High-resolution mapping of human brain metabolites by free induction decay 1H MRSI at 7?T

This work describes a new approach for high-spatial-resolution 1H MRSI of the human brain at 7?T. 1H MRSI at 7?T using conventional approaches, such as point-resolved spectroscopy and stimulated echo acquisition mode with volume head coils, is limited by technical difficulties, including chemical shift displacement errors, B0/B1 inhomogeneities, a high specific absorption rate and decreased T2 relaxation times. The method presented here is based on free induction decay acquisition with an ultrashort acquisition delay (TE*) of 1.3?ms. This allows full signal detection with negligible T2 decay or J-modulation. Chemical shift displacement errors were reduced to below 5% per part per million in the in-slice direction and were eliminated in-plane. The B1 sensitivity was reduced significantly and further corrected using flip angle maps. Specific absorption rate requirements were well below the limit (similar to 20?%?=?0.7?W/kg). The suppression of subcutaneous lipid signals was achieved by substantially improving the point-spread function. High-quality metabolic mapping of five important brain metabolites was achieved with high in-plane resolution (64 x 64 matrix with a 3.4 x 3.4 x 12?mm3 nominal voxel size) in four healthy subjects. The ultrashort TE* increased the signal-to-noise ratio of J-coupled resonances, such as glutamate and myo-inositol, several-fold to enable the mapping of even these metabolites with high resolution. Four measurement repetitions in one healthy volunteer provided proof of the good reproducibility of this method. The high spatial resolution allowed the visualization of several anatomical structures on metabolic maps. Free induction decay MRSI is insensitive to T2 decay, J-modulation, B1 inhomogeneities and chemical shift displacement errors, and overcomes specific absorption rate restrictions at ultrahigh magnetic fields. This makes it a promising method for high-resolution 1H MRSI at 7?T and above. Copyright (C) 2011 John Wiley & Sons, Ltd.