High-resolution human in vivo spectroscopic imaging using echo-time encoding technique.

A high-resolution human in vivo spectroscopic image was obtained using the 2.0-T NMR imaging system developed at KAIS (Korea Advanced Institute of Science). The in vivo spectroscopic image obtained has high spatial resolution as well as spectral resolution good enough for extraction of the detailed spectroscopic information. This high-resolution spectroscopic imaging was made possible by use of the recently developed echo-time encoded spectroscopic imaging technique which enabled us to optimize both the spatial and spectral resolutions necessary for the study of the nuclei of interest. In the present proton experiment, we have employed 16 encoding steps for the extraction of the spectral information which includes chemical shift (wk), field inhomogeneity and magnetic susceptibility (xM) mainly to discern distribution of the patent lipid protons as well as water protons. During the last several years, we have witnessed a number of interesting developments in the NMR spectroscopy and imaging areas (1-5). Among others, the spectroscopic imaging techniques of various types such as the 4-D Fourier technique (6-8) and the chemical-shift-selective (CHESS) techniques (9, 10) are found to be valuable assets in in vivo metabolic studies. In an attempt to resolve the spectra at each pixel in the reconstructed image plane, however, either the spectral or spatial resolution has been limited; e.g., in Dixon’s method (8) the spectral resolution has been limited, while the long imaging time which is necessary for Maudsley’s 4-D NMR imaging (7) has indeed been a difficulty for practical use when the high spatial resolution imaging is required. The above two techniques are, therefore, found to be of little use for high-resolution human in vivo spectroscopic imaging. More recently, a new technique (21-13) has been developed in an attempt to overcome the above difficulties, namely, the echo-time encoded spectroscopic imaging technique with which the spectral resolution can be arbitrarily selected without limitation on spatial resolution, thereby allowing us to choose any desired spectral and spatial resolutions by separately controlling the involved parameters, namely, the spatial and spectral resolutions and the imaging time. In Fig. 1, the pulse sequence of the proposed echo-time encoded spectroscopic imaging technique based on the Fourier imaging technique (2) is introduced. This technique has been used for chemical-shift artifact correction in proton imaging in a high field (22). The basic difference of the present method compared with the con-