# Potential Imaging Capability of Optical Coherence Tomography as Dental Optical Probe: A Mini-Review
## 1 Introduction  
Optical coherence tomography (OCT) is a non-invasive imaging tool that is emerging in various fields of medicine including dentistry. It can provide cross-sectional images of both hard and soft tissues with micrometric resolution [1]. Using the principle of low-coherence interferometry, OCT images are generated by measuring the intensity and time delay of the reflected or backscattered near-infrared light from the tissue structure, which is analogous to the underlying principle of medical ultrasound imaging. OCT can provide near real-time or video rate in situ images of tissues, owing to its high acquisition speed [2].  
OCT can be used as an alternative to invasive diagnostic methods (e.g., biopsy or exploratory surgery), histological examination, and other imaging modalities. A comparison of OCT and other imaging methods is presented in Table 1. Although the principle of OCT is comparable to that of ultrasonography, both methods use different source images. OCT, which has limited tissue penetration, can provide a higher resolution than ultrasonography [3]. The avoidance of ionizing radiation can also be considered an advantage of OCT imaging. Owing to the aforementioned advantages, OCT has been used in manymedical and dental research projects aiming to optimize its utilization as an experimental and diagnostic tool.  
In dentistry, OCT has been widely used as an experimental tool rather than a diagnostic tool due to portability issues and its high initial cost. The use of OCT for clinical purposes in dentistry is also still limited. Therefore, several studies have attempted to optimize the advantages of OCT technology in dentistry. As a modern imaging tool, OCT can assist dentists to improve diagnosis by providing non-invasive high-resolution images in real time without the use of ionizing radiation. In conservative dentistry, OCT has been commonly applied to hard tissue or restoration material (that is, diagnosis of caries [6; 7; 8; 9], detection of tooth cracks [8; 9; 10], and assessment of the tooth-restoration interface [8; 11; 12; 13]). Compared to clinical and radiological assessments, OCT can improve caries detection due to its micrometer resolution images [14]. Moreover, OCT can distinguish tooth demineralization from healthy tooth tissues. It can also be used to detect the gap formation between the tooth and restoration material, which may lead to microleakage [8].  
To date, OCT has been used to visualize periodontal tissues, including hard and soft tissues in the subgingival area [3; 15]. Clinically, a subgingival condition is generally examined using a periodontal probe since radiographical assessment cannot accurately depict the structure of this area in sufficient detail. However, the probing method, which can be considered as an invasive method, can be painful and at times inaccurate since it is performed without visual guidance. Therefore, the purpose of this review was to discuss the potential of OCT as a dental optical probe and answer the focus question: \"What is the imaging capability of OCT to visualize subgingival area?\". In this review, the feasibility of OCT to visualize the subgingival area, including periodontal tissue, peri-implant condition, and marginal fit of prosthetic crown restorations have been discussed.