Abstract: Gradient current pulses which generate pulse-shaped magnetic gradient fields in an MRI apparatus may be composed of gradient pulse samples which directly succeed one another and are generated by a power amplifier 51. The latter is controlled by an input signal 47 which assumes discrete values only. This input signal is generated by a digital PWM converter 45. Generally speaking, the discretization will introduce a difference between the discrete gradient pulse sample and the value to be discretized, but such a difference will be known. According to the invention this difference is stored until the next gradient pulse sample is formed. The value then found is corrected by means of the stored difference and the value thus calculated is discretized as described. In this way small deviations from the desired value still occur in the gradient pulse sample, but averaged over the total duration of the gradient current pulse this deviation appears to be so small that it can be ignored.

Abstract: Gradient pulses are generated in an MRI apparatus by a control circuit in which disturbing influences are liable to occur; consequently, the gradient current pulses are not shaped exactly as prescribed by the input signal on the input (33) of the control circuit. This effect can be counteracted by feedback, but some deviations still remain because of a variety of capacitive and/or inductive elements (3, 51) in the control circuit. These deviations cannot be sufficiently corrected because of the limited bandwidth in the control circuit. According to the invention the control circuit is provided with a feed forward loop (57) which includes a filter (61, 63) whose filter characteristic is the inverse of the impedance characteristic of a component (3, 51) conducting the gradient.