We use optical coherence tomography (OCT) to perform a comprehensive program of in vivo and in vitro structural imaging of hard and soft tissues within the oral cavity. We have imaged the different types of healthy oral mucosa as well as normal and abnormal tooth structure. OCT is able to differentiate between the various types of keratinized and non-keratinized mucosa with high resolution. OCT is also able to provide detailed structural information on clinical abnormalities (caries and non-caries lesions) in teeth and provide guidance in dental restorative procedures. Our investigations demonstrate the utility of OCT as a diagnostic imaging modality in clinical and research dentistry.
©1998 Optical Society of America
Many diseases of the oral cavity are accompanied by changes in tissue structure. Typical examples include caries and non-caries lesions in teeth, periodontitis, and oral and maxillofacial tumors. The ability to perform high resolution diagnostic imaging is therefore crucial to the detection and treatment of these diseases. Current imaging modalities in dentistry include traditional X-ray radiography, as well as modern techniques such as computed tomography (CT), magnetic resonance imaging (MRI) and spectroscopy (MRS), and ultrasound . In standard clinical practice, however, resolving sub-millimeter tissue structure is difficult with any of these modalities. Conventional X-ray radiography displays ≥ mm resolution, suitable for characterizing macroscopic structural changes in teeth. Ultrasound is limited by the wavelength of the source, and sub-millimeter clinical resolutions can be achieved only under optimal conditions. The increased image acquisition times necessary to achieve sub-millimeter resolution in MRI, as well as its physical footprint and expense, make it impractical in routine clinical dentistry settings.
Light has also been used to investigate the structural properties of teeth. Light offers several advantages not found in other imaging techniques, including its innocuous character (i.e, absence of tissue-damaging radiation) and ease of generation and detection. The use of visible and ultraviolet light as a means for investigating and characterizing the structure and composition of dental material has been performed by a number of groups. Zijp and ten Bosch have investigated wavelength dependent scattering and absorption in human and bovine dentin in the 400 – 700 nm range of the spectrum [2,3]. Other groups have also examined the ultraviolet photoluminescence from dentin  to detect and differentiate chromophoric content. Until recently, however, the use of light as a high resolution imaging tool in dental applications has been severely compromised by the turbid nature of dental tissue. Light that travels through turbid media undergoes arbitrary changes in propagation directions and loss of coherence due to scattering from the random distribution of inhomogeneous optical scatterers. For tissues with thicknesses exceeding a few scattering lengths, the resulting image quality is severely degraded due to these scattering events.
The recent development of optical coherence tomography (OCT) has made it possible to overcome these limitations by using interferometric cross-correlation techniques to detect the coherent backscattered components of short coherence length light . OCT is an emerging bio-imaging technology that promises to have broad and significant impact on clinical diagnostic imaging. Pioneered by Fujimoto and co-workers to perform in vivo clinical imaging of the human eye [6,7], OCT has been extended to measurements in vivo of the human skin [8,9], mammalian cerebral cortex , animal gastrointestinal and respiratory tracks , and the human gastro-intestinal tract, larynx, esophagus, cervix, and bladder , as well as cardiac function in Xenopus Laevis  and in vitro imaging of atherosclerotic plaque morphology in human coronary arteries , and human nervous, reproductive, and vascular systems . Izatt, et al. and Chen, et al. have also shown that OCT can be used to measure velocity profiles in blood vessels in vivo using Doppler velocimetry [16,17]. Recently, Colston, et al. [18,19] has applied OCT to in vitro and in vivo imaging of dental structure and Warren et al.  has performed OCT characterization of healthy teeth and caries lesions in vitro.
In this paper, we present a comprehensive investigation of OCT in vivo and in vitro imaging of the structure of hard and soft tissue in the oral cavity. We have performed extensive imaging of all of the various types of oral mucosa as well as the structure of healthy and diseased teeth. In particular, we investigate the ability of OCT to image caries and non-caries lesions in enamel and dentin on a variety of teeth using dual polarization OCT imaging . We also use OCT to monitor the progress of dental restorations in operative dentistry. Our results indicate that OCT is capable of high resolution imaging in a substantial number of clinical dental applications and point to the viability of OCT as a new imaging tool for the dental community.
