1. Introduction

During the past century, the nature of dental decay or dental caries in the US has changed markedly due to the introduction of fluoride to the drinking water, the use of fluoride dentifrices and rinses, application of fluoride topicals in the dental office and improved dental hygiene. In spite of these advances, dental decay continues to be the leading cause of tooth loss in the US. The nature of the caries problem has changed dramatically with the majority of newly discovered caries lesions being highly localized to the occlusal pits and fissures of the posterior dentition and the interproximal contact sites between teeth (Fig. 1). These early carious lesions are often obscured or “hidden” in the complex and convoluted topography of the pits and fissures or are concealed by debris that frequently accumulates in those regions of the posterior teeth. Moreover, such lesions are difficult to detect in the early stages of development. By definition, early caries lesions are those lesions confined to the enamel and have not yet penetrated into the inner dentin. In the caries process demineralization occurs as organic acids generated by bacterial plaque diffuse through the porous enamel of the tooth dissolving the mineral. If the decay process is not arrested, the demineralization spreads through the enamel and reaches the dentin where it rapidly accelerates due to the markedly higher solubility and permeability of dentin. The lesion spreads throughout the underlying dentin to encompass a large area, resulting in loss of integrity of the tissue and cavitation. Caries lesions are usually not detected until after the lesions have progressed to the point at which surgical intervention and restoration are necessary, often resulting in the loss of healthy tissue structure and weakening of the tooth. The caries screening and treatment paradigms that were developed in the past, based on radiography, for example, are adequate for large, cavitated lesions, however they do not have sufficient sensitivity or specificity for the detection of early non-cavitated caries, particularly in the early stages. Therefore, new imaging technologies are needed for the early detection of such lesions. Moreover, the treatment for early dental decay or caries is shifting away from aggressive cavity preparations that attempt to completely remove demineralized tooth structure toward non-surgical or minimally invasive restorative techniques. In non-surgical therapy, a clinician prescribes antibacterial rinses, fluoride treatments, and dietary changes to arrest demineralization and facilitate remineralization of the caries lesion before it becomes irreversible [2]. The success of this type of therapy is contingent on early caries detection and also requires imaging modalities that do not require ionizing radiation that can be used safely and accurately to monitor the success of such treatment.

 

Fig. 1. New caries lesions are found in either contact sites between teeth, interproximal, or in the pits and fissures of occlusal surfaces.

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In our previous article, we demonstrated that the near-IR (NIR) is ideally suited for the transillumination of interproximal caries lesions located at the contact points in between teeth [1]. A NIR imaging system operating at 1310-nm was used to acquire images through whole teeth and tooth sections of varying thickness. Simulated lesions, which model the optical scattering of natural dental caries, were placed in plano-parallel dental enamel sections. The contrast ratio between the simulated lesions and surrounding sound enamel was calculated from analysis of acquired projection images. The results showed significantly higher contrast between the lesion and the enamel at 1310-nm vs. either the visible range or 830-nm [3].

Over the past thirty years there has been an effort to develop optical methods for the detection and imaging of dental decay. These methods include fiber-optic transillumination, fluorescence based methods and optical coherence tomography. The development of high intensity fiber-optic illumination sources has led to the introduction of fiber-optic transillumination [4]. DiFOTI^™ (Digital Fiber-Optic Transillumination) from Electro-Optical Sciences, Inc., utilizes visible light, and has recently received FDA approval [5]. However, the strong light scattering of sound dental enamel at visible wavelengths, 400–700 nm inhibits imaging through the tooth. There are two fluorescence based caries detection systems that can be applied to occlusal decay that are being sold in the US, they are the Diagnodent^™ (Kavo, Germany) and QLF^™ (Inspektor Research Systems, Netherlands). Alfano [6] demonstrated that laser induced fluorescence (LIF) of endogenous fluorophores in human teeth could be used as a basis for discrimination between carious and noncarious tissue. Upon illumination with near UV and visible light and imaging of the emitted fluorescence in the range of 600–700 nm, carious/demineralized areas appear dark. Image contrast is provided by the attenuation of the fluorescent light excited in the sound underlying dentin as it passes through the highly scattering demineralized tissue. The Diagnodent^™ uses red light to excite porphyrin fluorescence from bacteria byproducts trapped in the pores of demineralized tissue for the detection of “hidden” occlusal caries lesions [7]. Although, this must be considered a major step towards better caries detection in occlusal surfaces, the principal limitation of this device is that it detects lesions in the later stage of development after which the decay has penetrated into the dentin and accumulated a considerable amount of bacterial byproducts and has a poor sensitivity (~0.4) for early lesions confined to enamel[8].

