# Quantitative Assessment Methods of Early Enamel Caries with Optical Coherence Tomography: A Review
## 4 Quantitative Assessment Methods of Early Enamel Caries with OCT
### Quantitative Assessment Based on Reflectivity  
Many studies have used reflectivity for the quantitative assessment of early caries, since the reflectivity of caries lesions can be obtained directly from OCT signals. A commonly used quantitative index related to reflectivity is the integrated reflectivity for caries detection. A line profile for each lesion depth is taken from B-scans, and the integrated reflectivity (\(\Delta R,\text{dB}\times\upmu\text{m}\)) can be calculated by integrating the reflectivity from the enamel surface to various depths [50]. The observed optical depth should be divided by the enamel refraction index (n = 1.63) to determine the real lesion depth when using the line profile. Most studies have confirmed that integrated reflectivity increases after demineralization and decreases after remineralization [49; 50; 57; 59; 60]. [PERSON] et al. [49] utilized a fixed depth algorithm and an edge detection algorithm to calculate integrated reflectivity. In the first algorithm the integration was performed to a fixed depth that needed to be greater than the maximum lesion depth, while the second algorithm could obtain the depth of the lesion. The results showed both algorithms were able to detect the difference in demineralization from 0 to 4 days, except that the fixed depth algorithm yielded a higher integrated reflectivity. [PERSON] et al. [50] calculated the mean integrated reflectivity of artificial caries prepared by applying a 14-day pH cycle model based on the perpendicular axis PS-OCT images, and the result was 450 \(\pm\) 110 arbitrary units. Meanwhile, it was demonstrated that the integrated reflectivity calculated by PS-OCT was linearly correlated with the relative mineral loss determined by DM (r = 0.755). [PERSON] et al. [59] detected demineralization around adhesive-bound orthodontic brackets in vivo using CP-OCT for a period of 1 year and acquired 2D projection images of \(\Delta R\) with automated algorithms. The results indicated \(\Delta R\) for both adhesives increased remarkably with time, varying in the range from 10.2 (10.5) to 29.7 (9.4) \(\text{dB}\times\upmu\text{m}\). PS-OCT was applied to monitor the process of remineralization of caries using an acid remineralization model by [PERSON] et al. [60]. There were significant alterations in the integrated reflectivity of the lesion region after remineralization, from 257 \(\pm\) 60.2 to 168 \(\pm\) 58.5 \(\text{dB}\times\upmu\text{m}\). [PERSON] et al. [61] scanned demineralized bovine teeth using 850-nm OCT and demonstrated that \(\Delta R\) of the enamel reduced with the time of demineralization. They proposed the percentage reflectivity loss (\(R_{\%}\)) as a quantitative index as follows:  
\[R_{\%}(\text{dB}\times\text{mm})=\frac{(R_{\text{sound}}-R_{\text{demineralized}} )}{R_{\text{sound}}}\times 100\% \tag{1}\] \begin{table}
\begin{tabular}{c c c c} \hline
**Reference** & **OCT System** & **Sample Preparation** & **Lesion Depth Result** \\ \hline \multirow{8}{*}{[PERSON] et al. [57]} & \multirow{8}{*}{1310-nm PS-OCT, axial/lateral resolution 11/30 μm.} & Sample number: 20 human posterior teeth for in vitro detection & \\ \cline{3-4}  & & Solution: pH 4.9 & \\ \cline{3-4}  & 1310-nm PS- & \multirow{2}{*}{\
\begin{tabular}{} \end{tabular} } & \multirow{2}{*}{
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\end{table} 
Table 1: _Cont._where \(R_{sound}\) is the reflectivity of sound enamel and \(R_{dmineralized}\) is the reflectivity of enamel lesion. The results showed that \(R_{\%}\) increased from 54.0 \(\pm\) 11.27 to 86.64 \(\pm\) 7.57 dB \(\times\) \(\upmu\)m with demineralization time. In a follow-on study it was demonstrated that \(R_{\%}\) was linearly correlated with both the mineral loss determined by transverse microradiography (TMR) (\(r=1.00\)) [62] and the percentage of fluorescence loss calculated (\(\Delta Q\)) by QLF (\(R_{\%}=45.56+\Delta Q,r=0.963\)) [63].