To perform OCT imaging of oral mucosa, we used a compact, dual wavelength, fiber-based OCT scanner. Two superluminescent diodes operating at 830 nm (Δλ=25 nm) and 1280 nm (Δλ =50 nm) served as the short coherent length light source, producing 1.5 mW and 0.5 mW powers respectively. These powers are below ANSI standard guidelines for safe light exposure. Depth scanning (penetration distance into the tissue) was achieved electronically via piezo-electric modulation of the fiber length in the reference and sample arms of the interferometer. The in-depth resolution of the OCT scanner was 13 μm (830 nm) and 17 μm (1280 nm), determined by measuring the point-spread function of the interferometer using a single reflective surface in the sample arm. For these experiments, a maximum scanning range of 3 mm was used. X-Y transverse scanning (up to 1 × 1 cm) was accomplished by automatic bending of the tip of the optical fiber behind the focusing lens. The transverse resolution was 17 μm (830 nm) and 22 μm (1280 nm), determined by stepping the focus point of the beam across a small pinhole and measuring the transmitted light. Positions of both infrared beams were tracked using a 640 nm CW diode which coupled into the same fiber and thus traveled together with the infrared beam. The operation of the OCT scanner was completely automated and controlled by a personal computer. Single transverse scans were accomplished in 2–5 seconds. In some cases, longer scan times (25 s) were used to provide a better signal-to-noise (higher contrast) image.
Scans simultaneously recorded both 830 and 1280 nm signals. We found that it is worthwhile to use two complementary colors (providing white or neutral gray when summarized in equal amounts) to represent these two-wavelength OCT images. The figures are presented in the “green-magenta” palette, where each pixel contains an information about the backscattered intensity at 830 nm in green color and about 1280 nm in magenta. This representation is in some sence similar to the human color vision (using two components instead of three) and help to increase the contrast and visibility of some structures in the same way as it takes place with real color images. For hard tissue images presented here we used single-wavelength OCT device operating at 1280 nm with about 2 mW superluminescent source.
Because the transverse scanning could be accomplished along any arbitrary direction in the lateral plane, we were able to scan 3D domains of arbitrary form within the tissue. The simplest demonstration of this capability was to acquire 3D images by recording a series of OCT B-scans in parallel tissue cross-sections. To form the 3D images, we developed tranparency software that permits observation of the reconstructed volume tomogram in an animated mode. Below, we present an example of this animation as a convenient and powerful method for 3D OCT tomogram viewing.
Our OCT unit also has the capability to record tomograms in the orthogonal polarization channel (with respect to the incident polarization) . Contributions to the interferometric signal are produced by those regions within the tissue that depolarize the light upon backscattering, or to the observation of sharp depolarizing boundaries deep within the tissue. To receive a signal form the tissue in the orthogonal polarization in a polarization maintaining (PM) fiber interferometer, we mounted a Faraday rotator in front of the optical scanner at the output of the probe arm. Light which passes into the tissue experiences a 45 degree rotation. Backscattered light that changes polarization experiences a 45 degree rotation upon collection back into the proper polarization for the PM fiber and can be interferometrically sensed by the photodetector. To facilitate switching between normal and orthogonal polarization modes without misalignment, the Faraday rotator was compact (1“ diameter by ½” thick) and used an electromagnet to apply the appropriate voltage.
To facilitate access to the tissue in the oral cavity, we constructed a specialized L-shaped probe terminating the scanning arm of the interferometer. It allowed us to probe any area of oral mucosa and almost all tooth surfaces (except for approximal (contact) surface). To obtain OCT images of the soft oral tissue in vivo, we selected 5 adult volunteers with the normal mucosa and performed OCT imaging in a clinical setting. The recorded tomograms were then analyzed and compared with data taken from textbooks on oral histology. To monitor the hard dental tissue, more than 20 patients were examined. Both normal and pathological structures have been imaged. Finally, on some patients, restorative procedures were also monitored in situ.
3. Oral Mucosa
The oral mucosa can be divided into three types: the masticatory mucosa (gingival and hard palate mucosa), the lining mucosa (alveolar, soft palate, labial, and buccal mucosa, as well as the mucosa of the mouth floor and the ventral surface of the tongue) and the specialized mucosa (lips, dorsum of the tongue). The histological organization and description of these tissues was taken from Oral Histology by Ten Cate  and more detailed description can be found therein. Here, we present tomograms of each type of mucosa and compare them with established histological representations.
