Eight-channel Tx/Rx Helmet Coil for Human Brain Imaging with Improved RF Homogeneity

INTRODUCTION Recently, an anatomically shaped helmet coil consisting of four curved microstrip transmission-line (MTL) elements was introduced (1). The MTL elements were combined either as a circularly polarized (CP) transmit/receive (Tx/Rx) coil (Type A) or as a CP transmit, four-channel receive array (Type B). Both types provided an almost perfect circular polarization in the central region and a linear polarization near the MTL elements. However, a transmission field (B1) with opposite rotation sense is generated between adjacent MTL elements causing reduced signal intensity and image contrast in peripheral head regions. To overcome this problem, a more advanced design with eight MTL elements was developed. To maintain a sufficiently open coil with ample space for audiovisual stimulation, the geometric length of three of the MTL elements was reduced, whereas the electric length was maintained by proper termination.