Although there are many interesting research results in ultra-high field magnetic resonance imaging (UHF-MRI) [1]. Most of them are in brain imaging studies. When it comes to body imaging, full potential of UHF-MRI have not been achieved yet. The biggest challenge is B1 inhomogeneity due to shorter wavelength. A range of new RF coil designs has been proposed to overcome this problem. One of such designs is the radiative antenna array [2]. High B1⁺ sensitivity were reported compared to the conventional RF coil. However, these arrays were on-body (or wrapped) coil with limited spatial coverage. In this work, 8-channel top-hat dipole antenna is presented for improved B1⁺ homogeneity along Z-direction with parallel transmit RF array. EM simulations have been performed in order to find optimal top-hat dipole antenna structure and there results were compared with the original dipole antenna array.

Adding metal to the ends of a dipole antenna will increase their transmission (TX) & reception (RX) efficiency. By adding a large piece of metal at the top we can increased the capacitance from the top of the antenna to the reference plane and thereby increased the current flowing to the top of the antenna. The “top-hat” can be a metal disk, radial wires, a metal sphere, etc. Their electrical lengths are equal to half-wavelength and lumped elements are used to preserve the tuning and matching. Figure 1 shows three types of the top-hat optimal shapes are found from the simulation.

Electromagnetic (EM) simulation analysis based on FDTD method were performed using Sim4Life V2.0 (SPEAG, Switzerland). Surface current distribution is measured for 3 types of top-hat dipole antenna with varying disc. B1⁺ field distribution and SAR field were also simulated. For 8 channel configuration, the array consisted of eight 50 cm-long top-hat dipole antennas, which are equally distributed in radial direction (52 cm D) almost identical to inner diameter of the Philips Achieva 7T (Philips Healthcare, Netherlands) magnet bore. To construct a uniform phantom (46 cm D × 52 cm H), the dielectric properties of liver tissue is used. For human model, Billie V1.3, 75 tissue parameter model was used [3]. All the 8 channels were driven with circular-polarization mode excitation and no RF shimming in the x-y plane was applied. All the results were normalized such that 1 x 8 watt RF power delivered to the array. All these top-hat dipole antenna were compared with original dipole antenna.

Figure 2a shows the surface current distribution measured by the three types and it is observed that there is increase in current near the end of the antenna when top-hat is attached. Figure 2b shows the B1⁺ profile along the z-direction generated by three types in center of the coronal plane with 8 TX channels combined. Figure 3 shows the B1 nonuniformity in the axial plane (ROI - 36 cm D) and in the coronal plane (ROI - 40 cm L × 36 cm W). Each bar plot shows the optimal shape in each type, the circle represent the best shape in all types for the final top-hat dipole antenna array design. This half-disc top-hat dipole antenna is used for the simulation of human model and Fig. 4 shows the B1⁺ field and SAR field in axial and coronal plane. There is homogeneous field in body region and also low SAR except the arm region. Table 1 shows the average B1⁺ and maximum SAR in phantom as well as human model. In conclusion, top-hat dipole antenna array shows improvement of B1⁺ homogeneity in z-direction as well as low SAR compared to original dipole antenna. Application of RF shimming on the x-y plane as well as high dielectric pads are currently being investigated as they have proved to be very effective in optimizing B1⁺ field as well as SAR reduction.