At field strengths of 7T and higher, the RF wavelength in biological tissue becomes similar to human head and body dimensions. Under this condition, the B₁ fields of loop and microstrip coils become asymmetric [1]. However, the B₁ fields of dipole antennas are still nearly symmetric even in body imaging at 7T [2]. The different behaviors of dipole and microstrip coils may be explained by the fact that they have similar magnetic (H-field) vectors but different Poynting vectors. We propose to manipulate the Poynting vector and thus the symmetry of the B₁ patterns of microstrip coils using capacitor-segmented ground (CSG) planes in order to achieve improved coil performances.

In a conventional microstrip coil [3, 4], the E-field inside the coil is perpendicular to the conductor and the ground plane, as shown in Fig. 1A. When the ground is segmented by capacitors (referred to as C_seg), a parallel E-field component is produced, as shown in Fig. 1B. This component rotates the Poynting vector toward the target space and makes the B₁ field pattern more symmetric, similar to a dipole antenna.

To validate this approach, we performed numerical simulations using HFSS (Ansys, Canonsburg, PA, USA). A conventional microstrip coil was modeled on a Teflon bar (25Χ5Χ2.5 cm³) (Fig. 1A, C). The widths of the conductor and ground were 1 cm and 5 cm, respectively. The ground plane of the CSG microstrip coil was uniformly segmented by seven identical capacitors (C_seg) (Fig. 1B, D). C_seg was set with different values (150 pF, 68 pF, 33 pF, 15 pF and 7.5 pF) to vary the length of the parallel E-field component. A cylindrical phantom with 30 cm length and 20 cm diameter was placed 2.5 cm below the coil. The phantom electrical properties were б=0.6 S/m and ξr=55, approximating average tissue properties at 7T. Input power was set at 1W in all simulations.

We also built a conventional and CSG microstrip coils with C_seg=7.5 pF. GRE images and B₁⁺ maps on a water phantom were acquired on a 7T human Philips Achieva scanner (Philips Healthcare, Cleveland, Ohio, USA). The parameters of the GRE sequence were: FA=30^o, TR/TE=100/10 ms, FOV=180 X 180 mm², matrix= 256X256, thickness=3 mm. B₁⁺ maps were measured with the DREAM method [5].

The Poynting vector distribution inside the water phantom is shown in Fig. 2A (sagittal plane). The conventional coil’s Poynting vector points into the phantom only in the region immediately adjacent to the coil. In comparison, particularly for C_seg =15 and 7.5 pF, the Poynting vector of the CSG coil was almost uniformly directed into the phantom. Figs. 2B and C show the current distributions along the ground plane and the S₁₁ plots vs. frequency. As C_seg decreases, its equivalent impedance increases and thus current in the ground decreases, so the current distribution becomes more like that of a dipole. Figs. 2D and E show the B₁⁺ and B₁^- fields, respectively. As expected, the B₁ field pattern became more symmetric (dipole-like) as C_seg decreases, thus demonstrating the ability to manipulate the symmetry of the B₁ patterns by tuning C_seg.

Fig. 3A shows the constructed microstrip coils. For the CSG coil, three capacitors were connected in parallel (2.4 pF+2.7pF+2.4pF=7.5pF) in each slit. Fig. 3B compares the measured S₁₁ curves and loaded Q-values. Figs. 3C and D show the GRE images and B₁ maps, respectively. Similar to the simulation results, the B₁ patterns of two coils are quite different: the conventional coil has a twisting B₁ field and an obvious lobe, while the CSG microstrip coil has a relatively symmetric B₁ field.

We have developed a capacitor-segmented ground (CSG) approach for microstrip coils to change their intrinsic B₁ field patterns. This concept has been validated by numerical studies and practical MRI experiments. The CSG method provides additional flexibility for manipulating the shape of the B₁ field, which may be advantageous for RF shimming and parallel transmission. Although microstrip arrays have relatively low coupling, strong cross-talk still exists in 16-channel arrays [6, 7]. If the coupling among microstrip elements can be reduced using different capacitor segmentation schemes between elements, it should provide an easy and efficient method to create densely-spaced microstrip arrays. Furthermore, the CSG microstrip coil can be seen as a transmission-line-folded dipole antenna. However, unlike the dipole, the CSG microstrip coil can be fed at the end as well as at the center and the selection of length is more flexible.