RF multi-channel head coil design with improved B ₁ ⁺ Fields uniformity for high field MRI systems

In ultra high field MRI systems (7 tesla and higher), the wavelength inside the body is short and smaller than the human anatomy at the Larmor frequency. At these shorter wavelengths, interference effects appear; the uniformity of the RF excitation B ₁ ⁺ field over the whole subject becomes inhomogeneous. The RF B ₁ ⁺ field generated by the RF coil is a maximum in the center along the length of the coil microstrip element and falls off towards both ends of the coil element and it leads to anomalous contrast in the MR images. In this study, double trapezoid-like shape along the length of the microstrip resonator is proposed to obtain gradual impedance variation and flatten transmission near-field profile along the length of the coil. The conventional and proposed 8 channel head coils were investigated with a phantom in a 7 tesla MRI scanner. The results show very flat field distribution with about 35% increased local transmission magnetic field strengths at the end sides as well as about 13% improvement at the center and the Q (quality factor) ratio between unloaded and loaded is also increased about 45% (from L.46 to 2.13) compared to the conventional structure.