# Evaluation of 8-Channel Radiative Antenna Arrays for Human Head Imaging at 10.5 Tesla
## 4 Discussion  
We focused on the following three points in this work. First, we evaluated how much degradation was present in an 8-channel dipole antenna array due to the interference among coaxial cables and antennas. Minimizing feed cable interferences is an important point, particularly for UHF head applications, due to the possibility for variance in the distance of the antenna element to the head tissue, which can result in possible stronger interaction of the antenna element with the coaxial feed cable than the imaging object. The second important point is a careful comparison among three different types of 8-channel head antennas. The third point is to demonstrate a close match between the simulation results and the physical experiment. It is important since the accurate simulation of RF coils with human models is ultimately used to estimate the RF coil safety at UHF [6]. The inductor-shortened dipole antenna array--albeit with a practically unrealistic coaxial feed cable routing (Figure 1e,f)--showed higher B1+ efficiency values compared to more practical tight cable routing (Figure 1g,h). This distant feed cable setup utilized for phantom imaging is not practical in most clinical research setups, and a substantial B1+ efficiency drop was observed with a more realistic coaxial feed cable setup. In this respect, the monopole and sleeve antenna arrays have a significant advantage in that the coaxial feed cables are routed away from the patient while keeping the performance of antennas optimal. In our initial 16-channel research [8], the asymmetric sleeve antenna array concept showed lower peak 10 g SAR and higher SAR efficiency compared to a dipole antenna array. Besides the benefits of the balanced coaxial feed point, this is the result of the principal advantage of the sleeve antenna concept with the important degree of freedom to alter the geometry of the sleeve parts, as well as monopole antenna parts. This allows for a practical compromise between the high E- and B-fields close to the ground plane of classical monopole arrays (Figure 4) and the more advantageous field patterns of dipoles for central slices. The ability of a balanced array feed point positioned away from the tissue/load results in a unique advantage of the sleeve antenna concepts for head applications. As an outcome, in the head array research presented here, we observed reduced peak 10 g SAR for the sleeve antenna array compared to the dipole and monopole antenna arrays. Generally, the feed point of an antenna has high current flow and produces strong E-fields in the imaging sample. In recent 10.5 T dipole antenna work by [PERSON] [6],  
Figure 6: B1-phase-shimmed GRE (**a**) and TSE (**b**) images of the 8-channel sleeve antenna array with human cadaver in the coronal plane. this issue was elegantly addressed by increasing the physical distance of the dipole antenna feed point to the sample, resulting in what the author termed a 'bumped' dipole antenna. Similarly, the E-fields spread out around the feed points of the sleeve antenna array, since the feed points of the individual antennas were pulled away from the subject for the experimental setup. Consequently, the asymmetric sleeve antenna array has an increased region of 10 g SAR towards the feed point (Figure 4f); however, this spreading out of 10 g SAR distribution leads to lower peak 10 g SAR and higher SAR efficiency values compared to the inductor-shortened dipole antenna array. The monopole antenna array showed the highest B1+ efficiency with the phantom and the human model compared to the dipole and sleeve antenna arrays close to the ground plane. However, it showed relatively higher peak 10 g SAR with the human model compared to the phantom. Based on comparison among Figures 2, 4 and 5, SAR efficiency of each array was consistently lowered with the human model compared to the phantom.