A semiadiabatic spectral-spatial spectroscopic imaging (SASSI) sequence for improved high-field MR spectroscopic imaging

Purpose: MR spectroscopic imaging (MRSI) benefits from operation at 7T due to increased signal-to-noise ratio (SNR) and spectral separation. The 180° radiofrequency (RF) pulses used in the conventional MRSI sequences are particularly susceptible to the variation in the transmitted RF (B₁) field and severe chemical shift localization errors at 7T. RF power deposition, as measured by specific absorption rate (SAR), also increases with field strength. Adiabatic 180° RF pulses may mitigate the effects of B₁ variation. We designed and implemented a semiadiabatic spectral-spatial spectroscopic imaging (SASSI) pulse sequence to provide more uniform spectral data at 7T with reduced SAR. Methods: The adiabatic Shinnar–Le Roux algorithm was used to generate a high bandwidth 180° adiabatic spectral-spatial (SPSP) pulse that captured a spectral range containing the main metabolites of interest. A pair of 180° SPSP pulses was used to refocus the signal excited by a 90° SPSP pulse in order to select a 3D volume of interest in the SASSI sequence. Results: The SASSI pulse sequence produced spectra with more uniform brain metabolite SNR when compared with the conventional nonadiabatic MRSI sequence. Conclusion: SASSI achieved comparable SNR to the current adiabatic alternative, semi-LASER, but with 1/3 of the SAR. Magn Reson Med 76:1071–1082, 2016.