Abstract
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.
1. 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.
2. 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].
3. 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.
4. 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 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 [21–29]. 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 [32–37]. 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 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 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 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].
The latest studies continue to focus primarily on early diagnosis of caries, assessment of the quality and thickness of dentin, and assessment of dental fillings [67–74]. The precise topics and conclusions of the articles from the last 5 years, according to field of dentistry are summarized in Tables 1, 2, 3, 4, 5, 6, 7, and 8. In the first table, there is set of publications [61, 75–94] that are exposing the facilities of OCT and the possibility of diagnostics in dentistry.
Table 2 collects publications [12, 19, 28, 58, 60, 62, 63, 67, 69–74, 95–148] which show the advancement in cariology and restorative dentistry that has taken place by using the OCT. Publications [39, 56, 149–158] presented in Table 3 are hastening the experiments and the results that were taken in endodontics. The publication [159] contained in Table 4 is the only recent publication connected directly with the pedodontics. Table 5 present the articles in the field of prosthetics [160–165]. Table 6 collects the articles [45, 65, 66, 166–180] about OCT in periodontology and diagnostics of oral tissues and implantology. The articles about diagnostics in orthodontics are presented in Table 7 [181–188]. Table 8 is collecting the other review articles that can be useful in extending the knowledge about OCT in dentistry.
5. 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 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].
6. 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.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.