Numerical and Experimental Evaluation of Short Folded Recieve-Only Dipoles for 9.4T Human Head Arrays

Improvement of signal-to-noise ratio (SNR) is a critical step in designing any MRI radio frequency (RF) coil. Increasing the number of surface loops in a human head receive (Rx) array improves the peripheral SNR, while the central SNR doesnt substantially change. Recent studies demonstrated that an optimal central SNR at UHF frequencies (298 MHz and higher) requires contribution of two current patterns associated with loops and dipoles. To incorporate multiple dipoles into a human head loop Rx-array, the dipole length has to be substantially reduced, which compromises its performance. Another issue of using short Rx-dipoles is a sensitivity of their resonance frequency to loading due to a large electrical field near the dipole. To reduce the sensitivity, we propose to fold dipoles towards the RF-shield. A novel array consisting of 8 transceiver surface loops and 8 folded Rx-dipoles was developed and tested. Addition of Rx-dipoles doesnt substantially change the $B_{1}^{+}$ field and the maximum local SAR of the array. At the same time, the new design improves both the central and peripheral SNR as compared 16-element array with Rx-only vertical loops and 8-element transceiver surface loop array.