Multi-parametric µMRI and Synchrotron radiation-based XPCT for studying human bone tissue in archaeological contexts

In this study, the microporosity and the microstructural topology of archaeological cortical bones from a Roman necropolis, dated to the 1st-3rd century CE, were investigated using two approaches: a direct method based on Synchrotron radiation-based X-ray phase contrast tomography (SXPCT) images and an indirect method, based on a model to extract pore size utilizing data derived from Magnetic Resonance micro-imaging (µMRI) weighted in T2 relaxation times and molecular diffusion. This study aimed to understand the interchangeability of these techniques, i.e. whether they provide the same or complementary information. To validate this, a multiscale approach based on High resolution SXPCT and µMRI was performed on the same samples and compared the results. The study was carried out on three archaeological tibia samples. One of these samples was affected by periostitis, one was healthy and in a good state of preservation and one showed strong signs of post-mortem degradation. This selection was made by surface inspection of the bone samples by an expert archaeologist. The potential value of subsurface and volumetric examination using two non-destructive tomographic techniques based on X-ray and Nuclear Magnetic Resonance will be investigated. Since the SXPCT technique is well established for microscopic studies of archaeological bones, but its cost is significantly higher than a µMRI examination and its access is limited, a comparison of the SXPCT and µMRI results was made, highlighting their agreement, differences and complementarities. The pore size dimension obtained by µMRI, is underestimated compared to that obtained by SXPCT, which agrees with the literature. This is due to the high value of magnetic susceptibility differences between the bone matrix and water inside the pores, which increases with the magnetic field. To extract pore sizes by µMRI, relaxivities were quantified at 9.4 T. The relaxivity value of the periostitis sample was significantly lower than that of healthy and taphonomic samples. Relaxivity depends on several factors, not only related to nano-micromorphology of pore walls, but also related to bone chemical and biochemical constituents. For this reason, it is suggested that the interplay between its values, together with the internal microstructure immediately below the bone surface, could indicate the origin of periostitis lesions, from trauma and hard work, or from bacterial infections. SXPCT allows 3D visualization of Haversian canals, which appear less ordered and have a smaller cross-sectional area in the periostitis sample compared to healthy and taphonomic samples.