RF head coil design with improved RF magnetic near-fields uniformity for magnetic resonance imaging (MRI) systems

Higher magnetic field strength in magnetic resonance imaging (MRI) systems offers higher signal-to-noise ratio, contrast, and spatial resolution in magnetic resonance (MR) images. However, the wavelength in ultrahigh fields (7 T and beyond) becomes shorter than the human body at the Larmor frequency with increasing static magnetic field (B₀) of the MRI system. At short wavelengths, an interference effect appears, resulting in nonuniformity of the RF magnetic near-field (B₁) over the subject and MR images may have spatially anomalous contrast. The B₁ near-field generated by the transverse electromagnetic RF coil's microstrip line element has a maximum near the center of its length and falls off towards both ends. In this study, a double trapezoidal-shaped microstrip transmission line element is proposed to obtain uniform B₁ field distribution by gradual impedance variation. Two multi-channel RF head coils with uniform and trapezoidal shape elements were built and tested with a phantom at 7-T MRI scanner for comparison. The simulation and experimental results show stronger and more uniform B ₁⁺ near-field with the trapezoidal shape.