The Use of Optical Coherence Tomography in Dental Diagnostics: A State-of-the-Art Review

Optical coherence tomography provides sections of tissues in a noncontact and noninvasive manner. The device measures the time delay and intensity of the light scattered or reflected from biological tissues, which results in tomographic imaging of their internal structure. This is achieved by scanning tissues at a resolution ranging from 1 to 15 μm. OCT enables real-time in situ imaging of tissues without the need for biopsy, histological procedures, or the use of X-rays, so it can be used in many fields of medicine. Its properties are not only particularly used in ophthalmology, in the diagnosis of all layers of the retina, but also increasingly in cardiology, gastroenterology, pulmonology, oncology, and dermatology. The basic properties of OCT, that is, noninvasiveness and low wattage of the used light, have also been appreciated in analytical technology by conservators, who use it to identify the quality and age of paintings, ceramics, or glass. Recently, the OCT technique of visualization is being tested in different fields of dentistry, which is depicted in the article.


Introduction
Medical imaging is the basis of effective medical diagnosis and is now the mainstream of a dynamically developing branch of science, which is biomedical engineering. Its development started after an accidental discovery of Wilhelm Conrad Roentgen, a professor of physics, who in 1895 observed little fluorescence during his research on electrical discharges and cathode rays. X-radiation turned out to be a fundamental discovery which is still used in medicine today.
Another milestone was the development of the first computed tomography (CT) device by Godfrey Newbold Hounsfield in 1967. The concept of tomography refers to a method that provides images showing sections of the tested structure. The first CT scanner initiated rapid development of medical imaging techniques. A common feature of different types of CT devices is noninvasive imaging of tissue structures and internal organs, as well as their functional parameters. The desire to minimize invasiveness of methods such as biopsy or exploratory surgery, which are painful and may cause deterioration in the patient's condition, was an impetus for the improvement of computed tomography equipment. As a result, completely new technologies were developed, such as magnetic resonance imaging (MRI), ultrasonography (USG), positron emission tomography (PET), single photon emission computed tomography (SPECT), and the latest and more widely used optical coherence tomography (OCT).
The method of optical coherence tomography using interferometry with partially coherent light was first presented in 1991 at the Institute of Technology of the University of Massachusetts [1]. The first in vivo measurements of the section of the human retina were made two years later in Vienna [2]. The first commercial optical tomography device was produced in 1996 by Zeiss-Humphrey [3].
The article depicts the types of optical tomographs and the schematic construction based on the academic knowledge and enunciates the up-to-date knowledge concluded in the articles accessible in the US National Library of Medicine National Institutes of Health (PubMed), Dentistry & Oral Sciences Source EBSCO, and the http://octnews.org website.

Types of Optical Coherence Tomography
Optical coherence tomography (OCT) uses a beam of partially coherent light to create tomographic images. Currently, there are two basic types of optical coherence tomography: time domain optical coherence tomography (TdOCT) and Fourier domain optical coherence tomography (FdOCT). The former technique was developed in 1991 by the abovementioned group of researchers from the Massachusetts Institute of Technology in the United States [1] for use in ophthalmic diagnosis. It can produce tomographic images of relatively low quality, resulting from long time of measurement, but it does not allow for three-dimensional imaging of objects [4]. Modern optical tomography with detection in the frequency domain (Fourier domain optical coherence tomography) reduces the capture time by more than a hundred times and creates three-dimensional images of the test object.
Optical coherence tomography enables the study of objects that are partially transparent for light from the near infrared range. In the OCT scanner based on this method, the information about the location of scattering (reflecting) layers along the sample beam is contained in the modulation frequency of the light intensity measured as a function of frequency. The electric signal resulting from detection of spectra of interfering beams is called the signal of spectral bands. Nowadays, two methods of practical realization of this type of detection are used. The first is spectral optical coherence tomography (SOCT). The other method is swept source OCT or optical Fourier domain imaging (OFDI) [4]. The common elements, used in both methods (SOCT and OFDI), are fixed reference mirrors (as opposed to time domain OCT).
This improves mechanical stability of the system. An interference image is obtained by the numerical Fourier transform of registered spectral bands. However, the method of detection of an interference signal is different. In SOCT, the light source generates a broadband light beam. A spectrometer is used to detect signals for individual optical frequencies. In OFDI, an ordinary photodetector is used instead of a spectrometer, because the applied fast tunable laser generates light of a narrow spectral line individually for each wavelength.
The recently introduced SS-OCT uses a short cavity swept laser with a tunable wavelength of operation instead of the diode laser used in spectral-domain OCT [5] The SS-OCT has improved image penetration using a wavelength of 1050 nm and has an axial resolution of 5.3 μm and an axial scan rate of 100,000 scans per second. Prototype models could reach faster scan speed of more than 400,000 scans per second [6,7]. The 12 × 9 mm scan enables simultaneous imaging of the macula, the peripapillary area, and the optic nerve head and the choroidal thickness. The 12 × 9 mm scan comprises 256 B scans each comprising 512 A scans with a total acquisition time of 1.3 s [8] SS-OCT also provides the capability of a wide field up to 12 × 12 mm images [9]. SS-OCT enables clear simultaneous visualization of the vitreous and the posterior precortical vitreous pockets and the choroid and the sclera [10].