3.1. Masticatory Mucosa
A tomogram of one type of masticatory mucosa, the hard palate mucosa, is displayed in Fig.1. The hard palate consists of bone covered with mucosa. Submucosa is absent here and the mucosa is tightly attached to the periosteum. The mucosa covering the hard palate exhibits a distinct keratinized layer called the stratum corneum (SC). In addition, capillaries loop up into the connective tissue papillae. In the tomogram, the 170 μm thick layer at the top of the tissue is referred to as the orthokeratinized stratified squamous epithelium. A characteristic feature of keratinized regions in the oral cavity is the presence of relatively high connective tissue papillae projecting into the overlying epithelium. The 200 μm thick region beneath the squamous epithelium is the lamina propria (LP). The papillae of the LP within the epithelium contain strong bundles of collagen fibers which are tightly interlaced and woven into the periosteum (bone covering tissue). In the OCT scan, a distinct boundary between the LP and the periosteum is visible.
Another example of masticatory mucosa is the gingival mucosa, a tomogram of which is shown in Fig. 2. Gingival mucosa (“gingiva”) is covered by keratinized stratified squamous epithelium. The thickness of the epithelium (EP), including the keratin layer and the epithelial papillae, ranges between 200–300 μm. The thickness of the LP is approximately the same. The EP contrasts weakly with the LP, appearing at a depth of 260–290 μm. The origin of the poor contrast between the EP and LP is not well understood, but our prior OCT studies of skin and mucous membrane also shown that keratinized EP typically poorly contrasts from LP. The total depth of OCT imaging in the gingival mucosa is 600–650 μm.
3.2. Lining Mucosa
The mucosa of the soft palate is an example of lining mucosa and differs structurally from the mucosa of the hard palate (masticatory mucosa) by the absence of a corneous layer in its EP and by the presence of submucosa. Fig.3 displays a tomogram of the mucosa of the soft palate. The nonkeratinized epithelium (330 μm) is seen as a very transparent strip. The LP smoothly passes into the submucosa which is well developed here (630 μm deep). The long shadows found in the connective tissue layers may be fatty elements, ducts of salivary glands, or muscular fibers.
Another example of lining mucosa is the alveolar mucosa. It is similar in structure to the soft palate mucosa but has a significantly thinner epithelium and the presence of weakly pronounced papillae of LP. Fig.4 displays an OCT image of vestibular alveolar mucosa. The EP is seen as a straight, transparent layer ~150 μm in thickness. The LP, seen as the brightly backscattering (500 μm thick) strip in the OCT scan, is a fibrous connective tissue structure and is separated from the EP by a basement membrane. It also contains muscle fibers and blood vessels which weakly backscatter and appear as dark structures in the scan above the darker, bony attachment.
Features in the structure of another lining mucosa, buccal mucosa, are quite specific and warrant some discussion. The buccal mucosa is commonly subdivided into three zones: the zona maxilaris, the zona mandibularis, and the zona intermedia. Figures 5 and 6 show OCT images of the zona maxilaris in the premolar area. The non-keratinized epithelium with small papillae is seen in the tomograms as a weakly scattering smooth layer with thickness increasing from 170 to 240 μm as it approaches the zona intermedia. The imaging depth of connective tissue layers in the upper parts of the zona maxilaris is about 650 μm, while closer to the zona intermedia the depth increases to 850 μm (Fig.6). Serial imaging may display specific features in the structure of these connective tissue layers. In the upper parts, the LP is clearly identified as a highly backscattering layer (Fig.5) which becomes less distinct near the zona intermedia (Fig.6). The weakly scattering shadows observed tomographically in the LP and the submucosal layers may be attributed to blood vessels as well as salivary glands. More specific identification of these special features requires parallel histological and tomographic investigations.
3.3. Specialized Mucosa
An example of specialized mucosa, the dorsum of the tongue, is shown in Fig. 7. The mucosa of the lingual dorsum has no submucosa and is directly attached to the muscular body of the tongue. Four types of lingual papillae are derivatives of mucous membrane and have common structural outline. Filiform papillae make up the majority of lingual papillae. Their surface is covered with keratinized EP. In Fig. 7, the keratinized EP is barely differentiated from the LP.
Our OCT investigations of this and other types of oral mucosa highlight the influence of keratinization on the OCT image. The presence of keratin in the EP influences the OCT appearance of mucosa. OCT images of those parts where EP evidences high keratinization (marginal gingiva, vermillion border of the lip, buccal zona intermedia, dorsal surface of the tongue, hard palate) substantially differ from images of those parts where the EP evidences low or no keratinization in its normal state (alveolar mucosa, labial mucosa, floor of the mouth, and soft palate). Keratinization may reduce the contrast and makes it difficult to distinguish the LP and submucosa from the EP. In non-keratinized mucosal epithelium, the LP and submucosa (if present) are well contrasted and muscular and bony tissues attached to the mucosa can be seen. OCT imaging also reveals blood vessels and glands in the LP and submucosa because their optical properties differ significantly from their environment (fibrous connective tissue).
