Short-echo, single-shot, full-intensity proton magnetic resonance spectroscopy for neurochemical profiling at 4 T: Validation in the cerebellum and brainstem

A short echo time (TE = 24 ms) semiadiabatic localization by adiabatic selective refocusing (LASER) sequence was designed and optimized for full-intensity proton magnetic resonance spectroscopy (¹H MRS) at 4 T. The sequence was combined with VAPOR water suppression and three-dimensional outer volume suppression for improved localization and suppression of unwanted coherences. Artifact-free, single-shot spectra were obtained from the human brain with a spectral pattern almost identical to that obtained with an ultra-short TE (TE = 5 ms) stimulated-echo acquisition mode (STEAM) sequence as a result of the train of adiabatic refocusing pulses in semi-LASER that reduce the apparent TE. Approximately 2-fold higher signal intensity relative to STEAM was demonstrated in phantoms and the human brain. To test the performance of the sequence in clinically relevant brain regions with a volume coil, semi-LASER spectra were acquired from three cerebellar and brainstem volumes of interest (VOIs) in 23 healthy subjects. Ultra-short echo STEAM spectra were acquired from the same VOIs to compare neurochemical profiles obtained with semi-LASER with those obtained with STEAM. Neurochemical profiles of the cerebellum and brainstem acquired by these two techniques were nearly identical, validating the accuracy of the metabolite concentrations obtained with semi-LASER at the longer TE relative to STEAM. A high correlation between metabolite concentrations obtained by these two proton ¹H MRS techniques indicated the sensitivity to detect intersubject variation in metabolite levels.