Operating Principle of Optical Coherence Tomography
OCT is a modular device. It consists of coupled hardware components. It contains the software and five basic modules: a partially coherent light source, an imaging apparatus, a measurement head, a module of data processing, and image generation as well as a computer control system. The light source used in the device determines its axial resolution and penetration depth of the light beam. The OCT imaging apparatus module is the central element of the system. This may be any measuring device capable of measuring the reflected or backscattered light with high sensitivity and resolution. Instruments that enable lossless signal transmission are also indispensable. Other elements of the described OCT system are the measuring head and the system for bringing the probe beam to the test structure. They take different forms, depending on the field of medicine for which they are intended. Their shape also depends on the structure of the imaging apparatus block. The purpose of this module is to acquire measurement data from the imaging apparatus. Another necessary step is analysis of the obtained values, their processing, and presentation. This is achieved through a variety of techniques in the field of image processing, such as noise reduction algorithms, motion and visualization correction algorithms, segmentation, and image resolution enhancement. The computer control system controls the entire OCT scanner. It enables to control scanning the reference arm of the interferometer and synchronize the operation of all components. Moreover, it allows for communication between the apparatus and the image processing block as well as the display of measurement results in real time as it is shown in Figure 1 [11].
OCT imaging is possible by measuring the intensity and time delay of the "echo" of the reflected or backscattered light. The method of OCT imaging is analogous to ultrasonography. However, they differ in terms of data measurement techniques. This is due to the fact that the speed of light is almost one million times greater than the speed of sound and, as a result, the distance measured by OCT is characterized by a much higher time resolution than USG. OCT resolution is 10 μm, and in ultrasonography, it is 150 μm. OCT, on the other hand, has a more limited tissue penetration ability. A light wave in OCT reaches a depth of 2 mm, whereas a sound wave in USG a depth of 10 cm. In the case of USG, electronic detectors can be used for detection of the returning acoustic wave reflected from an object. The use of such devices for detecting light waves is impossible, because the rate of signal changes is too high. The basis of optical tomography is the phenomenon of interference of two partially coherent light beams coming from a single source-the reference beam and the probe beam. Biological objects, such as tissues and organs, are for light waves, the centres with nonuniform distribution of a refractive index. The analysis of interference signal enables to locate the points at which the refractive index changes. These points are situated along the direction of propagation of the probe beam. The graph of reflected wave power density as a function of the position of the reflective point, which is the source of the wave, is called an A-scan. B scans give sagittal scans of the object and C scans-lateral scanning images at a constant depth. Combination of measurement results lying in one plane (numerous parallel directions of the probe beam) creates a two-dimensional image of the section of the test object [12].
The localization of the boundaries of layers with different refractive indices, that is, determination of the waveform of refractive index changes as a function of light beam penetration depth is realized by interferometric distance measurement systems. They use the property of light waves, which is the ability to overlap. This property is dependent on coherence of light. There are two types of light coherence: spatial-defining the phase correlation between wave sequences generated by different points of the light source and time-defining the phase correlation of wave sequences emitted by a single point of the light source at different points in time [13]. The time consistency of light is examined using the Michelson interferometer [14]. The schematic diagram of the operation of the Michelson interferometer is shown in Figure 2.
The light wave incident on the semi-transparent mirror BS (beam splitter) splits into two beams. The light source (LS) changing its direction into perpendicular after passing through BS is reflected by the movable mirror M1, again passes through BS, without changing its direction, and reaches the screen D (detector). The second beam formed by the passage of the primary beam through BS without changing its direction is reflected by the fixed mirror M2, then passes through BS changing the direction into perpendicular, and falls on screen D. The beam incident on the screen forms an interference image.

The Short History of OCT in Dentistry
Attempts to use optical coherence tomography in dentistry were first made in 1998 by researchers from the Laboratory of Medical Technology of Livermore, California, in collaboration with researchers from the University of Connecticut. In their work, they presented a prototype of dental optical coherence tomography and its in vivo application [15].
The device designed by them scanned hard tissues to a depth of 3 mm and soft tissues to a depth of 1.5 mm, which even now, 14 years after the creation of this sample design, is comparable to the possibilities of the latest generation apparatus. Two years later, the same group of researchers presented the first intraoral scans not only of the hard tissues but also soft tissues of the oral cavity, using another specifically designed CT prototype. In the published work, they demonstrated the possibility of imaging the gum margin, periodontal pockets, and attachments, both epithelial and connective, using an infrared beam of light [16]. The usefulness of optical coherence tomography in the recognition of lesions in the structure of both soft and hard tissues of the oral cavity was also presented in the same year 1998 by experimental and clinical studies conducted by Feldchtein et al. [17], which was actually the first mention of the possibility of OCT examination of hard tissue. In 2000, the same scientific center compared two OCT prototypes having different wavelengths of light: 850 and 1310 nm. Analysis of the quality of scans from individual devices and the evaluation of the possibility of reflecting the anatomical details of the oral cavity showed greater effectiveness of the apparatus using longer wavelengths of light [18]. Five years later, as an experiment, twenty-one dentists were asked to analyze fissure sealants, composite fillings, or tissue enamel based on OCT scans. Despite the lack of knowledge of the techniques of OCT scan interpretation, the dentists who took part in the study    Intensity changes in NIR images at wavelengths ranging from 1300 to 1700 nm correlate with increased mineral loss measured using TMR. NIR reflectance and transillumination at wavelengths coincident with increased water absorption yielded significantly higher (P < 0 001) contrast between sound enamel and adjacent demineralized enamel. In addition, NIR reflectance exhibited significantly higher (P < 0 01) contrast between sound enamel and adjacent composite restorations than visible reflectance. Even though it appears that most of the lesions manifested little change with fluoride varnish application in the 30 weeks of the study, CP-OCT was able to measure the depth and internal structure of all the lesions including the thickness of the important transparent surface zone located at the surface of the lesions, indicating that CP-OCT is ideally suited for monitoring lesion severity in vivo.
Optical coherence tomography use in the diagnosis of enamel defects OCT imaging enabled the identification of the type of enamel defect and the determination of the extent of the enamel defects in MIH with the advantage of being a radiation free diagnostic technique. The gaps along the dentinoenamel junction were additionally observed in SS-OCT. SS-OCT was capable of monitoring the cervical demineralization induced by a cariogenic biofilm and is considered to be a promising modality for the diagnosis of cervical demineralization. Vinyl polysiloxane (VPS) impression materials which are routinely used in dentistry can be used to enhance the detection of dentinal lesions on tooth occlusal surfaces. Two probes are constructed almost entirely from off-the-shelf components, while a third, final variant is constructed with dedicated components, in an ergonomic design. The handheld probes have unidimensional (1D) galvanometer scanners; therefore, they achieve transversal sections through the biological sample investigated-in contrast to handheld probes equipped with bidimensional (2D) scanners that can also achieve volumetric (3D) reconstructions of the samples. These latter handheld probes are therefore also discussed, as well as the possibility to equip them with galvanometer 2D scanners or with Risley prisms. For galvanometer scanners, the optimal scanning functions studied in a series of previous works are pointed out; these functions offer a higher temporal efficiency/duty cycle of the scanning process, as well as artifact-free OCT images. Bio-photonic detection and quantitative evaluation method for the progression of dental caries using optical frequency domain imaging method The physicians were able to diagnose the tooth volumetric and thickness changes at an initial stage by considering the obtained results as promising threshold parameters, which will be useful to barricade the progression of caries. To enhance the accuracy of the threshold parameters, quantitative (thickness and volumetric) information of multiple in vivo specimens will be evaluated, averaged, and normalized along with clinical trials in future studies. obtained clinically acceptable results, which proved the potential clinical application of OCT [19]. The possibility of assessing caries developing under fissure sealants, which is difficult to diagnose, was subject to similar verification. After 90-minute training, doctors assessed the correctness of the enamel structure under 5 different types of sealing materials. When analysing OCT scans, the doctors detected caries more frequently compared with clinical or radiological assessment [20]. In the following years, a leading center dealing with optical tomography became the University of California in San Francisco. A series of articles was published, broadening the knowledge on the aspects of OCT application in conservative dentistry. The described issues were related to imaging of caries incipiens, their remineralization, and monitoring of the progressing or stopped demineralization of the enamel surface or tooth structure underneath fillings [21][22][23][24][25][26][27][28][29]. The issue of enamel remineralization is still continued [12]. In 2010, an innovative work was presented on attempts of enamel remineralization with chitosan. The penetration depth of chitosan into the enamel structure was evaluated by optical tomography. An attempt of complete enamel remineralization using this method did not prove to be successful, but the exploratory efficiency of the used diagnostic method was once again confirmed [30]. In the same year, the enamel structure of primary teeth was analysed. Since caries is a disease that affects both primary and permanent teeth, the authors verified the effectiveness of the new method of caries diagnosis in the primary dentition. They proved a high potential of optical tomography in paediatric dentistry, as a technique for effective, painless, and noninvasive detection of early tooth decay [31]. The next studies described the effectiveness of optical coherence tomography in monitoring the range and efficiency of infrared and fractional CO2 lasers in caries removal [32][33][34][35][36][37]. The effectiveness of a diode laser and Nd-YAG laser in the development of root canals during endodontic treatment was also verified [38]. An attempt was also made to use OCT in endodontic

