Overcoming high-field RF problems with non-magnetic Cartesian feedback transceivers

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 radio-frequency coil circumference. Consequently, radio-frequency 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. Non-magnetic 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.

A 'Hi-Fi' Cartesian feedback spectrometer for precise quantitation and superior performance

The use of Cartesian electronic feedback for effecting a major improvement in the functioning of magnetic resonance instrumentation is reported. The dependences of both flip angle and signal strength upon probe loading, matching, and tuning are virtually eliminated. Thus, for a chosen probe, sample geometry and flip angle, the free induction decay signal strength is rendered solely dependent upon the number of nuclei. The instrument therefore becomes capable of absolute calibration. In addition, phase and amplitude distortion of selective pulses, introduced by crossed diodes, power amplifier heating, etc., is virtually eliminated, as are radiation damping and phase modulation caused by probe vibration. The use of multiple probes at the same frequency, for example quadrature probes and phased arrays, is also simplified as the effects of interactions between such probes are typically reduced by two orders of magnitude.

The NMR multi-transmit phased array: a Cartesian feedback approach

The use of Cartesian feedback is proposed to solve the problem of using an array of coils for the purposes of transmission in magnetic resonance imaging. The difficulties caused by direct and sample-mediated coil interactions are briefly examined, and the known solutions of using power-mismatched pre-amplifiers and transmitters noted. It is then shown that, without loss of transmitter efficiency, a high effective impedance may be created in series with each coil in the array by the use of Cartesian negative feedback. A bench experiment is described that confirms the theory. The solution is also viable for signal reception and is more efficacious than pre-amplifier damping, albeit over a smaller bandwidth.