Rilasso-tomografia quantitativa a risonanza magnetica per l 'analisi non distruttiva del degrado dei materiali lapidei

Stone decay is usually correlated to a variation of porosity and/or the Surface-to-Volume (S/V) ratios of the rocks. The variability of local porosity and V/S ratio (pore size) distributions in heterogeneous stone materials make difficult to obtain representative data by traditional methods. In recent years, Nuclear Magnetic Resonance (NMR) techniques established in the major field of Magnetic Resonance for fluids in Porous Media have been applied to the field of the Safeguard of Cultural heritage. In this paper Magnetic Resonance Quantitative Relaxation Tomography (MR-QRT) is presented. The visualization of the absorption and diffusion of liquid water in a decayed sample of marble coming from the Cathedral of "Santa Maria del Fiore " in Florence, and the quantitative analysis of the correlation between local porosity and S/V ratio will be given as examples of application. MR-QRT quantifies structural properties of the pore-space by means of NMR relaxation maps. Voxel-by-voxel, the relaxation curves of the 1H nuclei magnetization of the water saturating the pore-space are acquired. By combining 1H signal intensity and relaxation time of each voxel, it is possible to display and quantify how the porosity is shared among different classes of relaxation times, corresponding to different classes of S/V ratios. All this improves the knowledge of the pore-space structure. as an example, a voxel with low NMR relaxation time value contains water that is confined in high S/V ratio "pores", but does not necessarily corresponds to a low porosity voxel, and the method can distinguish between voxels having the same porosity but different S/V ratios.