Near-IR and CP-OCT imaging of suspected occlusal caries lesions
Near-IR imaging methods have great potential for improving the early diagnosis of occlusal lesions. (21) [61] Hariri et al.
Effects of structural orientation of enamel and dentine on light attenuation and local refractive index: an optical coherence tomography study.
Unlike enamel, refractive index and OCT signal patterns in dentine vary according to structural orientation, with dentine tubules playing the role. Attenuation of OCT signal intensity was small in enamel. The findings may contribute to a better understanding of the interactions of light with the dental tissue. Precise records of refractive indices and OCT signal patterns may be important for clinical diagnosis of caries and measurement of structural depth for operative purposes using this technology. Effects of dentine structural orientation on refractive index and scattering pattern must be considered when observing human teeth by OCT cross-sectional imaging. SS-OCT appears to be a more reliable and accurate method than bitewing radiographs for the detection and estimation of the depth of proximal lesions in the clinical environment.
(2) [96] Van Hilsen and Jones Comparing potential early caries assessment methods for teledentistry Although MID and CP-OCT were useful in detecting the presence of demineralization, examiners were not able to utilize these devices to adequately assess the depth of the demineralization. This study found that MID and CPOCT did not have markedly superior diagnostic values from simple CAM assessment for use in teledentistry.
3D assessment of void and gap formation in flowable resin composites using optical coherence tomography The flowable composite with SDR (stress-decreasing resin) technology performed better than the conventional composite; however, bulk filling a 4 mm-deep cavity will compromise the sealing of the bonding interface regardless of the type of composite. OCT is a unique method of characterizing materials and their behaviors nondestructively and precisely. Optical coherence tomography as an auxiliary tool for the screening of radiation-related caries The OCT technique was able to characterize radiation-related caries, from a morphological point of view. Also demonstrated was its potential benefit for use in the clinical monitoring of radiation-related carious process. Nondestructive assessment of current one-step self-etch dental adhesives using optical coherence tomography OCT is a unique tool to nondestructively evaluate the sealing performance of the restoratives through the cavity, provided that cavity walls have a certain minimum inclination with respect to the beam.
Park et al.
Assessment of interfacial defects at composite restorations by swept source optical coherence tomography OCT imaging has the potential to nondestructively assess the interfacial adaptation of composite restorations and to detect internal defects in the layered composite material.
Quantitative evaluation of dental abfraction and attrition using a swept-source optical coherence tomography system A valuable tool in the evaluation of the dynamic evolution of ex vivo artificially induced abfractions and attritions is able to measure minute changes in the tooth morphology, having the potential to be employed as an effective tool for monitoring the temporal evolution of dental wear. OCT can offer the possibility of providing in vivo volumetric measurements and identification of fractural lines in dentine. The 2D and 3D pictures prove the OCT ability in the evaluation of dental abfractions and attritions.
The system could measure a minimal volume of 2352 μm to 32,352 μm 3 , where each volume is acquired as 25,000 A scans in 2.5 s. Despite correctly evaluating the depth, this work showed that the lesion width calculated from SS-OCT reflectivity images did not accurately predict the demineralized width. The relative reflectivity could not accurately determine the mineral density of the demineralized lesions. SS-OCT detected subsurface fissure demineralization and could be used to determine if the decay process was advancing toward the enamel-dentin junction.
Use of 2D images of depth and integrated reflectivity to represent the severity of demineralization in cross-polarization optical coherence tomography Calculated lesion depths from OCT were compared with lesion depths measured from histological sections examined using polarized light microscopy. The 2D images of the lesion depth and integrated reflectivity are well suited for visualization of early demineralization.
Measuring initial enamel erosion with quantitative light-induced fluorescence and optical coherence tomography: an in vitro validation study OCT and QLF were able to detect demineralization after 10 min of erosive challenge and could be used to monitor the progression of demineralization of initial enamel erosion in vitro. (12) [106] Nakajima et al.
Detection of occlusal caries in primary teeth using swept source optical coherence tomography The results obtained from SS-OCT and conventional visual inspections were compared with those of CLSM. SS-OCT detects both cavitated and noncavitated lesions. The magnitude of sensitivity for SS-OCT was higher than those for visual inspection (sensitivity of visual inspection and SS-OCT, 0. 70  Demonstrative study by synchrotron radiation microtomography The present study proved the capability of the OCT method in visualizing the morphology and integrity of zirconia-doped tooth adhesive fillings to be used for a further in vivo tool development.
Characterization of transparent dentin in attrited teeth using optical coherence tomography Physiological changes in transparent dentin that involve deposition of mineral casts in the dentinal tubules lead to lower attenuation of OCT signal. OCT has a potential role to detect transparent dentin on the surface of attrited teeth and can be used in the future as a clinical adjunct tool. (15) [109] Ku et al.

