We compare terahertz-pulsed imaging (TPI) with transverse microradiography (TMR) and microindentation to measure remineralization of artificial caries lesions. Lesions are formed in bovine enamel using a solution of 0.1 M lactic acid/0.2% Carbopol C907 and 50% saturated with hydroxyapatite adjusted to pH 5.0. The 20-day experimental protocol consists of four 1 min treatment periods with dentifrices containing 10, 675, 1385, and 2700 ppm fluoride, a 4-h/day acid challenge, and, for the remaining time, specimens are stored in a 50:50 pooled human/artificial saliva mixture. Each specimen is imaged at the focal point of the terahertz beam (data-point spacing = 50 μm). The time-domain data are used to calculate the refractive index volume percent profile throughout the lesion, and the differences in the integrated areas between the baseline and post-treatment profiles are used to calculate ΔΔZ(THz). In addition, the change from baseline in both the lesion depth and the intensity of the reflected pulse from the air/enamel interface is determined. Statistically significant Pearson correlation coefficients are observed between TPI and TMR/microindentation (P < 0.05). We demonstrate that TPI has potential as a research tool for hard tissue imaging.
The aim of this study was to determine the ability of Terahertz Pulsed Imaging (TPI) to measure mineral changes in enamel lesions during de/remineralisation studies. A comparison was made between transverse microradiography (TMR) and microhardness measurements. Artificial lesions were formed in bovine enamel using a solution of 0.1 M lactic acid (pH 5.0) containing 0.2% Carbopol C907 and 50% saturated with hydroxyapatite. The 20 day experimental protocol consisted of four, one-minute treatment periods with dentifrices containing 10, 675, 1385 and 2700ppm fluoride, a 4 h/day acid challenge, and for the remaining time specimens were stored in a 50:50 pooled human / artificial saliva mixture. Terahertz images were generated by positioning the specimens at the focus of the beam and raster scanning the optics to collect the reflections from the air / enamel (AEI) and lesion / enamel (LEI) interface. Significant differences were observed in the intensity change from baseline of the AEI and LEI reflections upon treatment with the four dentifrices. A linear correlation was observed between ΔAEI vs ΔVHN (r2 = 0.997), ΔAEI vs ΔKHN (r2 =0.964), ΔII (ratio of LEI to AEI) vs ΔΔZ (r2 =0.875) and ΔLEI vs ΔΔZ (r2 =0.870). Statistically significant correlations (p<0.05 Pearson correlation coefficient) were also found between the TPI and microhardness / microradiography data. This study has demonstrated that TPI is a useful technology to measure in vitro (and possibly in situ) mineral changes in enamel and is sufficiently sensitive to discriminate between the levels of remineralization produced by the different dentifrices.