WET, a T1- and B1-Insensitive Water-Suppression Method for in Vivo Localized 1H NMR Spectroscopy

Abstract

Suppression of the water signal during 1H magnetic resonance spectroscopy by repeated sequences of a frequency-selective radiofrequency pulse and a gradient dephasing pulse requires nulling of the longitudinal component of the water magnetization and is therefore affected by T 1 relaxation, RF-pulse flip angles (which depend on B 1), and sequence timing. In in vivo applications, T 1 and B 1 inhomogeneity within the sample may cause spatially inhomogeneous water suppression. An improved water-suppression technique called WET ( water suppression enhanced through T 1 effects), developed from a Bloch equation analysis of the longitudinal magnetization over the T 1 and B 1 ranges of interest, achieves T 1- and B 1-insensitive suppression with four RF pulses, each having a numerically optimized flip angle. Once flip angles have been optimized for a given sequence, time-consuming flip-angle adjustments during clinical examinations are eliminated. This water-suppression technique was characterized with respect to T 1 variations, B 1 variations, off-resonance effects, and partial saturation effects and was compared to similar techniques. Effective water suppression has been achieved with this new technique in single-voxel spectroscopy examinations of more than 50 brain tumor patients at 1.5 T.