Field-frequency locked in vivo proton MRS on a whole-body spectrometer

The stability of the main magnetic field is critical for prolonged in vivo magnetic resonance spectroscopy (MRS) acquisitions, especially for differen ce spectroscopy. This study was focused on the implementation and optimizat ion of a field-frequency lock (FFL) on a whole body spectrometer, to correc t the main field drift during localized proton MRS of the human brain. The FFL was achieved through a negative feed-back applied in real time on the Z 0 shim coil current, after calculation of the frequency shift from a refere nce signal. This signal was obtained from the whole head with a small flip angle acquisition interleaved with the PRESS acquisition of interest. To av oid propagation of the important short-term time-correlated fluctuations of the head water frequency (mainly due to respiratory motion) onto Z0 correc tion, the sampling rate of the reference frequency and the smoothing window for the Z0 correction were carefully optimized. Thus, an effective FFL was demonstrated in vivo with no significant increase of the short-term varian ce of the water frequency. Magn Reson Med 1999 42:636-642, 1999. (C) 1999 W iley-Liss, Inc.