MRI APPARATUS WITH A FEED FORWARD LOOP INSERTED IN THE GRADIENT LOOP

摘要

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. If the control circuit is also provided with a feedback loop, an A/D converter 41 can be inserted in the signal path in the case of a digital embodiment, and also a delay member (35) which precedes the analog-to-digital converter in order to delay the signal from the feed forward loop (57) in such a manner that the error signal e(t) assumes a very low amplitude. The analog-to-digital converter (41) can then be implemented so as to have a small bit width, with the result that the conversion rate can reach the desired high value.