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. © 2015 Wiley Periodicals, Inc.