Detection of early changes in caries lesion using QLF-D and OCT
The QLF-D and SS-OCT could detect subtle changes in mineral loss and lesion depth with respect to demineralized time. Furthermore, these devices were useful for monitoring changes in mineral amount and lesion depth.  A method for monitoring enamel erosion using laser irradiated surfaces and optical coherence tomography Irradiation of the enamel surface with a pulsed carbon dioxide laser at subablative intensities results in significant inhibition of erosion and demineralization under the acid challenge employed in this study. In addition, these results suggest that it may be feasible to modify regions of the enamel surface using the laser to serve as reference marks to monitor the rate of erosion in vivo. (19) [113] Cara et al.
Evaluation of two quantitative analysis methods of optical coherence tomography for detection of enamel demineralization and comparison with microhardness Both methods for signal analysis from OCT allowed detection of demineralization with good performance. The AUC-OCT approach enables obtaining a linear relation with the microhardness results, for a quantitative assessment of mineral loss in human teeth.

Assessment of the sealant/tooth interface using optical coherence tomography
Optical inspection and X-ray investigation revealed no defects, while SS-OCT assesses exactly the position, the nature, and the dimensions of each type of these defects. Clinical assessment of non carious cervical lesion using swept-source optical coherence tomography SS-OCT results confirm that dentin mineral loss and occlusal attrition were associated with larger NCCLs and can be considered as an etiological fact or information and progress of these lesions. (23) [117] Anadioti et al.
Internal fit of pressed and computer-aided design/computer-aided manufacturing ceramic crowns made from digital and conventional impressions The combination of the digital impression and pressed crown produced the least accurate internal fit. The coatings showed different thicknesses (60-250 micrometers) and various levels of structural and interfacial integrity. OCT could detect a demineralization inhibition zone adjacent to the edge of the fluorideand calcium-releasing material. Localized demineralization was occasionally observed under thinner coatings. Protection of susceptible enamel surfaces by thin resin-basedbioactive coatings provides protection from demineralization. OCT can be used to nondestructively monitor the integrity of such coatings, as well as enamel changes beneath and adjacent to them. The images obtained clinically in real time using the dental SS-OCT system are suitable for the assessment of natural subsurface lesions and their surface layer, providing comparable images to a laboratory high resolution μCT without the use of X-ray.
Sun et al.
Sensing of tooth microleakage based on dental optical coherence tomography The results of this study show that microleakage can be detected with oral probing using SS-OCT in vivo. The calculated microleakage length was 401 μm and the width is 148 μm, which is consistent with the related histological biopsy measurements. The diagnosis of microleakage in teeth could be useful for prevention of secondary caries in the clinical treatment plans developed in the field of oral medicine. Noninvasive quantitative evaluation of the dentin layer during dental procedures using optical coherence tomography The study demonstrates the usefulness of OCT imaging in guiding such evaluations during dental procedures.
Validation of optical coherence tomography against micro-computed tomography for evaluation of remaining coronal dentin thickness We used optical coherence tomography (OCT) and microcomputed tomographic (micro-CT) imaging to scan teeth after deep dentin caries removal. The remaining dentin thickness (RDT) at pulpal horns was measured and compared. A strong correlation was found in measurements between OCT and micro-CT imaging. It was possible to clearly visualize pulp horns with RDT up to 1.5 mm in thickness. A refractive index value of 1.54 is valid to convert optical readings of RDT by OC.  Assessment of curing behavior of light-activated dental composites using intensity correlation based multiple reference optical coherence tomography These results show that MR-OCT has the potential to measure the curing time and monitor the curing process as a function of depth. Moreover, MR-OCT as a product has potential to be compact, low-cost, and to fit into a smartphone. Using such a device for monitoring the curing of the resin will be suitable for dentists in stationary and mobile clinical settings. Non-destructive evaluation of an internal adaptation of resin composite restoration with swept-source optical coherence tomography and micro-CT Micro-CT and SS-OCT may be useful nondestructive methods for evaluating internal adaptation. The microleakage measured by micro-CT was lower than that of SS-OCT; however, the two measurements were relatively high-correlated. When adhesion depends mostly on the dentin surface, a two-step self-etch adhesive system should be considered for long-term longevity. Mitigation of enamel erosion using commercial toothpastes evaluated with optical coherence tomography A significant increase in the mean roughness values was observed on eroded surface and also on treated surface as revealed by scanning electron microscopy. The use of SnF 2 /NaF toothpaste was the most effective method for reducing mineral loss. As quantitative methods, OCT and contact profilometry showed no statistical differences. OCT, which was used for this purpose for the first time, has the advantage of being noninvasive and therefore has the potential for clinical application. Internal adaptation in a high C-factor cavity was inferior to that in a low C-factor cavity for both conventional and bulk-filled composites. Internal adaptation, polymerization shrinkage, and stress were different among composite materials. Polymerization stress under the compliance-allowed condition showed significant correlations with internal adaptations in high and low C-factor cavities.
Monitoring of cariogenic demineralization at the enamel-composite interface using swept-source optical coherence tomography The carious demineralization around composite restorations was observed as a bright zone in SS-OCT during the process of bacterial demineralization. SS-OCT appears to be a promising modality for the detection of caries adjacent to an existing restoration. Optical analysis of enamel and dentin caries in relation to mineral density using swept-source optical coherence tomography Both enamel and dentin demineralization showed significantly higher IS200 and μμ than the sound tooth substrate from the sagittal scan. Enamel demineralization showed significantly higher IS200 than sound enamel, even with low levels of demineralization. In demineralized dentin, the μμ from the horizontal scan consistently trended downward compared to the sound dentin. This study demonstrated that PS-OCT can be used to nondestructively measure changes in lesion structure and severity upon exposure to an acidic remineralization model. This study also demonstrated that automated algorithms can be used to assess the lesion severity even with the presence of a weakly reflective surface zone.
Nondestructive assessment of early tooth demineralization using cross-polarization optical coherence tomography Cross-polarization OCT is ideally suited for the nondestructive assessment of early demineralization. (58) [58] Nazari et al.

