Fat suppression for ¹H MRSI at 7T using spectrally selective adiabatic inversion recovery

Proton magnetic resonance spectroscopic imaging (¹H MRSI) at 7T offers many advantages, including increased SNR and spectral resolution. However, technical difficulties associated with operating at high fields, such as increased B₁ and B₀ inhomogeneity, severe chemical shift localization error, and converging T₁ values, make the suppression of the broad lipid peaks which can obscure targeted metabolite signals, particularly challenging. Conventional short tau inversion recovery can successfully suppress fat without restricting the selected volume, but only with significant metabolite signal loss. In this work, we have designed two new pulses for frequency-selective inversion recovery that achieve B ₁-insensitive fat suppression without degrading the signal from the major metabolites of interest. The first is a spectrally selective adiabatic pulse to be used in a volumetric ¹H MRSI sequence and the second is a spatial-spectral adiabatic pulse geared toward multi-slice ¹H MRSI. Partial interior volume selection may be used in addition to the pulses, to exclude areas with severe B₀ inhomogeneity. Some differences in the spectral profile as well as degree of suppression make each pulse valuable for different applications. 7T phantom and in vivo data show that both pulses significantly suppress fat, while leaving most of the metabolite signal intact.