An 8-Channel Transceive Head Coil Array for 7T

Purpose/Introduction: In this study we present an optimized 8-channel transmit-receive head coil array to achieve maximum B1 efficiency and minimum SAR for brain imaging at 7T. Subjects and Methods: The coil consists of 8 transmit-receive shielded array elements, symmetrically distributed around the head region (Fig 1). The distance between the back shield and each array element as well as the dimension of each element were optimized in order to get the max B1 efficiency and minimum SAR value of the array using CST simulator [1]. PIN diodes in all elements allow for actively switching off of the head coil during reception by a receive-only coil. Additionally a transmit/receive switch was used for each channel to allow for switching between Tx and Rx mode using the head coil as a transceive coil alone. Results: Results of the EM simulations (Fig.2) show the maximum B1 efficiency of 0.4 [μT/sqrtW}]. The peak local SAR averaged over 10g cubes over the accepted power turned out to be 0.55 [1/kg] in CP+1 mode using CST human model [1]. any two channels at 297.2 MHz was lower than -20 dB. Fig.3 shows the measured B1 efficiency of the array using a sugar-water phantom after phase shimming (all 8 channels) with a maximum B1 efficiency of 0.46 [μT/sqrt{W}], whereas a maximum B1 efficiency of 0.34 [μT/sqrt{W}] was measured in an oil phantom and of 0.33 [μT/sqrt{W] in the human brain of a healthy volunteer respectively. Discussion/Conclusion: An 8-channel transceive head coil was constructed and evaluated on a 7T MRI system. The optimized coil array has been shown to produce high B1 efficiency and high SNR. This coil can be used both as a transceive coil alone (due to its high B1 efficiency and low SAR) and also as a transmit-only coil