Effect of hydration on assessment of early enamel lesion using swept-source optical coherence tomography
In summary, the strong relationship found between DH and lesion extent indicates the potential of this method for assessment of early enamel lesion using SS-OCT. However, further studies on DH for evaluation of a wider range of demineralized lesions as well as remineralization, accompanied by a clinically relevant drying method are necessary to optimize the suggested methodology. (59) [60] Shimada et al.
3D evaluation of composite resin restoration at practical training using swept-source optical coherence tomography (SS-OCT) SS-OCT could detect the internal gaps and voids within the restorations in tomography images synthesized based on the backscatter signal from within the restoration. It is suggested that the SS-OCT is promising diagnostic modality, as well as educational imaging device for the detection of internal gaps in adhesive restorations. Results were comparable with that of the widely used 1310 nm OCT system. In the case of restoration with filler material, the 1310 nm OCT imaging displayed better imaging capacity due to lower scattering than 840 nm imaging. Under the conditions of this ex vivo study, periapical radiographs and CBCT were unreliable for the detection of simulated incomplete VRFs. The widths of the fractures appeared to have an impact on the diagnostic accuracy of CBCT as the detection of VRFs of ≥50 μm was significantly higher than those of <50 μm. The detection of complete fractures was significantly higher for all systems than that of incomplete fractures.
Nondestructive observation of teeth post core space using optical coherence tomography: a pilot study In the cementum absent group, the internal structure of the root could be visualized clearly compared with the cementum present group. The root internal structure could be observed by OCT and the image became clearer when cementum was removed. in vitro studies [39]. The results of studies evaluating the errors in prosthetic treatment were also published: defects in the structure of the materials used in prosthetic restoration and microleakage at the contact surface of the reconstruction and the tooth as well as the appropriateness of using OCT to control the internal structure of the prosthetic restoration without the need for its removal [34,40]. Attempts were also made to visualize and measure the length of periodontal ligaments before and during orthodontic tooth movement. Incisors of rats were moved by applying successively varying sizes of forces and then the teeth were removed. The condition of the ligaments was imaged using optical coherence tomography and X-rays. OCT scans showed differences in periodontal ligament arrangement depending on the size of the applied force and their significant twist when using the greatest forces [41]. In subsequent studies, scans of the periodontium were performed and the lengths of both stretched and relaxed ligaments were measured. These structures were imaged using standard radio visual graphic intraoral images. However, they did not prove useful in the evaluation of periodontal elements obstructed in the image by tooth tissues. OCT enabled three-dimensional measurement and multilateral imaging of ligaments. The results obtained when using a CT scanner were different from those obtained by means of standard two-dimensional imaging. Periodontal fibres measured in X-ray images appeared to be much thinner than in reality [42].
Another application of OCT was an attempt to evaluate the salivary pellicle. In order to compare the results and to improve the resolution and specificity of images, an optical coherence microscope (OCM) was used. Salivary pellicle islands were visible in the samples incubated in saliva, which grow into complexes completely covering the enamel surface [43]. The aim of the next study was to evaluate the retention of the biofilm around orthodontic hooks depending on the ligaturing method using OCT and microbiological samples. Both microbiological and optical (OCT) analysis showed a significant difference in biofilm formation depending on the ligaturing method. The hooks ligaturated with elastic elements showed a greater amount of cariogenic Streptococcus mutans, whereas metal ligatures showed much less biofilm retention. The study found that optical coherence tomography may also be treated as a full-fledged quantitative indicator of bacterial plaque, which can be quickly and reliably visualized around orthodontic hooks [44]. Similar problems were presented in an ex vivo models. They proved the possibility of calculating the biofilm mass by measuring the distribution of light intensity scattering to a depth of the biofilm. An indirect possibility of characterizing the examined ecosystem on the surface of various types of composite The OCT showed a potential for quantitative estimation of lesion depth and mineral loss with cavitated dentin lesions in vitro. materials was also demonstrated [45]. The study on biofilm imaging, describing the impact of dental calculus, enamel decalcification, and plaque, was an attempt to use optical coherence tomography not only in dentistry but also clinical periodontics. These studies confirmed the possibility of detecting enamel decalcification despite the presence of dental calculus or plaque and their diversification in the scans [46]. Another direction of research using the OCT technique has become the assessment of restorations with composite fillings in conservative dentistry. The study demonstrated, based on analysis of OCT scans, the leakage of composite restorations of enamel defects. The fissures were on average 50 μm. The results were confirmed by X-ray images and optical microscopy. The study resulted in the development of their own spectral CT scanner, which was based on the Michelson interferometer. The created device, as well as the modern optical tomography instrument, divides monochromatic light into two beams, allowing for the reflection of the beams from semi-transparent mirrors and their subsequent interference. Using such a device, the researchers revealed the errors of composite reconstruction in the form of visible pits and fissures at the border between the filling and the cavity wall [47]. Enamel cracks at the border between the enamel and the composite filling reinforced with glass fibre were evaluated in a similar manner [48].The subject of evaluation was also the tightness of three selected composite fillings, cracks of composite reconstruction reinforced with Digital technique for in vivo assessment of internal and marginal fit of fixed dental prostheses Digital approaches to assess the misfit of fixed dental prostheses have been limited to in vitro evaluation. The present article describes a fully digital technique for the in vivo assessment of the fit of fixed dental prostheses by means of a chairside optical scanner and software for three-dimensional (3D) analysis. The 3D digital capture is performed in 3 steps: an extraoral scan of the restoration, an intraoral scan of the abutment tooth, and an intraoral registration scan of the restoration positioned on the abutment tooth.
Examination of ceramic restorative material interfacial debonding using acoustic emission and optical coherence tomography Sustainable cyclic load stresses in ceramic/dentinbonded specimens were substantially lower than the measured SBS. Predicted S-N curve showed that the maximum endured load was 4.18 MPa passing 10 6 fatigue cyclic.

