THE ANTISENSE COAXIAL SURFACE-COIL PROBE, AN IMPROVED METHOD FOR COIL-TO-COIL ELECTROMAGNETIC DECOUPLING OF COAXIAL SURFACE COILS

A probe design for improved coil-to-coil electromagnetic decoupling of coaxial surface transmitter and receiver coils in the presence of conductive sample loading is presented. The probe consists of large transmitter and small receiver surface coils. The receiver coil consists of two "antisense" loops with low mutual inductance located on both sides of the transmitter coil. Importantly, the axial position of the transmitter coil can be varied via a screw mechanism to achieve a state of near electromagnetic symmetry with the two antisense receiver loops even when one of said loops is unbalanced from its antisense partner by electromagnetic coupling to the sample under interrogation. The currents induced in the two antisense loops by the radiofrequency excitation pulse can be visualized intuitively as having equal strengths but being in antiphase, a concept referred to as "counter-rotating current" by Hyde and co-workers (Magn. Reson. Med. 3, 590, 1986). The actual current induced in the receiver coil by the transmitter is close to zero, indicative of good isolation between circuits. This arrangement, unlike other typical coil-to-coil decoupling techniques, does not require fractional wavelength coaxial transmission lines and/or crossed diodes. It readily achieves excellent isolation ({slanted equal to or greater-than}40 dB) between coaxial transmitter and receiver coils tuned to the same resonance frequency. The feasibility of this approach is demonstrated herein by measurements of the transverse radiofrequency magnetic field, coil quality factor, and NMR experimental parameters. Results of experiments in vivo using this probe design are presented. © 1992.