Automatic tuning and matching of a small multifrequency saddle coil at 4.7 T.

A new circuit design for automatically tuning and matching a saddle coil for small animal imaging is presented. This design allows working at (1)H, (19)F, and (3)He resonance frequencies in a 4.7 T spectrometer. It is based on a balanced circuit with commercial variable capacity diodes controlled by a computer using digital potentiometers. The change between two different frequencies can be accurately performed in a few seconds. System Q is compared, between 140-210 MHz, to the same coil tuned and matched with high Q variable capacitors. Differences lower than 5% were found with a loaded coil. The proposed design has initially been evaluated in (19)F and (1)H NMR images acquired with a five-tube phantom. An application is also shown for the acquisition of (3)He, (19)F, and (1)H lung images in a control rat.