Real-time RF pulse adjustment for B0 drift correction.

Although the magnetic field of an MR scanner is very stable under little or no load, it can become less stable under heavy-duty cycle conditions, such as in diffusion tensor imaging (DTI). Uncorrected, such field drifts lead to an apparent image shift along the phase-encoding direction and decreasing effectiveness of fat saturation pulses. A method is presented to adjust the center frequency of all RF pulses and the receiver in real time during the acquisition. No data postprocessing or changes to the sequence timing are necessary. In vivo acquisitions were performed to assess the prolonged effectiveness of fat saturation. Field drifts of approximately 2.5 Hz/min were measured and corrected during DTI acquisitions at b-values of up to 3000 s/mm2. The effectiveness of fat saturation diminished over the duration of an 18-min acquisition when the drift was left uncorrected. The proposed method corrects for apparent image shift and ensures continuously effective fat saturation over the duration of an acquisition. Copyright (c) 2006 Wiley-Liss, Inc.