Optimization of 1H‐MRS methods for large‐volume acquisition of low‐concentration downfield resonances at 3 T and 7 T

Purpose This goal of this study was to optimize spectrally selective 1H‐MRS methods for large‐volume acquisition of low‐concentration metabolites with downfield resonances at 7 T and 3 T, with particular attention paid to detection of nicotinamide adenine dinucleotide (NAD+) and tryptophan. Methods Spectrally selective excitation was used to avoid magnetization‐transfer effects with water, and various sinc pulses were compared with a band‐selective, uniform response, pure‐phase (E‐BURP) pulse. Localization using a single‐slice selective pulse was compared with voxel‐based localization that used three orthogonal refocusing pulses, and low bandwidth refocusing pulses were used to take advantage of the chemical shift displacement of water. A technique for water sideband removal was added, and a method of coil channel combination for large volumes was introduced. Results Proposed methods were compared qualitatively with previously reported techniques at 7 T. Sinc pulses resulted in reduced water signal excitation and improved spectral quality, with a symmetric, low bandwidth‐time product pulse performing best. Single‐slice localization allowed shorter TEs with large volumes, enhancing signal, whereas low‐bandwidth slice‐selective localization greatly reduced the observed water signal. Gradient cycling helped remove water sidebands, and frequency aligning and pruning individual channels narrowed spectral linewidths. High‐quality brain spectra of NAD+ and tryptophan are shown in 4 subjects at 3 T. Conclusion Improved spectral quality with higher downfield signal, shorter TE, lower nuisance signal, reduced artifacts, and narrower peaks was realized at 7 T. These methodological improvements allowed for previously unachievable detection of NAD+ and tryptophan in human brain at 3 T in under 5 min.