Determination of pore space in enamel by X-ray microtomography

Objectives: To investigate measurement of accessible pore space in enamel by X-ray microtomography (XMT). Theory: XMT gives 3D measurements of linear attenuation coefficient (LAC). For enamel, mineral concentration can be calculated from LAC to a very good approximation. If an X-ray attenuating species enters accessible pores, its concentration can be calculated, as mass/unit volume of the tissue, from the increased LAC. If the density of the species is known, the volume occupied can be determined. Methods: Three specimens (~3x4x5 mm) cut from crowns of permanent teeth were coated with varnish, leaving natural enamel surface exposed. A high purity (99.999%) Al wire (1 mm diam.) was mounted on each specimen. Following a baseline XMT scan, each specimen was immersed in KI solution (1 or 3 mol dm^-3) for £ 47 days. On typically 4 occasions, each specimen was removed, blotted, and rescanned. After reconstruction of each XMT data set, the specimen LACs were standardized against LAC measured for the Al standard. For each period of immersion, mean LAC was determined for 19897 voxels (~0.07 mm³) at 3 sites in 2 specimens and 9 sites in the third. Results: Mineral concentrations for sound enamel were 2.65-2.76 g_HApcm^-3 (based on composition of stoichiometric hydroxyapatite). From increases in LAC, £ 6 vol.% of enamel was determined to be accessible to KI solution. Successive measurements of accessible pore space at each site varied, but did not correlate with immersion time in KI solution. However, the maximum pore space determined at each site varied inversely with the mineral concentration at that site. Conclusions: XMT measurements of KI uptake by enamel can provide an estimate of accessible pore space, but development of the method is needed to improve accuracy. Acknowledgements: We thank the MRC (Programme Grant no. G9824467) and the EPSRC (Grant No. GR/R28911/01).