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 profiles of "adiabatic" full-passage (AFP) RF pulses are utilized for frequency-selective excitation of the water resonance. In the proposed WS schemes, dubbed WASHCODE, hyperbolic secant (HS) pulses were used as the AFP pulses. Besides the high immunity of WS efficiency toward B(1) inhomogeneity, these sequences also exhibit extraordinary insensitivity to the dispersion of the water T(1) relaxation times. The actual performance of the proposed WS schemes was achieved in particular by optimizing the frequency offsets of WS HS pulses and the time intervals between them. To reduce the RF power requirements of these WS sequences for in vivo applications, HS pulses with the minimum possible frequency bandwidths were employed, which also substantially reduced the adverse effects on the observed proton MR spectra. The proposed WS schemes were evaluated by simulations based on the Bloch equations. Several WS sequences which looked particularly promising were verified experimentally on the human brain on a 3 T MR scanner using very short echo-time STEAM for volume selection and a standard single-loop surface coil for both signal transmission and reception. Routinely, water-suppression factors ranging from 2000 to 4000 were achieved in vivo without additional adjustment of parameters for individual subjects and without violating legal safety limits.

Very short echo time proton MR spectroscopy of human brain with a standard transmit/receive surface coil

A method for localized proton spectroscopy of the human brain is proposed which can be used with a standard transmit/receive planar surface coil producing an inhomogeneous RF field. Water suppression is accomplished by a train of full passage adiabatic pulses with optimized frequencies and delays, which account for variation in the water resonance frequency and the spin-lattice relaxation time. The robust method requires minimal pulse calibration and provides high-quality spectra even at very short echo times and in the absence of outer volume saturation and, therefore, is well suited for clinical in vivo spectroscopy. Performance of the method is demonstrated on a test object and on MR spectra from the human brain at 3 T.

In vivo 1H NMR spectroscopy of rat brain at 1 ms echo time

Using optimized, asymmetric radiofrequency (RF) pulses for slice selection, the authors demonstrate that stimulated echo acquisition mode (STEAM) localization with ultra-short echo time (1 ms) is possible. Water suppression was designed to minimize sensitivity to B1 inhomogeneity using a combination of 7 variable power RF pulses with optimized relaxation delays (VAPOR). Residual water signal was well below the level of most observable metabolites. Contamination by the signals arising from outside the volume of interest was minimized by outer volume saturation using a series of hyperbolic secant RF pulses, resulting in a sharp volume definition. In conjunction with FASTMAP shimming (Gruetter Magn Reson Med 1993;29: 804-811), the short echo time of 1 msec resulted in highly resolved in vivo 1H nuclear magnetic resonance spectra. In rat brain the water linewidths of 11-13 Hz and metabolite singlet linewidths of 8-10 Hz were measured in 65 microl volumes. Very broad intense signals (delta v(1/2) > 1 kHz), as expected from membranes, for example, were not observed, suggesting that their proton T2 are well below 1 msec. The entire chemical shift range of 1H spectrum was observable, including resolved resonances from alanine, aspartate, choline group, creatine, GABA, glucose, glutamate, glutamine, myo-inositol, lactate, N-acetylaspartate, N-acetylaspartylglutamate, phosphocreatine, and taurine. At 9.4 T, peaks close to the water were observed, including the H-1 of alpha-D-glucose at 5.23 ppm and a tentative H-1 resonance of glycogen at 5.35 ppm.