4. Polarization Imaging of Normal Dental Hard Tissue
Three hard tissues: enamel, dentin, and cementum comprise the solid structure of teeth. Enamel comprises the outer shell of the crown (the part of the tooth exposed above the gingiva to the oral environment). Dentin comprises the inner shell of the crown and the root shell (the part of the tooth below the gingiva and connected to a bony socket) which is continuous with the crown dentin.
The outer shell of enamel is internally contiguous with the inner shell of the crown dentin at an interface known as the dento-enamel junction (DEJ). Root dentin is covered by a thin (20–200 μm) sheet of a third hard tissue known as cementum which is contiguous to the base of the crown enamel at a circumferencial line known as the cemento-enamel junction (CEJ). The inner central portion of the tooth (both crown and root) is cavernous and known as the pulp, being filled with water-like fluid which houses nerves, blood vessels, and both specialized and undifferentiated cells.
Enamel is only slightly porous and is composed of rod-like elements, approximately 5 μm in diameter and 0.1–4 mm in length, depending upon where they are located on the crown. These elements are composed mainly of hydroxyapatite crystals. They laminate together in the crown, separated from each other by only a thin protein sheath. They snake in sigmoid fashion from the DEJ to the outer surface of the crown, meeting this outer surface at angles which vary according to location.
Dentin is composed of a collagen scaffolding, highly mineralized by hydroxyapatite, which form a multiplicity of fluid and cell-filled tubules (1.0–2.5 μm in diameter) which traverse the entire thickness of the dentin in sigmoid fashion from the pulp surface to the DEJ. Unlike enamel, dentin is highly light scattering and gives the tooth its yellowish appearance through the more translucent enamel.
Fig. 8 displays a tomogram of tooth #11 which has been scanned perpendicular to its facial surface from the midline of its incisal edge to the center of that surface. (In all of these discussions, we use the two-digit ISO/FDI numbering system for identifying teeth.) This tooth is from a 52 year old male who evidences considerable enamel wear on the incisal edge (left vertical margin of the figure) of this tooth and further evidences sclerotic (heavily mineralized) dentin beneath this worn incisal edge. Normal dentin is evidences below (to the right on the tomogram) this sclerotic dentin. Note the horizontal DEJ approximately 1 mm below the facial surface (top margin of the tomogram) and the left to right downward sloping lingual surface DEJ 1.5 – 2 mm below the facial surface. One is tempted to speculate that the highly backscattering lines evident in the facial enamel are fault lines congruent with lines of incremental growth. Fig. 9 displays this identical area as imaged through the orthogonal polarization channel. This image shows distinct differences from the normal polarization image. First, the high reflectivity 'echoes' present in the normal OCT image are suppressed in the orthogonal polarization image. In addition, the overall amount of coherent backscattered light is reduced in the orthogonal polarization image. Moreover, the fault lines present in the facial enamel show better contrast in Fig. 8.
To demonstrate the capability of our device for 3D imaging, we recorded the volume surrounding a composite resin dental restoration in vitro by taking a series of 100 parallel B-scans (total scan time of 3 min.). The data were processed using routines to provide a transparent presentation of the image in an animated mode. One frame of the animation is presented in Figs. 10. The full animation can be run by clicking the Fig. 10.
5. Hard Tooth Tissue Lesions
5.1. Caries Lesions
Dental caries is a multifactorial pathological process, initially characterized by local demineralization of the hard tooth tissue which eventually often leads to morphologic cavitation and further tissue destruction (the formation of a lesion). This initial local demineralization appears in tomograms as an inhomogeneous, highly backscattering region. Several classifications of dental caries lesions are used. Caries lesions may be classified according to the tissue(s) in which they are located (e.g., enamel and/or dentin), the tooth surface areas they affect (e.g. occlusal, smooth surface), and/or their suspected activity levels or association with dental prostheses (e.g., active vs. arrested, acute vs. chronic, primary vs. secondary).
A fissure caries lesion is one of the forms of occlusal caries lesions. The significant depth of fissures through enamel (1–2 mm) and pigmentation in this region often make it difficult to clinically diagnose the lesion. Obviously, OCT can be useful in finding this type of lesion. Tomographically, it is represented by a strongly backscattering region on the tooth surface in the fissure area. Fig. 11 demonstrates a typical image of a caries lesion in the fissure. The defect occupies a region 250–270 microns thick, at this point in time still confined to the enamel.