Examination of ceramic/enamel interfacial debonding using acoustic emission and optical coherence tomography
The acoustic emission technique combined with OCT MPa images as a preclinical assessment tool to determine the integrity of cemented load bearing restored ceramic material. Sustainable cyclic load stresses in ceramic/enamel-bonded specimens were substantially lower than the measured SBS. Predicted S-N curve showed that the maximum endured load was 10.98 (about 34.48 N) passing 10 6 fatigue cyclic.

Material defects in ceramic crowns identification by optical coherence tomography and microCT
OCT technology can be considered an early diagnosis method of faults contained in the table structure of the ceramic crowns before inserting them in the oral cavity, by reducing the risks of a prosthetic treatment. Within the limitations of this in vitro study, marginal discrepancies of inlay restorations were quantitatively and noninvasively evaluated by the OCT system. The following conclusions may be drawn: direct inlays presented smaller marginal gap values than indirect inlays. The marginal gap values were increased for all restorations after cementation. Evaluation of oral mucosa collagen condition with cross-polarization optical coherence tomography The OCT signal SD in cross-polarized images reflects two boundary conditions of collagen disorganization, namely, loss of fibre properties at active inflammation which attenuates the signal and fibrosis that occurs due to synthesis of a new remodeled collagen which amplifies the OCT signal.
Towards a bimodal proximity sensor for in situ neurovascular bundle detection during dental implant surgery The proximity to the neurovascular bundle can be tracked in real time in the range of a few millimeters with NIR signals, after which higher resolution imaging OCT to provide finer ranging in the submillimeter distances. Regarding the ability of the two OCT systems to visualize periodontal structures, the system operating at 1325 nm shows a better performance, owing to a longer central wavelength that allows deeper tissue penetration. The results with the system at 930 nm can also be used, but some features could not be observed due to its lower penetration depth in the tissue.
Imaging of 3D tissue-engineered models of oral cancer using 890 and 1300 nm optical coherence tomography 890 nm OCT retains some of its known advantages of higher contrast between anatomical tissue layers when used to observe dysplastic and malignant 3D oral mucosa constructs. However, 1300 nm OCT is confirmed to possess a greater ability to image the full thickness of the model epithelia, and in particular, it is more suited to imaging through the keratinized layer. The effectiveness of optical coherence tomography for evaluating peri-implant tissue: a pilot study Cement remnants at the submucosal area can be detected in some cases, which can be helpful in preventing peri-implant diseases. Still, though there are some restrictions to its application, OCT could have potential as an effective diagnostic instrument in the field of implant dentistry as well.

Non-invasive detection of periodontal loss of attachment using optical coherence tomography
The conventional time domain OCT system acquisition speed is limited by the speed of the mechanical scanning system. In order to overcome this issue, a novel electro-opticbased scanning system is proposed and demonstrated.  The biofilm network was dramatically destroyed after the professional dental cleaning. OCT noninvasive methods can act as a valuable tool for the 3D characterization of dental biofilms.
In vivo assessment of periodontal structures and measurement of gingival sulcus with optical coherence tomography: a pilot study OCT has the potential to be a reliable tool for in vivo periodontal tissues evaluation and for reproducible sulcus depth measurements in healthy sites. Further technological advances are required to reduce the procedure time and promote evaluation of posterior oral regions. (12) [177] Salehi et al.
Characterization of human oral tissues based on quantitative analysis of optical coherence tomography images These OCT features can reliably differentiate between a range of hard and soft tissues and could be extremely valuable in assisting dentists for in vivo evaluation of oral tissues and early detection of pathologic changes in the tissues. Quantifying dental biofilm growth using cross-polarization optical coherence tomography CP-OCT has the ability to nondestructively monitor biofilm growth and elucidate the growth characteristics of these microcosms on different dental material compositions. CP-OCT was able to quantify the mass of the biofilm by measuring the overall depth-resolved scattering of the biofilm.  Automatic method of analysis of OCT images in the assessment of the tooth enamel surface after orthodontic treatment with fixed braces This paper presents an automatic quantitative method for the assessment of tooth enamel thickness captured on the OCT scans. This method has proven to be an effective diagnostic tool that allows evaluation of the surface and cross section of tooth enamel after orthodontic treatment with fixed thin-arched braces and proper selection of the methodology and course of treatment.
Enamel thickness before and after orthodontic treatment analysed in optical coherence tomography The range of variations in the enamel thickness after treatment with fixed thin-arched braces is not subjected to modification of a factor such as the type of adhesive system. The OCT is an effective diagnostic tool to evaluate the thickness of the enamel tissue before and after the completed orthodontic treatment. The results demonstrated that enamel fractures were observed only in the samples bonded with ceramic brackets, and the type of pliers did not influence the incidence and extent of enamel damage. Moreover, the type of debonding technique (with side-cutting pliers or anterior bracket removal pliers) and the type of bracket did not influence the amount of adhesive remaining after debonding. The burs at low speed removed the remaining adhesive more effectively during cleanup procedures.

