FIGURE 2:

Schematic illustration of basic RF coil structures and instantaneous field patterns for fundamental coil structures when empty: (a) birdcage coil, (b) loop coil, and (c) dipole antenna. Black lines indicate conductor paths, blue lines indicate the direction of the B₁ field, and red lines indicate the direction of the electric field. The field pattern of the birdcage coil rotates about the coil’s central axis while that of the loop and dipole would oscillate at the Larmor frequency.