Overcoming highfield RF problems with nonmagneti

Authors: D I Hoult D Foreman G Kolansky D Kripiakevich

Publish Date: 2007/11/17

Volume: 21, Issue: 1-2, Pages: 15-

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Abstract

In extending human MR to field strengths approaching 10 T the wavelength of electromagnetic radiation at the proton Larmor frequency becomes less than human body size and conventional radiofrequency coil circumference Consequently radiofrequency magnetic fields are better generated by an array of small coils than by one large coil In this article the primary problem of array coil interactions during transmission is examined and a standard proposed whereby secondary induced currents should be less than 1 of the primary coil current The use of cancellation methods and of power amplifiers with high output impedance to reduce interactions is examined in the light of this standard and found wanting Nonmagnetic Cartesian feedback transceivers functioning at the magnet entrance are then proposed as a solution that both reduces instrumentation cost and increases the bandwidth over which the standard may be met The compromises inherent in instrument design are detailed and examples given of the innovative circuitry used It is shown experimentally that when connected to interacting coils two Cartesian feedback instruments function stably in accord with theory and such that the proposed standard is typically attained over a bandwidth of 22 kHz during transmission much greater during signal reception—enough for all current MR protocols

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