Effectiveness of varnish with CPP-ACP in prevention of caries lesions around orthodontic brackets: an OCT evaluation
The major limitation of this study is that it is a study in which demineralization was obtained with the use of chemical products and did not occur due to the presence of Streptococcus mutans and its acid byproducts. Application of CPP-ACP-containing varnish irrespective of being associated with brushing and mouthwash, or not, reduced depth of caries lesions around orthodontic brackets.
Longitudinal monitoring of demineralization peripheral to orthodontic brackets using cross polarization optical coherence tomography CP-OCT was able to measure a significant increase in demineralization (P < 0.0001) at the base of orthodontic brackets over a period of 12 months.
Assessing near infrared optical properties of ceramic orthodontic brackets using cross-polarization optical coherence tomography Noninvasive, near infrared (NIR) cross-polarization optical coherence tomography (CP-OCT) has potential to effectively image through portions of ceramic brackets; however, further investigation into the optical effects of resin integration in the base portion of the brackets is warranted.
glass fibre, which were imaged using optical coherence tomography (OCT), scanning electron microscopy (SEM), and optical microscopy (OM) [49]. The results enabled to describe the internal cracks of composites, which were not accessible during SEM or OM imaging. It was also observed that the assessment by means of optical coherent tomography required no special sample preparation, making it less expensive compared with the assessment in the scanning electron microscope [50]. In a further step, the efficiency of optical coherence tomography and confocal microscope in the evaluation of composite materials was compared [51].
There are also publications extending the above issue and evaluating marginal adaptation, porosity, and internal integrity of composite fillings. The potential of OCT and high resolution scans, allowing for critical assessment of the structure of fillings, previously inaccessible using common diagnostic methods, has thus been proven [52]. Similar studies evaluating polymerization shrinkage showed significant differences in its size depending on the tested materials [53]. Composite fillings restoring bovine enamel defects and their marginal adaptation with the use of self-etching techniques were also studied. The findings confirmed the thesis that optical coherence tomography is an effective tool in the accurate assessment of tightness of composite fillings [54]. The study of Senawongse et al. [55] made it possible to visualize the adhesive connection between the bonding system and the dentin, analyse carious lesions within the crown and root of the tooth, and assess secondary caries [56,57]. From a clinical point of view, the studies identifying the relationship between the quality of OCT scans and the level of tooth hydration are very important [58,59]. It directly affects the strength of the enamel prisms to injuries and the colour of the tissue, which is to be reproduced during conservative or prosthetic restorations. The use of OCT for educational purposes was also presented. The mistakes in the fillings made by dental students were discussed based on performed scans [60].
A further development of work on using an optical scanner and analysis of images was the research which used the potential of OCT to evaluate light scatter and the magnitude of the local refractive index depending on the state of the enamel and dentin. Optical properties of the prisms of the human enamel and dentin tubules were imaged [61].
OCT was also used to evaluate enamel cracks. The results were verified using a stereomicroscope and histological samples of individual enamel layers. Enamel cracks were identified by CT as intensified signals appearing in exactly the same places where damage to the histological samples and stereomicroscopic images was visible. The results showed that OCT very accurately identified cracks and their size, so measurements of the scanned teeth yielded results that were equally reliable to those obtained from stereomicroscopy and histological examination of subsequent enamel layers [62].
In order to improve the quality of OCT scans and facilitate their interpretation, gold nanoparticles were applied. They are normally used as contrast in SEM imaging to visualize the hybrid layer and dentin tubules [63]. This was a significant advancement in dentin imaging because until then only a qualitative and quantitative evaluation of tooth decay had been possible, without distinguishing histological structures [64].
Attempts were also made to use optical coherence tomography in maxillofacial surgery for separating normal and dysplastic fragments of oral epithelium and distinguishing between solid and bullous lesions [65,66].
Optical coherence tomography: a new era in dentistry It can be used for noninvasive investigations for both in vivo and in vitro structural imaging within the oral cavity.
Optical coherence tomography for non-invasive ex vivo investigations in dental medicine-a joint group experience Complementary studies are possible embracing OCT with more traditional methods, such as confocal microscopy and micro-CT. Combination of principles is expected to evolve due to their limitations when considered separately.
Novel digital imaging techniques to assess the outcome in oral rehabilitation with dental implants: a narrative review New optical imaging techniques may be considered possible approaches for monitoring peri-implant soft tissue health. MRI and ultrasonography appear promising non-ionizing radiation-imaging modalities for the assessment of soft tissue and bone defect morphologies. Optical scanners and OCT may represent efficient clinical methods for accurate assessment of the misfit between the reconstructions and the implants.
Optical coherence tomography-a imaging modality in dentistry beyond X-rays OCT offers noninvasive, noncontact, in vivo, and real-time subsurface images with high-depth resolution. OCT represents a valuable method for investigation and assessment of the health status of soft oral tissues and of hard dental structures. OCT can be used for evaluation of dental treatments reducing their failure rate and saving time and resources, by eliminating incorrect restorations before their insertion in the oral cavity. Describes the use of OCT for detecting dental caries, tooth fractures, and interfacial aps in intraoral restorations. OCT can be a reliable and accurate method and a safer alternative to X-ray radiography.
Novel digital imaging techniques to assess the outcome in oral rehabilitation with dental implants: a narrative review Optical scanners and OCT may represent efficient clinical methods for accurate assessment of the misfit between the reconstructions and the implants.

