Newer Methods:
Various other sequences / methods have been employed:
- binomial sequences (1 -1, 1 -2 1, 1 -3 3 -1) but= also non-linear excitation profile, phase roll
- selective excitation (with spinlocks, rectangular or shaped pulse)
- principle= selectively excite water with -90 first, then use hard pulse 90:
- water sees -90 +90 = 0 degree pulse, but magnetization of interest = standard 90.
Example:
- Shifted pulses (SSpulses)= one pulse 1 -2 1 version, good water supression, but difficult to set up. Non linear exitation profile (sin2Dw)t
Recently= combination of the selective schemes with pulsed field gradients!
Real improvement in supression: Flat baselines, no significant disadvantages, excellent supression
- Watergate: water filter using four principles: selective excitation, spin-echo, and gradient dephasing/refocusing, and diffusion filter. Can be appended to any pulse sequence, i.e. removes water directly before acquisition, when water/signal of interest is in observable plane
Example: 1D-90 pulse-sequence with watergate filter
Notes:
Watergate is the first supression method which could compete against presaturation. It is relatively straightforward to be used, and can easily be appended to any NMR-experiment/pulse sequence. However, watergate still requires a good deal of setup/pulse optimization. Reward is the significantly better water supression than with presat. In principle, it has the same restriction as presat, i.e. it fully saturates or better excites the water it filters out!
PJ Hore, J. Magn Reson, 55, 283-300 (1983).
Selective excitation schemes:
Sklenar & Bax, J. Magn. Reson. 75, 352-357(1987); 75,378-383(1987); 74,469-479 (1987).
SH Smallcombe, JACS 115, 4776-4785,1993
Piotto, Saudek, Sklenar J Biomol NMR 2,661-665 (1992).
Sklenar, Piotto, Leppik, Sudek, J. Mag. Reson. A102,241-245(1993).