Improving RF field strength and uniformity in a local region by controlling individual transmitting RF phase in a 7T tranceive array coil

Abstract

To improve the radiofrequency (RF) field strength and uniformity in a local imaging region, individual RF transmitting phases were controlled in surface coil elements in a 7T transceive array coil; the RF field distribution was compared with the conventional in-phase approach at 7 T. Optimal RF transmitting phases in the individual coil elements in a four-channel transceive array coil were numerically calculated using the electromagnetic (EM) field solver to obtain uniform RF field in a local imaging region. In 7 T phantom experiments, the RF field uniformity and mean SNR were evaluated on gradient echo images. In addition, a flip angle map was obtained by the double angle method. Experimental results clearly show that the EM calculation yielded an improved RF field strength and uniformity in the specific imaging region along the center of the coil array when the RF transmitting phase offset was 180° between the left and right rows of surface coil elements. In a 7 T experiment, B1 field uniformity and mean SNR with this phase offset were better than the other cases and the FA map showed a more focused and symmetric distribution as compared to other cases. Even though the RF field distribution in the transceive array coil is still strongly affected by dielectric properties at 7 T, the 180° phase offset between the left and right rows of the coil elements gave the highest magnetic flux density in the specific imaging.