Discussion
The common objectives of the discussed studies were increased diagnostic capabilities in the oral cavity, more accurate understanding of physiological and pathophysiological processes related to soft and hard tissues of the oral cavity, and monitoring the effects of treatment.
OCT capabilities commonly applied in many fields of medicine (such as ophthalmology) are not yet fully used in dentistry, mainly due to the low availability of customized intraoral equipment and insufficient range of OCT rays, which penetrate into the tissue to a depth of only a few millimeters depending on the apparatus type. Lesions within the tooth tissue usually reach deeper and are often measured in centimeters, which makes it necessary to perform hundreds or even thousands of scans to illustrate the entire lesion. Latest studies [56,168,169] are using the intraoral probes, which show that this obstacle is being slowly eliminated in the intraoral diagnostics.
To maximize the efficiency of the dental diagnostic OCT, the wavelengths of light responsible for generating the image should be subjected to testing. In the near infrared light range, the central wavelength determines the maximum depth of penetration into the tissue due to scattering and absorption properties [71]. A wavelength below 1000 nm provides the greatest imaging efficiency because light scattering properties are similar to the size of tissue particles. Hydrated tissues dissipate much more energy than hard tissues containing a small percentage of water. For this reason, universal dental OCT should offer the possibility of controlling the wavelength depending on the type of the tested tissues. A different wavelength must be used for imaging the periodontal and tooth tissue per se.
However, the technical limitation of the dental OCT is not the only problem. A very important issue is the golden standard that lacks the methodology in many publications.
Only few experiments design the study in a manner that compares the obtained results to other more or less conventional methods. There are studies that practice the golden standard by comparing it, for example, to the transverse microradiography [52], microscope [58], standard histopathology [61], confocal laser scanning microscope and light microscopy [70], micro-OCT [74], cone beam computed tomography [82], synchrotron radiation microtomography [103], laser [108], SEM [114], and microfocus X-ray computed tomography [116]. It is important to focus on this topic during analyzing and citing the published results.
Another problem arising in dental diagnosis is the quality of individual teeth. The enamel can vary in its structure in a single subject. Likewise, dental fillings or prosthetic materials having a different composition reflect or absorb light at varying degrees, which has a decisive effect on the image quality and the possibility of its correct interpretation. Materials whose reflectance index is similar to that of the background will give a similar image. In addition to image quality, the possibility of performing objective measurements of the obtained scans is very important. To date, publications have been mainly focused on the possibility of obtaining images of individual structures and their acquisition rate, which is especially important in in vivo studies. The authors of the present paper attempted to develop an algorithm for rapid and accurate measurements of tooth tissues. This algorithm works fully automatically, without any operator intervention, enables to quantify the changes in the structure of enamel, allows for quantitative assessment of the effectiveness of cleaning the tooth surface and the effectiveness of the use of selected methods of enamel development. The analysis time of a sequence of 2D images does not exceed 5 seconds when using the Core i5 CPU M460 @ 2.5 GHz 4 GB RAM. The results of the mean thickness of the tooth enamel and Imaging of the oral cavity using optical coherence tomography The intensity of backscattered light is measured as a function of depth in the tissue. Low coherence interferometry is used to selectively remove the component of backscattered signal that has undergone multiple scattering events, resulting in very high resolution images (<15 microns). Lateral scanning of the probe beam across the biological tissue is then used to generate a 2D intensity plot, similar to ultrasound images. This imaging method provides information that is currently unobtainable by any other means, making possible such diverse applications as diagnosis of periodontal disease, caries detection, and evaluation of restoration integrity.
Study on application to the field of dentistry using optical coherence tomography (OCT) This review discusses not only the basic principles of operation, types, advantages, and disadvantages of OCT but also the future applications of OCT technology and their potential in the field of dental diagnosis. minimum and maximum values as well as standard deviation are analysed automatically and saved to text files * .txt and Excel * .xls. Automatic analysis of tooth enamel thickness provides a number of further possibilities. These include area analysis of enamel thickness (for each individual tooth area separately) and enamel texture analysis. Imaging and quantitative measurement of the enamel structure before installation of braces and after their removal enables to expose the tooth tissue damage extent depending on the used brackets and method of attachment. This makes it possible to deduce which brackets and what technique of their installation is the safest for tooth enamel. This solution has been published in work [72]. There are also a few other possibilities for using the quantitative analysis of the intraoral structures and tissue conditions such as dental enamel and dental caries [86], dental abfraction and attrition [98], enamel erosion [101], enamel demineralization [109], thickness of dentin layer [121], and soft tissues [173].

Conclusions
OCT is a very important tool for the study of various tissues in vivo and in vitro. Despite problems with equipment, the possibility of early diagnosis of caries in conservative dentistry in adults and children has already been proven. It is a unique improvement in relation to X-ray diagnostics exposing patients to X-ray radiation, which is often unable to visualize the early stages of caries.
OCT allows for soft-tissue imaging, which is important in the treatment of periodontal diseases, inaccessible to direct clinical assessment, and offers great perspectives for early diagnosis of lesions in the oral mucosa. Early differentiation of the observed lesion is of great importance in the treatment of a patient due to the frequent occurrence of tumours in the oral cavity. The use of long light waves will also enable the early diagnosis of tumours of the jaw bones.
OCT provides tissue sections in a noncontact and noninvasive manner and allows for real time tissue imaging in situ, without the need for biopsy, histological procedures, or the use of X-rays, so after solving the problems related to the availability and quality of equipment, it will be the method of choice in modern dental diagnostics.