Dual-Tuned Floating Solenoid Balun for Multi-nuclear MRI and MRS

Abstract

Common-mode currents can degrade the RF coil performance and introduce potential safety hazards in MRI. Baluns are the standard method to suppress these undesired common-mode currents. Specifically, floating baluns are preferred in many applications because they are removable, allow post-installation adjustment and avoid direct soldering on the cable. However, floating baluns are typically bulky to achieve excellent common-mode suppression, taking up valuable space in the MRI bore. This is particularly severe for multi-nuclear MRI/MRS applications, as two RF systems exist. In this work, we present a novel dual-tuned floating balun that is fully removable, does not require any physical connection to the coaxial cable, and has a significantly reduced footprint. The floating design employs an inductive coupling between the cable’ solenoid and a floating solenoid resonator rather than a direct physical connection. Unlike the previous float solenoid balun, this balun employs a two-layer design further to improve the mutual coupling between the two solenoids. A pole-insertion method is used to suppress common-mode currents at two user-selectable frequencies simultaneously. Bench testing of the fabricated device at 7T demonstrated high common-mode rejection ratios at Larmor frequencies of both ¹H and ²³Na, even with a compact dimension (diameter 18 mm and length 12 mm). This balun’s removable, compact, and multi-resonant nature enables light-weighting, allows more coil elements, and improves cable management for advanced multi-nuclear MRI/MRS systems.

INTRODUCTION

In an MRI system, RF signals flowing through the transmission line can induce common-mode currents on the shield of the coaxial cable, caused by the unintended electric potential differences across the cable shield [1]. The common-mode current will result in safety issues for the patient as well as degraded signal-to-noise ratio (SNR) and/or transmit efficiency [2,3]. Thus, the balun circuit is necessary for suppressing the common-mode current.

Multi-nuclear MRI/MRS, including ²³Na, ³¹P, and ¹³C, offers valuable insights for clinical and biological analysis purposes [4–11]. Due to the low concentration of for low-gamma (X-) nuclei in MRI imaging, ¹H imaging for localizer and B₀ shimming is still necessary [5,7]. Thus, in multi-nuclear MRI, multiple sets of RF systems exist; one is for proton (¹H), and the others are X-nuclei. Therefore, baluns are needed to suppress common-mode current at two frequencies, corresponding to the Larmor frequencies of ¹H and X-nuclei [12–20].

Although two separate baluns (one for ¹H and the other for X-nuclei) in series can suppress common-mode currents at both frequencies, this approach doubles the number of required baluns and consumes valuable space in the MRI bore. Such a design is undesirable as RF coils trend towards being denser and more flexible.

A refined design to replace two separate baluns is to use a single balun that operates at two frequencies [17,21]. The double resonant frequencies could be realized by incorporating an additional LC tank circuit (also known as the pole insertion method). However, implementing such a pole insertion method presents challenges when transitioning to a floating design. Note that floating balun [18,21–23] is preferred and widely used in MRI RF coils for the following reasons. First, it does not require direct soldering on the cable and avoids potential damage to the cable. Second, it allows for removal and post-installation adjustment along the cable. Modifying a conventional floating balun to a dual-tuned floating balun is challenging because achieving excellent common-mode suppression often requires a larger footprint. The conventional floating Bazooka balun (FBB) [22], which is 25 mm in diameter and 80 mm in length, is used in 1.5 T systems (64 MHz), a frequency similar to the Larmor frequency of ²³Na at 7T (78.6 MHz). The common-mode suppression capability, often assessed by the common-mode current rejection ratio (CMRR), is further compromised in the presence of two frequencies, requiring a larger design to achieve comparable suppression performance.

In the previous design, the dual-tuned balun was based on the conventional FBB [22] and the pole-insertion design [21]. As shown in Figure 1A, adding a tank circuit (consisting of a capacitor C_H and a parallel inductor L_H) in series with capacitor C_L makes the whole circuit become a double resonant circuit. C_H is mainly responsible for tuning the high frequency (¹H), while C_L is mainly for tuning the low frequency (X-nuclei). In such a design, the balun is completely floating and can easily be placed in various positions on the cable. As mentioned above, such a floating design avoids the potential physical damage to the cable due to soldering and provides flexibility for placing it in different positions of the cable [22]. However, this mutual inductance between the cable shield and float resonator is relatively weak, whereas it plays the backbone role in determining the CMRR for the floating balun. While moving to the double-tuned application, CMRR is further worsened for each resonant frequency, which therefore requires an even greater dimension of the balun [21].

Figure 1.