Optical coherence tomography (OCT) is a noninvasive technique for creating cross-sectional images of internal biological structure. The intensity of backscattered light is measured as a function of its axial position in the tissue. Several groups have used OCT to image oral tissues [9–10]. Demineralization (decay) in the enamel was resolvable to a depth of 2–3-mm into the tooth. Polarization sensitivity (PS-OCT) provides additional contrast between sound and demineralized tissues [11–12]. However, OCT is not well-suited for imaging entire tooth surfaces or interproximal surfaces in between teeth due to time restraints and the enormous quantity of data that would be collected. The NIR imaging system described in this article is well-suited for imaging interproximal lesions and for acquiring images of entire occlusal surfaces, contrasting the areas of demineralization and removing the confounding influence of stains, pigmentation and fluorosis. We can envision using PS-OCT as an adjunctive tool to acquire specific depth resolved images of the lesion depth and severity in the carious areas delineated in the NIR images.

In this paper, we demonstrate that NIR light at 1310-nm can also be used to acquire images of early occlusal caries lesions which constitute the majority of newly developing lesions. Images were acquired from extracted human molars and premolars and the NIR images are compared with radiographs and images taken with optical coherence tomography.

2. Materials and methods

2.1 Sample preparation

Thirty freshly extracted molars and premolars were acquired from dental offices in the San Francisco Bay area and the teeth were sterilized with gamma radiation. The teeth were stored in a moist environment with 0.1% thymol to prevent fungal and bacterial growth. It is important to maintain hydration of the tooth and desiccated teeth yield reduced light penetration. There was no special preparation of the teeth before imaging.

2.2 Near-IR (NIR) imaging

The NIR imaging set-up is shown schematically in Fig 2. Light from a single-mode fiber-pigtail coupled to a 1310-nm superluminescent diode (SLD) with an output power of 15-mW and a 35-nm bandwidth, Model SLED1300D20A (Optospeed, Zurich, Switzerland), was coupled to a 20-mm NIR fiber-collimator (µLS Micro Laser Systems, Garden, Grove, CA). The 20-mm collimated beam was focused by a 150-mm focal length cylindrical lens at a point just above the dentinal-enamel junction which would be just above the gingival margin (gum-line) of the tooth. This configuration resulted in a fairly uniform intensity level across the occlusal plane. We found that broadband SLDs were advantageous to avoid speckle. An InGaAs focal plane array (FPA) (318×252 pixels) the Alpha NIR^™ (Indigo Systems, Goleta, Ca) with a Infinimite^™ video lens (Infinity, Boulder, Co) was used to acquire all the images. The acquired 12-bit digital images were analyzed using IRVista^™ software (Indigo Systems, Goleta, Ca).

 

Fig. 2. Imaging setup: (A) InGaAs FPA with video lens, (B) tooth, (C) cylindrical lens, (D) collimator, (E) fiber-pigtail from SLD.

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The NIR light enters the teeth just above the gum-line and is highly scattered by the dentin. The diffusely scattered light migrates upward towards the surface of crown, into the occlusal surface of the tooth, Fig. 3. The enamel of the crown (outer white part) is transparent at 1310-nm and varies from 1–3-mm in thickness. As shown in the diagrams of Fig. 3 any demineralization results in attenuation of the diffuse light and interferes with imaging within the transparent enamel areas. Areas of demineralization appear dark. As the dental decay spreads deep into the fissures of the enamel it spreads very slowly Fig. 3A. When the decay reaches the dentin it spreads laterally underneath the surrounding enamel, Fig. 3B. It is this later decay that dentists call “hidden caries” because it cannot be seen from the surface and it is not yet of sufficient severity to appear on an x-ray, e.g., Fig. 7.

