**2. Optical Properties of Teeth**

Biological tissues' optical properties have important theoretical significance and application value for the diagnosis of diseases. The variation in optical properties of tooth tissue during demineralization are the basis of OCT caries detection [29].

(1) Refractive Index

Refractive index describes the reflection and refraction of light as well as the temporal properties of light transmission within a tissue. The refractive index of tooth enamel is 1.62 to 1.65 [30,31]. Demineralization in caries lesions alters the refractive index of enamel [32], and can be considered as an index of its scattering properties [33].

Since hydroxyapatite in tooth enamel is an anisotropic crystal, tooth enamel has birefringent properties [18]. When polarized light propagates through biological tissues, its polarization state is altered by the scattering properties of the tissue. By detecting changes in the polarization state of backscattered light from dental tissue, microstructural information of the tissue can be obtained, and caries lesions can be detected.

#### (2) Scattering Properties

Dependent on the structure of hard dental tissue, two types of scattering occur when light is transmitted inside the tooth. Dental enamel is an ordered array of hydroxyapatite crystals surrounded by a protein/lipid/water matrix. The diameter of hydroxyapatite crystals is about 30 ∼ 40 nm, and the crystals are clustered together into enamel rods with a diameter of 4 μm [34]. In the near-IR (NIR), the crystal diameter is much smaller than the source wavelength, so it is in accordance with the Rayleigh scattering law. The diameter of the enamel rods is comparable to the source wavelength, so it obeys the Mie scattering law.

Enamel has the greatest transparency in the NIR close to 1310 nm. The attenuation coefficients of sound enamel at 1310 nm and 1550 nm are 3.1 cm−<sup>1</sup> and 3.8 cm−<sup>1</sup> [35], respectively, which are much lower than in the visible region. Thus, the NIR spectrum is suited for tooth imaging to identify lesions [36]. Micropores form in the lesion as the mineral crystals are partially dissolved during the demineralization process, and behave as scattering centers. During initial lesion development, the scattering coefficient has an exponential increase with increasing mineral loss. As the severity of the lesion increases, the scattering gradually increases [29].

#### (3) Absorption Properties

The absorption of enamel is quite weak in NIR. The absorption coefficient (*μa*) of enamel at 1310 nm is around 0.12 cm−<sup>1</sup> [36], which is much smaller than the scattering coefficient. Therefore, scattering plays a major role in light transmission through the tooth.

#### **3. Preparation of Artificial Carious Lesions**

The study of early caries using OCT requires tracking of the caries process, but natural early caries takes 6–18 months, and there are many influencing factors. Therefore, the preparation of artificial caries using isolated teeth is the main way to study early caries. The key to preparing artificial caries is to simulate the chemical and microbial environment of natural caries formation. There are two main methods to prepare artificial caries: chemical and biofilm methods. Chemical methods can be used for enamel surface demineralization by adjusting the chemical state and pH of a using a partially saturated acidic buffer [37,38] or an acid gel [39]. This method is easy and the most widely used, while a biofilm method is performed by placing a cariogenic suspension on the surface of the tooth, altering the metabolic environment of the bacteria and creating a carious sample that more closely resembles natural caries [40,41]. However, the biofilm method is generally used for studies related to the pathogenesis of dental caries because it is difficult to control the experimental conditions.

## **4. Quantitative Assessment Methods of Early Enamel Caries with OCT**

Most of the current caries detection studies based on OCT use frequency domain OCT, which eliminates axial scanning of the reference arm and overcomes the disadvantage of slow scanning speed of time-domain OCT (TD-OCT) [42]. Moreover, some functional OCT systems are employed for caries detection due to the birefringent properties of dental hard tissues. Polarization-sensitive OCT (PS-OCT) as a type of functional OCT system can provides additional polarization-sensitive information about the sample compared to conventional OCT systems [43–46]. In addition, as a new type of PS-OCT, Cross Polarization OCT (CP-OCT) not only reflects the polarization-sensitive structural information of the sample, but also better reveals the superficial microstructure of the sample by attenuating the effect of strong reflected light on the sample surface [47].
