Enhanced suppression of residual water in a "270" WET sequence

In certain water suppression experiments, the residual water, which comes f rom a region away from the center of the RF coil and experiences a much sma ller flip angle than the designed one, may appear. The residual water in th e WET sequence can be reduced significantly by using a composite 90(x)(degr ees)90(y)degrees 90(-x)degrees 90(-y)degrees pulse, which de-excites molecu les experiencing a small hip angle. The composite pulse, however, has two n ull excitation points near on resonance, causing a severe loss of spectrum intensity and baseline distortion toward the null points. Since the residua l water experiences a very small hip angle, it can be treated as a linear s pin system; i.e., the intensity of the residual water is proportional to th e pulse strength and width. Based on this principle, the residual water can be reduced dramatically by replacing the 90 degrees pulse in the "270" WET sequence with a 270 degrees pulse for one out of every four scans, without noticeable loss of intensity and baseline distortion. (C) 2000 Academic Pr ess.