Design of dual-band Coaxial-transmission-line coils with independent tuning capabilities

Introduction

RF coils using coaxial-transmission-line (CTL)^1–3 have lessened the burden on anatomy posture constraints. It is made of a coaxial cable with the port connected the inner conductor and loaded with a gap at the outer shield located opposite to the port. Although they are not truly flexible since lumped component are also integrated within the design, such technology is low cost and can be adapt to anatomy variation. The study of CTL has revealed multimode operating frequencies associated with the design parameters. While the fundamental frequency is often used to implement receivers^1,3 RF array coils, the second operating mode has a high impedance characteristic and can be used to design compact transceivers² array. Besides, a dual-band RF coil can be implemented with the SCC to study non-proton nuclei (X-nudei) for magnetic resonance spectroscopy and imaging. By adding gaps in the inner layer or the outer layer, one can tune the resonant frequencies of the CTL⁴. However, the resonant frequencies from the multimode CTL cannot be tuned independently to the desired frequencies. Recently, independent tuning of both bands can be achieved with two asymmetric gaps at the outer shield conductor of the CTL⁵. The location of the two asymmetric gaps in the outer shield can be carefully selected for the CTL coil through repetitive optimization using full wave electromagnetic solver to operate at the targeted dual band frequencies. While this technique enables independent tuning of both resonant frequencies, in practice the position of the asymmetric gaps in the outer shield can not be easily changed to compensate for load variation, or cable bending to retune the resonant frequencies. In this work, we propose an alternative CTL design where the first and second mode resonance frequency can be independently tuned by simply using lumped components, which is not feasible using the conventional CTL RF coils.

Methods

The evolution process of the proposed dual-band CTL with independent tuning capabilities is depicted in Fig. 1. The process starts with CTL² introduced previously, where C_m is the matching capacitor and C_t1 is the tuning capacitor [see Fig. 1 (a)]. In such design the tuning capacitor C_t1 control simultaneously the first (f₁) and second (f₂) resonant frequencies. In Fig. 2(b), another tuning capacitor C_t2 is added to the gap at the outer shield. Analysis has demonstrated that both resonant frequencies (f₁ and f₂)) are also controlled by C_t2. By isolating the two arm of the outer shielding and shorting the inner conductor to the outer conductor, we obtained the design in Fig. 1 (c) namely “shorted end coaxial transmission line” (SECTL). Such design provides independent tuning capability such that the resonant frequency f₁ is only dependent of C_t1 and f₂ is only dependent of C_t2. The proposed SECTL design is used to implement a dual-tuned (¹H/¹³C) designed at 10.5T where the Larmor frequencies of the proton ¹H and the X-nudeus ¹³C operate at 447 MHz and 112 MHz, respectively. To validate the capability of the proposed dual band SECTL with independent control of each resonant frequency, we evaluated the scattering matrix of the design fora fixed value of C_t1 while varying C_t2 and vice versa. Furthermore, the transmit B1 field strength of the dual-band SECTL is also obtained at both frequencies.

Fig. 1.

Evolution process of the proposed dual-band shorted end coaxial transmission line with independent tuning capabilities. (a) CTL coil introduced in²; (b) CTL coil with tuning capacitance (C_t2) at the gap of the outer shield; (c) CTL with shorted end.

Fig. 2.

Simulated frequency response of the proposed shorted end coaxial transmission line loaded with a cylindrical phantom. Results shown a dual tuned (¹H/¹³C) response at 10.5T where the Larmor frequencies of the proton ¹H and the X-nucleus ¹³C operate at 447 MHz and 112 MHz. Note different matching is optimized at each band with different matching capacitors.

Results

The proposed SECTL is placed 0.5 cm on top of a cylindrical phantom (with conductivity = 0.6 S/m and permittivity 50) to imitate human brain tissue (see Fig. 2). The parameters of the SECTL are the same as the “RG-405 .086” commercial coaxial cable. The diameter of the SECTL is set to 6 cm. A full wave simulation using High-Frequency Structure Simulator shows that the SECTL is designed for the dual-tuned (¹H/¹³C) at 10.5T (C_t1 = 8.5 pF; C_t2 = 1.3 pF; C_m = 3.5 pF for ¹³C signal, and C_m = 150 pFfor ¹H signal). Note that a separate matching capacitor (C_m) is required for each band, meaning that a full duplex switch is required at the feeding point for practical use. A parametric study is performed on the SECTL. By simply adjusting the value of C_t1 while other parameters are fixed, simulations results shown that f₁ can be tuned in a wide frequency range while f₂ remains fixed [see Fig. 3 (a)]. On the other hand, C_t2 only control the tuning of f₂ [see Fig. 3 (b)]. The simulated transmit field strength of the proposed SECTL designed for the for the dual-tuned (¹H/¹³C) at 10.5T is illustrated in Fig. 4. The simulated (at both resonant frequencies) in all 2D planes cutting through the center of the phantom show similar field map compared to the one obtained from the conventional CTL^1,2,3.

Fig. 3.

Simulated scattering parameters of the proposed shorted end coaxial transmission line showing the effect of parametric study. (a) The first resonant frequency f₁ is controlled by C_t1; (b) The second resonant frequency f₂ is controlled by C_t2.

Fig. 4.

Simulated RF transmit field strength map of the proposed shorted end coaxial transmission line designed for dual-tuned (¹H/¹³C) at 10.5T.

Conclusion

In this abstract, we have introduced a dual band coaxial transmission line RF coil with independent tuning capability. Analysis supported by parametric studies based on hull wave simulation has demonstrated how both resonant frequencies can be controlled individually by simply varying the value of the lumped components in the design. Broadband tuning range is obtained which can be cover most the proton and X-nudeus signal.

Synopsis.

Recently, intensive research has been conducted on coaxial-transmission-line (CTL) RF coils for nuclear magnetic resonance (NMR) at frequency ranging from high field to ultra-high field. The flexibility of CTL is to some extent required to image dynamic anatomy of the human body. Analysis on the CTL coils based on transmission line theory has revealed multimode operating frequency. Consequently, CTL coils can be useful to implement dual-nuclear RF coil resonator designed for Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS). In this abstract, we design a dual-band CTL with independent tuning capabilities.