## 6 RMS Characterization of Dental Lesions
### RMS Characterization of Early Enamel Lesions  
The early detection of enamel lesions, as well as variations in the composition/structure of both enamel and dentin, represent important issues in modern dentistry. In this light, RMS is a valuable tool to detect various lesions at an early stage of formation, since it allows for the collection of high-resolution maps and is highly sensitive to small changes in enamel composition [61].  
The first clinically visible stage of carious disease is characterized by enamel demineralization without cavitation, known as white spot lesions (WSLs). However, in the absence of effective treatment, cavitation may occur, thereby increasing the necessity of invasive restorative treatments [87]. WSLs appear as milky white opaque spots on dental enamel. The translucency of enamel is an optical phenomenon that depends on the size of the intercrystalline spaces. In early stages, active caries require air drying to be visible, as the dissolution process of crystals at the outer enamel surface begins. Further enlargement of intercrystalline spaces results in a white patch that is visible without dryness. The effect of dehydration on enamel translucency is a result of the replacement of water content around enamel prisms with air. In a heterogeneous system, such as enamel prisms surrounded by a fluid medium, scattering occurs due to differences in the refractive indices (RI) of the two involved components. As the RI of enamel is approximately 1.65, while that of water is 1.33 and that of air is 1.00, a greater difference among RI values produces a greater scattering at the enamel/air interface [88]. [PERSON] et al. and [PERSON] et al. successfully provided Raman spectral imaging characterization for early caries [38; 89]. Indeed, the high spatial resolution (300 nm) makes it an excellent tool for analyzing human enamel components, enabling the detection of WSLs at an early stage of development [21]. In order to define the levels of enamel demineralization after exposure to acid environments, some authors have analyzed the width of the bands related to phosphate vibrations (such as the band centered at 960 cm\({}^{-1}\)) [90].  
In a recent study, chemical mapping of dental enamel was carried out over cross sections of specimens with natural and artificial WSLs [61]. The authors observed a severe decrease in the intensity of the PO4\({}^{3-}\) peak in the area corresponding to the body of the lesion, with respect to sound enamel. A similar trend was shown by C, which abruptly decreases in the lesion zone, while it starts to gradually increase in the intermediate zone, reaching its maximum value in sound enamel beyond the subsurface lesion. These findings are related to some alteration in crystal size and shape and, consequently, to reduced apatite perfection of the enamel prism. Finally, false color images derived from the CO3\({}^{2-}\)/PO4\({}^{3-}\) ratio, used to analyze changes in enamel inorganic components in each zone of the lesion, revealed an increase in the ratio in the lesion, in comparison to sound enamel. Hence, it can be stated that in WSLs, enamel becomes less compact due to the increase of inter-prismatic voids, and less mineralized due to the decrease in mineral density and the increase in the organic matrix content [61].