Researchers interested in RF coil array design.Radiative dipole and monopole coil arrays are uniquely simple to design and can penetrate deep into the body, and are increasingly used for ultrahigh field MRI ^1-3. One technical challenge in building densely-spaced radiative arrays with many coil elements is to suppress the strong electromagnetic (EM) coupling between elements. In telecommunication applications, various decoupling methods for monopole and dipole antennas have been described. These approaches fall into three categories: (I) Eigenmode feed networks ⁴; (II) External resonators ⁵; (III) Slits in ground the ground plane, i.e., defected ground structure (DGS) ⁶. Compared with other decoupling methods, the DGS method causes the least disturbance to the monopoles’ radiation patterns. However, direct implementation of this method in ultra-high field MRI is limited by the relatively long MRI wavelengths, which would require impractically deep slits. To address this limitation and enable this decoupling method to be used for MRI, we propose a Tunable-Defected-Ground-Structure (TDGS) using lumped elements, and demonstrate its capability to decouple monopole arrays at 7T.

Fig. 1A shows the diagram of traditional DGS using a 1/4 λ slit. With the combination of distributed capacitance and inductance, the DGS can be thought of as a bandstop filter and this effectively reduces current flow between the monopoles. Fig. 1B shows the proposed TDGS method using a small slot and a capacitor (Cd) in series. This configuration is more suitable for MRI since it provides tuning capability without compromising the ground plane.

To test the feasibility of the proposed method, we comparatively investigated the coil performance of 2-ch monopole arrays without and with TDGS. The width and length of each monopole element were 1 cm and 25 cm, respectively. The distance of the two monopole elements was about 8 cm. Simulations were performed using HFSS and Designer (ANSYS, Canonsburg, PA, USA). Based on the simulation results, we also built a 2-ch TDGS-decoupled monopole array. This array was used for both transmission and reception, and was matched to 50 Ω and tuned to 298 MHz. Imaging was performed on a human 7T Philips Achieva scanner (Philips Healthcare, Cleveland, Ohio, USA).

Figs. 2A and 2B show the simulated S₁₁ and S₂₁ plots of two coupled monopoles and two TDGS-decoupled monopoles, respectively. As expected, S₂₁ is only -7 dB when no decoupling treatment was used; but it was reduced to -28 dB by using TDGS. Figs. 2C and 2D show the current distribution over the surface of monopoles and ground, when the left element was driven with 1 W power, and the right monopole was terminated to 50 Ω. Similar to the S-parameter results, current flows on the right monopole become negligible when TDGS was utilized. Instead, a large portion of surface current is trapped by the capacitor-terminated slot, which is consistent with the bandstop filtering characteristic of L/C tank circuits.Fig. 3A shows the constructed 2-ch monopole array with TDGS. Fig. 3B shows measured S-parameter plots. Similar to the simulation results, the S₂₁ was only about -25 dB and while S₁₁ was -28 dB. Fig. 3C shows gradient echo images of each channel on a cylindrical water phantom (16 cm in diameter, 20 cm in height). The MR images were acquired with the following parameters: FA = 25⁰, TR/TE= 100/10 ms, FOV=180 Χ 180 mm², Voxel size= 0.94 Χ 0.94 Χ 2 mm³. During the MRI experiments, the phantom was placed very close to the monopole elements (distance <1 cm). The MR images show that the original individual monopole image intensity profiles are preserved using TDGS, reflecting that the TDGS method does not significantly disturb the single-element EM fields. This is different from the results with previous ICE-decoupled monopole array ⁵.We proposed a Tunable-Defected-Ground-Structure (TDGS) decoupling method for monopole arrays. The proposed concept was successfully validated by EM simulation, bench test and MR experiments. By using the TDGS method, the cross-talk between two closely-spaced monopoles was reduced from -7 dB to -25 dB. Based on the acquired MR images, it was found that the TDGS method had little effect on the original B₁ fields of the individual monopole elements.