Design and construction of the dual-tuned FSB. (A) Schematic diagram of the dual-tuned FBB [21,22]. (B) Equivalent circuit schematic for dual-tuned FSB. The inner solenoid is floating and terminated with C_L and pole-insertion circuit (C_H and L_H) to generate two resonant frequencies. The coaxial cable (grey) is winded around the inner solenoid to form the cable solenoid. (C) CAD model (sectional view) showing the mechanical structure for the dual-tuned balun. (D-E) Side view and front view of the fabricated balun for RG-174-like cable (Huber + Suhner G_02232_D). (F-G) Side view and front view of the fabricated balun for RG-223 cable.

In this work, we introduce a novel dual-tuned floating solenoid balun (FSB) designed to achieve an efficient CMRR in a compact size. This design builds on the recently proposed floating solenoid concept [24], which winds the cable into a solenoid to create a strong mutual coupling with a floating solenoid resonator. Although the FSB has been demonstrated for single-frequency applications, its suitability for dual-frequency applications remains unclear. The dual-tuned capability is achieved using the pole-insertion method [17,21]. Unlike the single-layer design described by Lu et al. [24], where the floating solenoid and cable solenoid are arranged in the same layer, this work arranges the floating solenoid inside the cable solenoid, leaving more space for each solenoid and increasing the self-inductance of each solenoid. We first investigate how the inductance of the add-on LC circuit affects the CMRR of the balun, then compare the single-layer and double-layer designs. Finally, we compare this design to the previously reported dual-tuned floating design [21,22] and demonstrate its application for different nuclei and static magnetic fields.

MATERIALS AND METHODS

Concept and Hardware fabrication

Recently, a novel FSB has been proposed, which uses an alternative winding method for cable and a floating solenoid resonator to increase their mutual inductance [24]. Previous work arranges the two solenoids in the same layer. However, this work employs a double-layer solenoidal design, as shown in Figure 1B. The comparison between the single-layer and double-layer design will be elaborated in the Results section. The inner floating solenoid acts as an inductor (L_f), connecting in parallel with a capacitor C_L and a pole insertion parallel circuit consisting of L_H and C_H, resulting in two resonant frequencies. The inductance of the cable solenoid was denoted as L_c.

Figure 1C shows the CAD side view design, where the support and casing were designed in SolidWorks and printed using resin materials (Tough 2000) on a Form3 printer (Formlabs, Somerville, MA, USA). The inner solenoid was constructed using AWG-16 enameled copper wire (1.36 mm diameter). One end of this copper wire passes through the inner hole of the model, enabling a connection with the other end via two capacitors (C_L ​and C_H​) and one inductor (L_H​). To find the optimal value of L_H​, we tried inductors varied from 22 nH to 90 nH (Coilcraft, AIAC-1812 series), with C_H​​ and C_L adjusted to tune the two frequencies to the Larmor frequencies of ²³Na and ¹H at 7T. Trimmers were used for C_H​​ and C_L during the optimization of L_H​. Once the optimal L_H was determined, C_H​​ and C_L were chosen from 5% tolerance non-magnetic capacitor pools (1111C series, PPI, Huntington, NY).

Figures 1D and 1E depict the actual fabrication for the RG-174-like coaxial cable (Huber + Suhner G_02232_D), and Figures 1F and 1G depict the fabrication for the RG-223 cable. Both designs wound the coaxial cable concentrically on the inner solenoid. For the RG-174-like cable, the dual-tuned balun has a total length of 24 mm and an overall outer diameter of 16 mm, with the inner solenoid (diameter 8 mm) of six turns and the outer cable of five turns. For the RG-223 cable, the balun has a total length of 30 mm and an overall outer diameter of 26 mm, with the inner solenoid (diameter 10 mm) of six turns and the outer cable of four turns. An outer casing covered with copper foil (35 μm thick, 3M) was used to act as a shield to prevent crosstalk with external electromagnetic fields for both baluns.

For comparison, we also fabricated the same-sized dual-tuned FBB for 7T ²³Na and ¹H. Similar to the proposed design, FBB is designed and manufactured for both RG-174-like cable and RG-223 cable, respectively.

Bench test

CMRR measurements were performed utilizing a calibrated four-port Vector Network Analyzer (VNA) (model E5071C, Keysight, Santa Rosa, CA). The CMRR of the fabricated 7T ¹H/²³Na balun was directly assessed by connecting each end of the cable’s shield to separate ports on the VNA, as described in previous works [21,25]. The CMRR was determined by measuring the magnitude of the S₂₁ parameter between the two VNA ports, with the VNA calibrated using response calibration such that S₂₁ = 0 dB corresponded to the condition where no balun was present. Given that this balun does not affect insertion loss, it was excluded from the testing in this study.

We first compared how CMRRs at low and high frequencies of a 7T ²³Na/¹H balun change with respect to L​_H. Then, we compared the CMRR of the double-layer design with that of a single-layer design, both using the optimal L​_H of 22 nH​. Note that for the single-layer design, the balun accommodates fewer turns of cable solenoid and floating solenoid. For example, the double-layer design can accommodate six turns of the float solenoid (at the cost of a slightly smaller diameter) and five turns of the cable solenoid. In contrast, the single-layer design can only accommodate three turns for each. Finally, we compared the proposed dual-tuned FSB to the dual-tuned FBB of the same dimensions.

