Monitoring of caries lesions in enamel by Terahertz Pulsed Imaging

Objectives:

The aim of the present study was to evaluate the non-destructive imaging technique; Terahertz Pulsed Imaging (TPI) to quantify the depth of caries lesions in vitro after demineralisation and subsequent remineralisation.

Methods:

Ten enamel specimens (5x5mm) were prepared from bovine incisors. The samples were mounted in epoxy resin and polished using silicone carbide paper (2500grit). For the demineralisation study, specimens (n=5) were immersed in a 0.1M lactic acid buffer (pH 4.5) containing 1% CMC, 3mM CaCl₂ and 1.8mM KH₂PO₄ for 1, 3, 7, 14 and 21 days. For the remineralisation study, samples with 50ìm caries lesions (n=5) were placed into artificial saliva (Ca/P ratio 0.25) containing 0.02ppm sodium fluoride for 1, 2, 3, 6 ,10 and 16 days. All samples were imaged using a TPI Imaga 1000 system (Teraview Ltd). Each sample was positioned at the focus of the THz beam and the optics raster scanned in the x-y plane to collect a grid of data points with a spacing of 100µm.

Results:

TPI analysis of the demineralised specimens calculated average lesion depths of 16, 38 and 72µm after 1, 3 and 7 days in the demineralisation solution. However, after 14 days, surface erosion was clearly visible. This data was consistent with X-ray Micro CT analysis of the lesions.

Upon remineralisation, an increase in the magnitude of the reflected THz pulse from the lesion surface and a decrease from the lesion/sound enamel interface was observed (78% with respect to day 0 at day 16). This was consistent with a refractive index change as a result of the increasing mineral content of the lesion.

Conclusion:

The data suggests that TPI is a potentially useful, non destructive imaging technique to quantify both the depth of caries lesions and the subsequent changes in mineral content due to remineralisation.