A smooth surface caries lesion is represented by a cervical caries lesion in Fig. 12. To the left of the lesion there is healthy tooth surface with a well defined dento-enamel junction (DEJ). To the right, the gingiva can be seen. The depth of the tissue defect indicates the lesion is confined to the enamel.
The capabilities of OCT are especially important when a caries lesion is completely hidden under the visually observed tooth surface. Such is often the case with a secondary caries lesion, one which develops at the interface between the tooth tissue and the restorative material. Fig. 13 reveals a caries lesion immersed in dentin under a composite resin prosthesis placed through the occlusal surface.
The pervasive use of fluoride in dentistry (drinking water, toothpaste, topical applications, rinses, etc.) has positively affected the remineralization of the surface area of the caries lesion resulting in an observable change in lesion morphology. There are now many cases of chronic caries lesions with a small cavitation leading to a large volume of caries tissue. These lesions are easily imaged by the OCT technique (Figs. 14a,b).
To compare the capabilities of OCT with the most common method of dental hard tissue diagnosis, we produced several x-ray images of OCT examined teeth with caries lesions. The images were digitally recorded using a CCD camera in a standard radiography process. Fig.15 (X1) is an X-ray image corresponding to Fig.11. Comparison of these images indicates that radiography does not provide structural information with as great a level of detail as does OCT.
5.2. Noncaries Lesions
We have also used OCT to monitor several hard tooth tissue lesions of a noncaries nature. An abfraction lesion is a loss of tooth tissue, sometimes in the occlusal region of the tooth, but more often in the cervical region, usually on the facial or buccal surface [23,24]. The lesion is thought to result from repetitive deformation of the tooth under occlusal load. The cervical lesion is typically V-shaped, (i.e., a horizontal wedge). The enamel structure in the defect is characterized by increased mineralization and narrowing of the space between enamel prisms. Fig.16 shows such a cervical lesion in enamel supported by an intact, healthy dentin structure (gingiva can be seen to the right). OCT imaging allows one to clearly differentiate histologically this abfraction lesion from a caries lesion (Fig.12).
6. OCT in Evaluation of Operative Dentistry Restorations
Several different materials are presently used in operative dentistry for dental restoration. Their physical properties (e.g., compressive strength, modulus of elasticity, coefficient of thermal expansion/contraction, index of refraction, etc.) are more desirable as they better approximate those of the dental tissues with which they form interfaces. OCT appears to be a promising technique for examining the structural quality of these restorations. We have OCT imaged several teeth in vivo that have been restored with various restorative materials.
Figures 17 a–c show tomograms of restored teeth using amalgam, composite resin, and compomer, respectively. Note that the light scattering from these materials is greater when compared to that of healthy hard tooth tissue. The amalgam (by virtue of its metallic composition) completely obscures the tooth interior beneath it in an OCT image. However, the other two materials, by virtue of their small absorption coefficients, allow us to distinguish such internal landmarks as the DEJ in the tomogram.
Within the compomer restoration, we can examine the homogeneity of the material and the gap between the material and the tooth tissue, both important indicators of structural quality. Fig.18a demonstrates the presence of an air bubble within the composite resin material. Fig. 18b demonstrates a gap between the restorative material and the tooth tissue. It is not completely clear is it an air gap or it is filled by transparent glue. Either defect might require replacement of the restorative material.
The OCT technique is also capable of dynamic monitoring of the restorative process, an important quality control opportunity. In the following series of tomograms recorded from nearly the same position on the tooth surface, we demonstrate step-by-step the course of a restorative procedure. Fig. 19a is a preoperative image of a caries lesion in the cervical area of the root of a tooth. Drilling with a diamond bur removes the damaged dentin and creates a so-called smear layer on the cut tooth surface, Fig. 19b. Upon acid etching this smear layer is eliminated and the dentinal tubules are thus opened (Fig. 19c). The restored tooth is shown in Fig. 19d, demonstrating a perfect seal between the composite resin restorative material and the tooth tissue.
In vivo and in vitro imaging of hard and soft tissue of the human oral cavity has been demonstrated using OCT. We have imaged and differentiated several types of oral mucosa and healthy and damaged tooth structures and, in addition, have demonstrated the efficacy of OCT as a diagnostic tool in dental restorative procedures. OCT accurately depicts dental tissue structure and is able to detect small anomalies in that structure. These results suggest that OCT is a potentially useful modality for dental clinical and research applications.
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