## 4 Discussion  
PSOCT-based representations of depolarization were presented for imaging enamel demineralization induced by an artificial lesion model and compared with PLM images of corresponding thin sections. Even though optical coherence tomography is not capable of resolving histological features as represented by microscopy, previous work has demonstrated the potential of assessing the related intrinsic characteristics from the scattering and attenuating properties [32]. Here, PSOCT enables the nondestructive detection of scattering and polarization properties linked to early  
Figure 4: (**A**) Measured lesion depth by PLM over the duration of acid exposition, i.e., stage of demineralization. (**B**) Scatter plot of the measured lesion depth by PLM and mean DOP at the central region of interest (ROI) of the slab. Linear regression (red line) shows a coefficient of determination of \(R^{2}=0.7118\). Figure 3: PLM image (40\(\times\) magnification) of an enamel slab at 25 days of demineralization with characteristic zones of lesion formation. Scale bar: 80 \(\upmu\)m.  
demineralization, which typically cannot be detected in bitewing radiographs. For this purpose, we adapted the noise-immune DOP as proposed by [PERSON] et al. [PERSON] et al. (2018) and found a visible depolarization contrast at DOP representations for early demineralization stages that were confirmed by PLM images. Besides the common B-scan representation providing a section plane similar to PLM, en-face imaging provided an overview of the extent of the demineralization area. These sample-specific variations, resulting in variations of the measured lesion depth over the investigated time period (Figure 4A), could be due to variations of the surface and sample morphology, i.e., microcracks or surface roughness. In addition, flattening and polishing during the manual preparation process of the slabs presumably made the surface more susceptible to dissolution compared to clinical conditions [PERSON] et al. (2018). The simplified demineralization model produced characteristic zones of demineralization (see Figure 3) that can be found in natural enamel lesions [PERSON] et al. (2018); [PERSON] et al. (2018). It should be noted that this model did not aim to simulate in situ conditions considering remineralization and cycle-based processes, but rather to assess the suitability of DOP imaging for monitoring the initial caries process under constant conditions and the relation to progression stage. Hereby, the regression analysis shows a clear linear correlation between the lesion depth and the calculated mean DOP values. However, high DOP values were associated with a high variation of lesion depths, whereas this relationship was more consistent with a linear decrease of the DOP at demineralization depths >150 um. Since the results of PLM showed that those demineralizations were dominated by the body of the lesion, we hypothesize that changes of polarization that contributes to the DOP representations are mainly determined by structures that exhibit a pore volume of 5%-25% (sound enamel: 0.1%) [PERSON] et al. (2018). This hypothesis is supported by depolarization measurements based on polarization-resolved Raman microscopy at initial enamel caries that showed that the depolarization ratio of a hydroxyapatite-associated band intensity is also primarily determined by the body of the lesion [PERSON] et al. (2018). A further demineralization with an increased exposition time to the acetic buffer solution could expand the correlation to lower depolarization values and higher lesion depths. Based on the observed heterogeneous demineralization process, it should be considered that this correlation could be impaired by an inaccurate matching of the selected thin section for PLM and the analyzed DOP region. In order to minimize specimen-specific variations, DOP analysis of the same specimen for the entire demineralization protocol will be part of further investigations.