Gradient system providing continuously variable field characteristics.

Peripheral nerve stimulation limits the use of whole-body gradient systems capable of slew rates > 80 T/m/s and gradient strengths > 25 mT/m. The stimulation threshold depends mainly on the amplitude of the induced electric field in the patient's body, and thus can be influenced by changing the total magnetic flux of the gradient coil. A gradient system was built which allows continuous variation of the field characteristics in order to permit the use of full gradient performance without stimulation (slew rate 190-210 T/m/s, G(max) 32-40 mT/m). The system consists of a modular six-channel gradient coil designed with a modified target field method, two three-channel amplifiers, and a six-channel gradient controller. It is demonstrated that two coils on one gradient axis can be driven by two amplifiers in parallel, without significant changes in image quality. Scaling of the field properties and stimulation threshold according to the current polarity and ratio of both coil sets was verified in both phantom and volunteer studies.