SOLVENT SIGNAL SUPPRESSION IN NMR

In proton in vivo spectroscopy, SSS is concerned not only with water in the selected volume, but also with that in the surrounding tissue. Another problem is the strong signal from fatty acids. SSS must coexist with the constraints of localization. This raises specific problems when localization is based exclusively on the rf field gradient, since the pulse sequence is then responsible for both localization (depth pulse sequence) and SSS. It is difficult to satisfy both simultaneously. The SSS method must tolerate inhomogeneity of B0 and of the rf field (especially large with surface coils, but significant also in other cases), and limitations on power dissipation in the tissues. SSS by water saturation may be poor in these conditions, so frequency-selective excitation is often preferred. The spectral response should be position-independent, both for the quality of SSS and for quantification of metabolite signals. Together with the need to suppress the fatty acid proton signal, this leads to a nearly absolute requirement for spin-echo techniques. Excellent frequency-selective refocusing (E2) sequences exist; their spectral response is independent of pulse angle, and therefore of position; they do not contribute to phase dispersion, and can even compensate for it. They also contribute to localization. The requirements on the excitation (E1) sequence are thereby reduced, and even a hard pulse can be used. When B0 gradients with good switching characteristics are available, they may be used for spatially selective excitation or for phase-encoding spectroscopy. In this case, localization does not require an rf field gradient, and the design of the SSS method is easier. A surface receiver coil may be chosen for sensitivity.