FAT SUPPRESSION FOR 1H MRSI AT 7T USING SPECTRALLY SELECTIVE ADIABATIC IR

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. 1H Magnetic resonance spectroscopic imaging (1H MRSI) is a useful technique for measuring metabolite levels in the brain in vivo. 1H MRSI at higher fields, such as 7T, offers the advantages of increased SNR and spectral separation. Short Tau Inversion Recovery (STIR) [1,2] is a commonly used fat suppression method which employs a 180¿ inversion pulse prior to the 90¿ excitation pulse in a PRESS sequence with inversion time (TI) adjusted such that the longitudinal magnetization of short T1 species (e.g. fat) passes through a null when primary excitation occurs. Although most metabolites have T1s significantly longer than fat, they nonetheless suffer from a partial signal loss on the order of 50% at 7T. Fortunately, the increased spectral separation at 7T makes it possible to design a spectrally selective adiabatic inversion pulse that only inverts lipid resonances, leaving most of the metabolites untouched. Two versions of such a pulse have been designed and inserted into a standard PRESS sequence. The first is a highly selective adiabatic spectral pulse that inverts the fat in the entire volume for slab-selective PRESS. The second is an adiabatic spatial-spectral (SPSP) pulse that allows simultaneous selectivity in both space and frequency while maintaining insensitivity to B1 inhomogeneities. The pulse has a spectral profile that selectively inverts the fat and, being slice-selective, is amenable for multi-slice MRSI. At 7T, multislice MRSI is particularly attractive, as compared to volumetric methods, due to the ability to individually optimize the B0 homogeneity of each slice using dynamic shimming [3,4]. In vivo 7T data demonstrates that both spectral and spatial-spectral adiabatic fat-selective inversion pulses successfully suppress fat while not degrading signal from most of the metabolites of interest in the brain.