*4.9. Quantitative Assessment Based on the Dehydration Parameter*

Recent research has revealed the impact of hydration on OCT images by conventional polarization-insensitive OCT [83,84]. A method for assessing early enamel lesions with the dehydration parameter (DH) based on the integrated reflectivity has been presented [85]. The dehydration parameter refers to the positive difference between the two OCT signals of tooth enamel under dry and hydrated conditions, i.e., the integrated area between the two signals.

Nazari et al. [85] detected sound and demineralized bovine enamel blocks after 3, 9, and 15 days of demineralization using 1310-nm SS-OCT with axial/lateral resolution 11/17 μm, and calculated the dehydration parameters. The experimental results showed DH for sound and demineralized bovine enamel ranged from 272(204) to 3304(751), and a strong linear correlation (*R*<sup>2</sup> = 0.9922) between the dehydration parameter and the square root of demineralization day. The benefit of this method is that the DH calculation does not involve determination of the cut-off depth, and the evaluation results are not influenced by

surface reflections. Although this method has the potential to quantitatively assess early enamel caries, there are few relevant studies, and optimization of the methodology requires evaluation of large amounts of demineralized and remineralized enamel.
