In comparison, local birdcage coil (volume resonator) has been industry standard for human head imaging at 7 T. Since the wavelength \({\lambda }_{{{\mbox{head}}}}\) (at 297 MHz) in biological tissues approaches human head dimension, node and antinode of SW appear in the human head. The human body and the inner metallic surface of the MRI bore (the waveguide) become electrically large, so the volume resonator can radiate power inside, and the waveguide carries TE 11 mode travelling wave (TW) at 297 MHz.
