Suppression of linearly responsive residual water

Abstract

The residual water, coming from a region away from the center of the RF coil and therefore experiencing a much smaller flip-angle than the designed one, may not be well suppressed in certain water-suppression experiments. To suppress the residual water efficiently, a composite 90°x90°y90°-x90°-y pulse is used in the WET sequence, which de-excites molecules experiencing small flip-angles. The composite pulse, however, has two null excitation points near on-resonance, causing a severe loss of spectrum intensity and baseline distortion toward the null points. A phase inversion at each null point is found in the excitation profile of the composite pulse. This phenomenon is explored in detail. Because the residual water experiences a very small flip-angle, it can be treated as a linear spin system, i.e., its peak intensity is proportional to the flip-angle. Based on this principle, the residual water can be reduced dramatically by replacing the 90° pulse in the WET sequence with a 270° pulse for one out of every four scans. Unlike the composite pulse, this scheme does not introduce noticeable loss of spectrum intensity and baseline distortion.