Les bases physico-chimiques du contraste en imagerie par résonance magnétique

Service Hospitalier Frédéric-Joliot, DRIPP, CEA, Orsay, France.

Résumé

L’Imagerie par Résonance Magnétique (IRM) a atteint au cours de la dernière décennie un niveau de développement technologique qui en fait une technique de mesure fiable et non destructrice. Cet article rappelle, dans une première partie, le principe de la localisation spatiale et de la mesure du signal en IRM. Dans une seconde partie, il examine différentes informations apportées par l’IRM, principalement dans le domaine biomédical, sur la relaxivité, le mouvement des spins et l’information chimique.

Abstract

Magnetic Resonance Imaging (MRI) underwent major technological improvement for last ten years, so as to reach the status of a reliable measurement technique. Spatial location is obtained by using small linearily space-varying magnetic fields. Imaging pulse sequences are based on gradient-recalled-echo and spin-echo techniques. Now, due to hardware improvement, spatial resolution down to 10^-4 mm³ has been reached, as well as subsecond imaging acquisition times. We examine the physical information (relaxivity and spin motion) and the chemical information provided by MRI, mainly in the biomedical domain. Relaxation rates, the more important contrast factors in biological systems, are due to several mechanisms : solvatation of macromolecules, proton exchange, molecular diffusion, electronical paramagnetism. They contain complex information uneasily used to characterize biological mechanisms. Phase maps or signal attenuation maps in the presence of intense gradient fields may be established to quantify coherent fluid motion as well as brownian diffusion. Finally, spectroscopic acquisitions may be performed, taking advantage of the spatial localization techniques, then suppressing the gap between spectroscopy and MRI.