RESULTS

Impact of L_H on CMRR

Figure 2A–B illustrates the measured S₂₁ (CMRR) for FSB with varying L_H values to highlight how L_H affects the CMRR at two distinct frequencies. Figure 2A displays measured S₂₁ at 78.6 MHz for different L_H values. The S₂₁ is −17.24 dB when L_H is 22 nH. However, with L_H of 90 nH, the S₂₁ increases to −12.16 dB, indicating a worse CMRR. Figure 2B shows the effect of various L_Hs at 298 MHz. In contrast, the CMRR becomes better as L_H increases. For instance, the CMRR is −27.69 dB when L_H is 22 nH, while it is −33.34 dB when using a 90 nH inductor. These results demonstrate that the L_H value significantly impacts the CMRR at both low and high resonant frequencies. A lower L_H value results in better CMRR at the low resonant frequency and worse CMRR at the high resonant frequency. Vice versa, a higher L_H value results in worse CMRR at the low resonant frequency and better CMRR at the high resonant frequency. Such a finding is also consistent with the previous work [21].

Figure 2.

Bench test results demonstrating L_H’s effect on CMRRs at two frequencies for 7T ¹H/²³Na. (A) Measured CMRRs (S₂₁) for balun in the frequency band 70 to 90 MHz (CMRR values at 78.6 MHz listed). (B) Measured CMRRs for balun in the frequency band 250 to 350 MHz (CMRR values at 78.6 MHz listed).

Therefore, the lower L_H value of 22 nH was chosen as the optimum design for this solenoid balun. Achieving a better CMRR at higher frequencies is typically easier, but tends to degrade at lower frequencies, as shown in Figure 2B. Thus, selecting the L_H value requires balancing this trade-off: we aim to enhance CMRR at lower frequencies, as higher frequencies already have adequate CMRR and can tolerate some reduction. Another reason we chose a smaller L_H is because it avoids an extremely small C_H in practice.

Single-layer vs. double-layer

Figures 3A and 3C show the CAD models of single-layer [24] and double-layer design for 7T ¹H/²³Na MRI, respectively, of the same size. A sectional view of the double-layer design is included to visualize the internal structure better.

Figure 3.

CAD drawings and bench test results showing the difference between single-layer and double-layer Designs. (A) CAD model for the single-layer design in which cable solenoid and floating solenoid are winded in the same layer, with the wire or cable alternatively distributed. (B) Bench test result of the single-layer design at two frequencies. (C) CAD model for double-layer design in which two solenoids are arranged in two separate layers. (D) Bench test result of the double-layer design at two frequencies. Both single- and double-layer designs employ the same pole-insertion circuit to generate two resonant frequencies.

Figure 3D shows the results for the double-layer design, where the RF signal is reduced to −21.1 dB at 78.6 MHz and −26.8 dB at 298 MHz. In contrast, Figure 3B shows the single-layer design results, with the RF signal reduced to −10.6 dB at 78.6 MHz and −21.6 dB at 298 MHz. This indicates that the double-layer design has better CMRR than the single-layer design, especially at the lower resonance frequency. We believe this is because of the relatively lower mutual coupling in the single-layer design, where fewer turns of the cable solenoid and fewer turns of the float solenoid were used due to limited space. Therefore, the double-layer design is preferred for the dual-tuned FSB.

Comparison with conventional dual-tuned FBB

Figure 4 compares the proposed design with the conventional dual-tuned FBB [21,22] of identical dimensions. When used for RG-174-like cables, the RF signal is reduced to −21.1 dB at 78.6 MHz and −26.8 dB at 298 MHz (Figure 4A), meaning that more than 99% of the common-mode signal is suppressed at both frequencies. In contrast, the conventional design in Figure 4B, with the same dimensions, shows reductions of only −1.2 dB at 78.6 MHz and −8.3 dB at 298 MHz, which means only 12.9% of the common-mode signal is suppressed at the low frequency and 61.5% at the high frequency. A similar finding was observed for the RG-223 cable. The proposed design reduces the RF signal to −24.2 dB at 78.6 MHz and −22 dB at 298 MHz, while the conventional design shows reductions of −4.4 dB at 78.6 MHz and −13.9 dB at 298 MHz. These results indicate that the proposed dual-tuned FSB provides superior CMRR and requires less space than the conventional design. This benefit applies not only to RG-174-like cables but also to RG-223 cables.

Figure 4.

