Evaluation of a Monopole and Dipole Hybrid Antenna Array With Individual Floating Shields for 10.5 Tesla Human Head MRI Applications

The frequencies required for ultra-high field (UHF) magnetic resonance imaging (MRI) applications dictate the necessity for higher channel count of transmit arrays to improve the uniformity and efficiency of the transmit magnetic (B $_{1}^{+}$ ) field using a parallel transmit system (pTx). Based on the comparison of 8-channel arrays, our previous study suggested the concept of a monopole and dipole hybrid antenna (MDH) array for human head imaging at 10.5 Tesla (T) MRI. Here we expanded it to a 16-channel MDH array to obtain more uniform B $_{1}^{+}$ field distribution. Then, we further expanded it to a MDH array with individual floating shields (MDHFS) to extend coverage through the z-direction of MRI. To evaluate these arrays, we compared the B $_{1}^{+}$ efficiencies of the 16-channel MDH and MDHFS arrays with the same dimensions both in electromagnetic field simulations and experiments at 10.5 T MRI. Both results revealed that the MDHFS array (0.52/ $0.36~\mu $ T/ $\surd $ W for the simulation and experiment, respectively) showed ~35% lower peak B $_{1}^{+}$ efficiency compared to the MDH array (0.76/ $0.59~\mu $ T/ $\surd $ W for the simulation and experiment, respectively)). However, compared to the MDH array (~10 cm), the MDHFS array (~20 cm) had longer coverage in the entire phantom. Especially, for human head imaging, simulations indicated a 43.5% lower specific absorption rate (SAR) and 22.1% higher SAR efficiency at 10.5 T with the MDHFS array (0.26 W/kg and $1.04~\mu $ T/ $\surd $ (W/kg), respectively) compared to the MDH array (0.46 W/kg and $0.81~\mu $ T/ $\surd $ (W/kg), respectively).