IN-VIVO IMAGING OF METABOLITES USING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Nuclear Magnetic Resonance Spectroscopy (MRS) exploits transitions bet ween nuclear energy levels, in the radiofrequency range. In Biology an d Medicine, P-31 and H-1 are the most used nuclei. The in vivo MRS-vis ible molecules are small soluble metabolites of concentration larger t han 1 mM and involved in the energetic and lipid metabolisms. Physiopa thological studies performed in vivo by MRS on animals or human beings , require to distinguish signals arising from different organs or from lesioned and normal tissues. Spatial localization of the signal can b e obtained using two kinds of methods, both deriving from Magnetic Res onance Imaging techniques. The first one consists in acquiring a spect rum from a single voxel. The second one, Spectroscpic Imaging, allows spectra from all the voxels in a slice to be acquired simultaneously. Images of the spatial distribution of each resonance line detected in the spectra can thus be obtained. Proton MRS is mainly used for brain studies because of the presence, in other organs, of overlaping signal s from fatty acids which frequently obscure the spectra. The spatial r esolution achieved in vivo by H-1 MRS is about 5 mm(3) and 1 cm(3) in animal and human rat brain studies respectively. P-31 MRS can be appli ed to organs other than brain. Due to the low sensitivity of this nucl eus, the spatial resolution is about 30 cm(3) in human brain studies. Depending of the physiopathology studied, a compromise has to be made between spatial and time resolutions. The clinical studies aimed to ev aluate the interest of MRS for the diagnosis and the therapy control.