A microstrip helmet coil for human brain imaging at high magnetic fields
A novel helmet coil for magnetic-resonance (MR) investigations of the human brain is suggested. It is based on a pure microstrip transmission-line (MTL) design. The coil consists of thin strip conductors (Cu) on a curved, low-loss dielectric material (polypropylene) generating an overall domelike structure. The design concept exploits the combination of the field profile generated by the current distribution along each MTL that is modified by termination and the geometric profile due to the curved MTL shape. Because of the semi-open transmission-line structure, most of the electromagnetic energy is stored in the dielectric material. Standing, quasi-transverse electromagnetic (TEM) waves are created in a plane nearly perpendicular to the axis of the MTL leading to a reduced radiofrequency (RF) stray field outside the volume circumscribed by the helmet. Because of the symmetry of the assembly, a high degree of circularly polarized RF is obtained within the brain. A quadrature transmit/receive (T/R) coil and a four-channel T/R array were built as initial prototypes for operation at 3 T. Computer simulations and imaging experiments verified that the MTL helmet coil permits imaging of the entire human brain including parallel-imaging capabilities.