An RF coil assembly for an MRI system includes a resonator formed by a cylindrical shield and pairs of opposing conductive legs disposed symmetrically around a central axis and extending the axial length of the shield. One set of conductive leg pairs is tuned to operate at the Larmor frequency of .sup.13C and another set is tuned to operate at the Larmor frequency of .sup.1H. Drive circuitry operates the RF coil assembly to produce .sup.1H spin magnetization which is transferred to .sup.13C magnetization by the nuclear overhauser effect and to acquire MR data from the .sup.13C spins. Multinuclear measurements can be made simultaneously at different Larmor frequencies.