# Biophotonics in Dentistry
## 3 Applications in Dentistry
### Optical Coherence Tomography  
Optical coherence tomography (OCT) is a non-invasive, non-destructive, high-resolution, cross-sectional imaging technique widely used in various clinical applications, including ophthalmology, dentistry and gastroenterology [108]. It may be considered equivalent to ultrasound imaging but with light waves instead of ultrasound. OCT is a powerful technique that captures real-time images of tissue in situ without the need for biopsies, achieving image resolution of 1-15 \(\upmu\)m, comparable to that of optical microscopy [109]. OCT is another interferometry method using low-coherence light. A photodetector is used to capture interference of reflected and scattered light. Different systems of OCT have been developed over the years, including doppler OCR (D-OCT), polarisation-sensitive OCT (PS-OCT), spectral-domain OCT (SD-OCT) and endoscopic OCT. The applications of OCT in dentistry and other biomedical applications are vast. OCT was first applied in dentistry in 1998 as a means of capturing in vivo images of hard and soft dental tissues [110]. The study obtained high-resolution in vitro images and was significant in showing the potential for OCT in diagnosing periodontal tissue. This was also one of the first studies to use the OCT technique to image hard tissue, and a greater understanding of the structure of teeth was achieved. Another application of OCT in dentistry is the detection of microleakage of restorations and fillings, as well as evaluation of their integrity [109]. A quality assessment of various dental treatments using OCT was conducted by [PERSON] et al. (2008) [111]. The study evaluated the use of OCT to study material defects and microleakage of dental prostheses. The applicability of OCT in this application was demonstrated successfully. Later, OCT was used as a means of investigating microleakage after endodontic treatment using lasers, and the conclusion reached was that the use of lasers improved prognosis and reduced microleakage [112]. More recently, in 2015, a study using swept-source OCT (SS-OCT) demonstrated the detection of in vivo microleakage, and the study concluded OCT to be a reliable technique for location and size analysis of a microleakage [113]. This is significant because the early detection of microleakage could prevent secondary caries that arise from accumulated bacteria stuck within microleakages. One limitation of the study was that it provided information for large-crack detection only; therefore, more research is needed on different materials and different crack sizes. Within this study, OCT was used to observe the growth of cracks and the resultant damage to a ceramic/enamel interface under fatigue shear testing [113]. Figure 11[113] shows the images of the ceramic/enamel interface obtained via OCT.  
Periodontal disease and dental caries are long-prevailing issues, and there is ongoing research concerning rapid, non-invasive diagnosis and treatment. In 2010, OCT was presented as a novel non-invasive approach to diagnose periodontal disease, with the aim of replacing clinical examinations or synergistically acting as a diagnosis aid [114]. PS-OCT in particular is useful for imaging caries and monitoring existing lesions on teeth. A common conventional technique for the detection of carries is radiography; however, the resolution of radiography is lower than that of OCT, and it only provides two-dimensional images [108]. Transversal microradiography, which has also been utilised in this application, has limitations in that it requires a thin sectioning process for imaging [115]. Exposure to radiation is an additional concern with the use of radiography in dentistry. Ultimately, compared to OCT, conventional methods do not provide the resolution, sensitivity and contrast required; therefore, early detection of lesions, periodontal disease and oral cancer can be difficult with conventional imaging techniques until they are well advanced, and remineralisation is not possible [108; 115].