A BIRDCAGE COIL TUNED BY RF SHIELDING FOR APPLICATION AT 9.4-T

The design and performance of an inductively fed low-pass birdcage rad iofrequency (RF) coil for applications at 9.4 T are described where tu ning is accomplished by mechanically moving a concentric RF shield abo ut the longitudinal axis of an RF coil. Moving the shield about the RF coil effectively changes the mutual inductance of the system, providi ng a mechanism for adjusting the resonant frequency. RF shield tuning eliminates adjustable capacitors on the legs of the RF coil, eliminate s current imbalances and field distortions, and results in improved B- 1 field homogeneity and high quality (Q) factors. RF shield tuning and inductive matching provide an isolated resonance structure which is b oth physically and electrically unattached, Experimental analysis of s hield position on both B-1 field homogeneity and resonant frequency is provided. Computer simulations of B-1 field homogeneity as a function of shield position and shield diameter are also presented. Magnetic r esonance microimaging substantiates the usefulness of this design. (C) 1998 Academic Press.