# Evaluation of 8-Channel Radiative Antenna Arrays for Human Head Imaging at 10.5 Tesla
## 2 Materials and Methods
### Construction of 8-Channel Dipole, Monopole, and Sleeve Antenna Arrays  
The coil formers for the dipole and sleeve antenna arrays were modeled (Fusion 360, Autodesk, Mill Valley, CA, USA) and then 3D printed (F410, Fusion3 Design, Greensboro, NC, USA). The dipole and sleeve antenna arrays have the same elliptically shaped physical dimensions, with a short and long axis of 20 cm \(\times\) 22 cm and a length of 20 cm with eight equally spaced antenna elements distanced at 8.8 \(\pm\) 1.4 cm between individual elements. The individual dipoles and sleeve antennas of the 8-channel dipole and sleeve antenna arrays were tuned to 447 MHz with wound inductors. In Figure 3c, the monopole antenna array was built with a 24 cm \(\times\) 24 cm dimension and a length of 20 cm on a circular-shaped acrylic former [20; 21]. Eight quarter-wave (\(\sim\)13 cm at 10.5 T) monopole antennas were connected to a 35 cm \(\times\) 36 cm dimension copper ground plate, which was segmented and reconnected with 330 pF capacitors to reduce eddy currents. As shown in our previous research [6; 8; 17; 19], all arrays were operated without an intra-element decoupling circuit.  
A lattice balun with two ceramic capacitors (10 pF, 100B series, American Technical Ceramics, Huntington Station, NY, USA) and two inductors (12 nH, A04T_L, Coilcraft  
\begin{table}
\begin{tabular}{c c c c} \hline  & **Dipole** & **Monopole** & **Sleeve Antenna** \\ \hline B1+ efficiency (uT/W) & 0.59 & 0.72 & 0.59 \\ Peak 10 g SAR (W/kg) & 0.55 & 0.74 & 0.31 \\ SAR efficiency (uT/W/kg) & 0.80 & 0.83 & 1.05 \\ \hline \end{tabular}
\end{table}
Table 1: Quantitative comparison of B1+ efficiency, 10 g SAR, and SAR efficiency among the 8-channel dipole, monopole, and sleeve antenna arrays with human model in the ROI marked for the highest value.  
Figure 3: CAD models (**a,b**) and photographs (**c,d**) of the 8-channel monopole and sleeve antenna arrays.  
Inc., Cary, IL, USA) was used for all matching networks of the 8-channel dipole antenna array. Lattice balun circuits at the feed point and one floating cable trap on the feed cable reduced the sheath current for each element of the 8-channel dipole antenna array. For the comparison in the experiment, the coaxial cables of the dipole antenna array were positioned as a further and tight fit setup. The coaxial feed cable path of the 8-channel dipole antenna array was adjusted to maximize distance from the resonant structure to reduce the interaction between the coaxial cables and elements of the array.  
The sleeve antenna array was basically designed with the combination of monopole antennas and floating cable traps [24; 25]. To support a better comparison with the inductor-shortened dipole antenna array, the length of the sleeve antenna array elements was also shortened with inductors. For the sleeve antenna array [8], the length of each monopole conductor of the antenna was set to 15 cm with inductor shortening, and combined with a 5 cm floating cable trap for an overall length of 20 cm. For the dipole and sleeve antenna arrays, similar floating cable traps were equipped on the coaxial cables [8; 26; 27; 28].  
Supplementary Table S1 summarizes bench measurements obtained from each of the arrays and indicates the range of reflection coefficient (S\({}_{11}\)) and coupling coefficient (S\({}_{21}\)) values when the arrays are loaded with a cylindrical phantom. All S\({}_{11}\) and S\({}_{21}\) were measured in bench measurements using a 16-channel network analyzer (ZNB78, Rohde & Schwarz, Munich, Germany). The S-parameters of each channel of the 8-channel dipole, monopole, and sleeve antenna arrays were measured in dB-scaled values.