Resonant trapezoidal gradient generation for use in echo-planar imaging.

Two passive filter circuit designs are compared which enable series resonant trapezoidal drive of the modulated read gradients used in both echo-planar and echo-volumar imaging. Using simple circuit simulation software, details of the expected power requirements and performance have been evaluated for peak operating currents of up to 1000 A. The circuits evaluated were of a 7th order design incorporating a 110 microH gradient coil operating at fundamental frequencies between 800 Hz and 2000 Hz. Results indicate that the simpler circuit design is not necessarily the most efficient. In addition, by suitable modification of the input waveform algorithm, it can be shown that the shape of the resonant gradient waveform may be varied entirely under software control from triangular to trapezoidal, the minimum ramptime being specified by the highest harmonic accommodated in the circuit. The energy storage, non-dissipative nature of the circuit design also means that the gradient ramptime is effectively independent of amplifier power capabilities, making a fast rise trapezoidal waveform as easy to implement as either triangular or sinusoidal waveforms. Using the approach outlined above, trapezoidal gradient modulation at peak current amplitudes in excess of 320 A and risetimes less than 60 microseconds have been easily produced in this laboratory for use in echo-planar imaging and for investigations into ways of optimising gradient performance before the onset of peripheral nerve stimulation.