Low-power water suppression by hyperbolic secant pulses with controlled offsets and delays (WASHCODE)

A class of chemical-shift-selective (CHESS) water suppression (WS) schemes is presented in which the characteristic frequency-domain excitation profil es of "adiabatic" full-passage (AFP) RF pulses are utilized for frequency-s elective excitation of the water resonance. In the proposed WS schemes, dub bed WASHCODE, hyperbolic secant (HS) pulses were used as the AFP pulses. Be sides the high immunity of WS efficiency toward B-1 inhomogeneity, these se quences also exhibit extraordinary insensitivity to the dispersion of the w ater T-1 relaxation times. The actual performance of the proposed WS scheme s was achieved in particular by optimizing the frequency offsets of WS HS p ulses and the time intervals between them. To reduce the RF power requireme nts of these WS sequences for in vivo applications, HS pulses with the mini mum possible frequency bandwidths were employed, which also substantially r educed the adverse effects on the observed proton MR spectra. The proposed WS schemes were evaluated by simulations based on the Bloch equations. Seve ral WS sequences which looked particularly promising were verified experime ntally on the human brain on a 3 T MR scanner using very short echo-time ST EAM for volume selection and a standard single-loop surface coil for both s ignal transmission and reception. Routinely, water-suppression factors rang ing from 2000 to 4000 were achieved in vivo without additional adjustment o f parameters for individual subjects and without violating legal safety lim its. (C) 2001 Academic Press.