CMRR comparisons between the dual-tuned FBB and the proposed design for two types of coaxial cables. (A and C) Measured CMRR plots versus frequency of the dual-tuned floating Bazooka baluns (FBB) for RG-174-like cable and RG-223 cable. (B and D) Measured CMRR plots versus frequency of the dual-tuned floating solenoid baluns (FSB) for RG-174-like cable and RG-223 cable.

DISCUSSIONS

This study introduced and validated a novel dual-tuned FSB with a compact size that enables excellent common-mode current suppression abilities at two resonance frequencies for multi-nuclei MRI and MRS. A double-layer design was employed, with the floating solenoid placed inside the cable solenoid. The dual-tuned FSB was also compared with the conventional FBB, showing CMRR values of −21.1/−26.8 dB versus −1.2/−8.3 dB at 78.6/298 MHz for the RG-223 cable. Note that dB is a logarithmic unit used to measure CMRR. The difference between −10 dB and −20 dB is relatively significant, as −10 dB corresponds to 10% residual common-mode signal, while −20 dB corresponds to only 1% residual, indicating substantial improvement. However, the difference between −20 dB and −30 dB is less critical, as −20 dB is already quite effective.

RG-174-like cables are widely used in high-density coil arrays and flexible coil designs, particularly in receive-only coils and transmit/receive (TR) array coils. It is important to note that the original RG-174 is magnetic, so it is typically replaced by similar products designed explicitly for non-magnetic applications. In addition to RG-174-like cables, RG-223 cable is also commonly used in transmit-only (Tx) and transmit/receive coils, especially in volume Tx coils that require high transmission power. We observed that the solenoid balun provided better CMRR at both resonant frequencies than the Bazooka design for both RG-174-like and RG-223 cables. However, we also noticed that the benefits of the FSB diminish compared to the conventional FBB design as the cable diameter increases. This is because a thicker cable usually results in fewer turns of cable solenoid and floating solenoid, given the same longitudinal space.

Through investigations, we find that L_f​ and L_c​ play distinct roles in the FSB. Together with the terminated components (capacitors and inductors), L_f​ determines the resonant frequencies of the balun. According to the theory described by Lu et al. [24], the frequency is primarily influenced by L_f​​ and is not affected by L_c​. In practice, we observed that L_c can slightly alter the resonant frequency, likely due to changes in parasitic capacitance between L_f​ and L_c​. We recommend selecting a relatively large L_f​​. Note that large L_f​​ is achievable even in very limited space, as one can always use thinner wire for the float solenoid. Nevertheless, L_f​ should not be excessively large to ensure that the terminated capacitors are not impractically small, especially true for the high frequency of the dual-tuned balun. Once the L_f​ is determined, L_c strongly affects the achievable best CMRR (especially at lower frequency), although it has little effect on the resonant frequency. This is because L_c​ strongly affects the mutual inductance between the two solenoids, which directly affects the CMRR. This is also the main reason we chose a double-layer design, as it accommodates more turns for the cable solenoid. We suggest tightly winding the cable solenoid to maximize L_f​ in the given space. If better CMRR is needed, increasing L_c is always an option.

Importantly, this design exhibits excellent repeatability during fabrication. The CAD model incorporates a dedicated slot to ensure precise turns for the inner solenoid. For the outer-wound cable, the specified number of turns can be tightly compacted, as defined by the CAD model. Consequently, each winding process remains consistent and reproducible.

In this study, we took ¹H/²³Na for 7T MRS as an example. However, the dual-tuned design can be extended to different combinations of nuclei (²³Na, ³¹P, and ¹³C) at various static fields (3T, 7T, and 9.4T). By only changing C_L and C_H (balun size and L_H unchanged), the unwanted common-mode RF signal can be suppressed for ¹H/X-nuclei at least to −17/−10 dB across all static fields, as shown in Figure 5. For the same balun size, a lower static field has worse CMRR owing to its corresponding lower Larmor frequency. Therefore, the dual-tuned FSB for the lower static field should be manufactured with greater dimensions (greater diameter and/or longer length) to increase the inductive coupling effect and reach excellent CMRRs. It is worth noting that this design still has some limitations. The cable requires a 90-degree bend, and the balun necessitates a more complicated disassembly and reassembly process.

Figure 5.

Measured CMRR plots versus frequency of dual-tuned FSB for various nuclei (²³Na, ³¹P, and ¹³C) under various static magnetic fields (3 T, 7 T, and 9.4 T).

CONCLUSION

This study introduced a novel dual-tuned FSB for multi-nuclei MRI and MRS. The compact, double-layer design, with the float solenoid placed inside the cable solenoid, enables superior common-mode current suppression ability at two distinct resonance frequencies. This innovative approach not only reduces the space required within the MRI bore but also enhances the flexibility and efficiency of coil design, particularly for high-density and flexible arrays. The balun’s ability to achieve high CMRR across multiple frequencies demonstrates its potential to improve imaging quality and patient safety in complex multi-nuclei MRI systems.