Analysis of eddy currents in nuclear magnetic resonance imaging.

The eddy currents in nuclear magnetic resonance (NMR) imaging are analyzed from the solutions of Maxwell's equations and their effects are examined over various experimental conditions from whole-body diagnostic imaging to recently developed NMR microscopy. The analysis is focused mainly on the frequency characteristics and intensity variations of the eddy-current-induced field which depends on the overall system size, ratio of the gradient coil size to the magnet bore diameter, and the pulse-sequence-dependent parameters such as input current waveform and repetition time. From the analysis, the frequency response of the eddy-current-induced field is that of a high-pass filter whose cutoff frequency is inversely proportional to the square of the overall system size. The intensity ratio of the generated field to the induced field is not affected by the overall system size, but is sensitively related to the ratio of the gradient coil size to the magnet bore diameter.