Abstract: A universal-tuned RF probe (8) is disclosed which covers all current nuclear MRS and MRI applications and anticipates the utilization of multiple nuclides. This probe comprises a switchable thin multi-layer coil (29, 30, 31), electronically controlled switches (11, 12), an impedance matching circuit (13-20), and a resonant frequency tuning circuit (21-28). By quick switching, all different nuclei are detected independently, therefore their performances can be optimized individually. Fast switching (of less than 10.sup.-9 seconds) allows multiple frequencies to operate simultaneously. A switchable thin multi-layer coil (29, 30, 31) is used to cover a wide range of resonant frequencies and maintain identical RF B1 magnetic fields for all the different nuclei.

Abstract: Apparatus for tuning an NMR probe coil (10) to a desired frequency (.omega.) and matching the coil (10) and the apparatus to a desired input impedance (20) comprises a differential capacitor (54, 70, 80, 120) having first, second and third terminals. The capacitance across the first and third terminals of the differential capacitor (54, 80, 120) is constant and is preselected to obtain the desired resonant frequency (.omega.). The capacitance across first and second terminals of the capacitor (54, 70, 80, 120) is variable and is adjusted to obtain the desired input impedance (Z.sub.0) without affecting the capacitance across the first and third terminals thereof.