Methodological developments towards quantitative short TE in vivo :sup:1:/sup:H NMR spectroscopy without water suppression

Abstract

Water suppression (WS) has been used exclusively as a routine for almost two decades in the in vivo 1H NMR spectroscopy to avoid the experimental and postprocessing difficulties caused by the dominant water signal, which is 3 to 5 orders magnitude larger than the signals of metabolites of interest. However, WS has some disadvantages as compared to the spectroscopy without WS. This thesis is devoted to methodological developments towards short echo time (TE) in vivo proton NMR spectroscopy without WS. The methodology developed in this thesis integrates experimental and software approaches for the optimization of the spectroscopic techniques, which concerns the two major parts, namely, the spectral measurements and spectral quantification. An experimental method was developed to eliminate frequency modulation sidebands induced by gradient pulses, a major obstacle of in vivo 1H NMR spectroscopy without WS. First order phase errors were eliminated by radio frequency pulse sequence optimization. The dominant water signal was modeled and extracted with a scheme developed on the basis of the Matrix Pencil Method, The extracted water signal was used as a reference for the lineshape correction. With these approaches the imperfections of the spectra were removed. Spectral quantification scheme was developed by combining the baseline characterization by a wavelet transform based technique and the time domain spectral fitting using full prior knowledge of the metabolite model spectra. The model spectra were obtained by spectral simulation instead of in vitro measurements. The performance of the methodology was tested and verified by Monte Carlo studies, phantom measurements and in vivo measurements on rat brain, and compared with existing methods. More than 10 metabolites can be quantified from spectra measured on a 4.7 T NMR imaging system. The estimated concentrations of major metabolites are in good agreement with literature values.