trap at positions where cable currents are most present at two or more frequencies. The weight of the trap can also be mitigated by using a different material to print the former or an alternative design that uses less materials without compromising the former’s integrity. During tuning the front boards can be deformed and potentially break if compressed too much. The faraday cage increases the diameter of the trap. The idea of having a copper mesh around the trap was tested but resulted in a greatly diminished attenuation. However, the idea of a copper mesh faraday cage will be revisited to determine if the size of the faraday cage can be minimized.

V. Conclusion

The proposed cable trap is a variant of the floating cable trap that can effectively attenuate cable currents for ¹H in a 3 Tesla MRI setting, be assembled and disassembled without re-tuning the device, and retain its position on a cable without slipping. Future research includes testing multi-tuning capabilities of the trap, minimizing the size of the faraday cage, testing lighter designs for the former, and testing the device in a 3T MRI environment.