2.3 Visible and x-ray imaging

Reflected light visible images were acquired of each occlusal surface using a color 1/3” CCD camera with a resolution of 450 lines, Model DFK 5002/N (Imaging Source, Charlotte, NC), equipped with the same Infinimite^™ video lens. Radiographs were acquired by the teeth section directly on Ultra-Speed^™ D-speed or F-speed film (Kodak, Rochester, NY) using 75 kVp, 15 mA, and 12 impulses.

 

Fig. 3. Occlual image contrast model, areas of occlusal decay appear darker as diffuse light from within the tooth is attenuated by occlusal caries. (A) shallow caries in fissures, (B) caries that penetrate deep and spread in the underlying dentin may create a larger opacity on the FPA.

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2.4 Polarization optical coherence tomography (PS-OCT)

PS-OCT was used as a tool to provide further information about the occlusal lesions on the teeth examined in this study without requiring the destruction of the tooth. An all-polarization maintaining fiber system operating at 1310-nm was used with a 20-mW superluminescent diode with a bandwidth of 50-nm. The reflected signal was collected in both polarization channels, the slow-axis of the fiber carried the signal in the incident parallel (P) polarization while the perpendicular (S) polarization was in the fast-axis of the fiber. The system has been described in detail elsewhere[12]. Demineralized areas in the tooth cause a large increase in the reflected signal in the fast axis (perpendicular polarization) due to depolarization from scattering, allowing easier discrimination of caries lesions [12].

3. Results

Figure 4 shows a NIR image taken using the setup of Fig. 2, of a molar tooth with a large central pit. The reflected light image shows a dark pit at the center surrounded by brown areas. A dentist would find this tooth suspicious and probe the area of the pit with an explorer— a sharp metal probe—to see if the material in the pit is soft before deciding to open up the tooth with the drill. Such a primitive visual/tactile approach has a low sensitivity below 50% for detecting occlusal caries lesions. This tooth is covered with white blotches most likely due to fluorosis (hypomineralization), a developmental defect that does not need to be treated. Unfortunately, early dental decay (demineralization) also appears white and it cannot easily be discriminated from fluorosis—see Fig. 5a for an example of demineralization without fluorosis. The area of demineralization (dark areas) can be clearly discriminated from the area of fluorosis (white areas) in the NIR image. Areas of fluorosis are white in both the reflected light image and the NIR image while demineralization is white in the reflected light image and opaque in the NIR image. The NIR image shows a large opacity around the pit area that suggests a deep lesion that has spread under the enamel in the dentin, similar to the scenario described in the diagram of Fig. 3b. A radiograph was taken using D-speed film which is significantly more sensitive than the F-speed film typically used in the clinic. The radiograph shows a small opacity indicating that decay has penetrated into the dentin. If a lesion is large enough and deep enough to show up on an x-ray, it is fairly extensive and requires the clinician to open up the tooth with the drill for restoration. Therefore, the caries lesion of Fig. 4 is extensive enough to be seen on a radiograph and the NIR image clearly shows the location of the lesion and it’s breadth in the NIR image suggests that it is extensive enough to warrant intervention.

 

Fig. 4. (A) Reflected light image (visible) of molar with extensive flourosis and a large lesion (yellow circle) in the central pit, (B) Radiograph (D-speed film), (C) NIR image of tooth, the lesion is the large opacity in the yellow circle.

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Figure 5 shows a less severe lesion with more superficial decay. The reflected light image (A) shows white and stained (brown) areas peripheral in the network of fissures in the surface. In the visible reflected light image it is difficult to localize the position of the demineralization due to the similar color of the sound enamel and the graduation of pigmentation. The radiograph shows no decay. In the NIR image, the demineralized areas show up with high contrast and are easily differentiated from the sound enamel. The staining and pigmentation does not intefere with the NIR image. A sound tooth without decay is shown in Fig. 6 for comparison. Even though the tooth is not demineralized there are areas that appear with varying graduations of white that can be easily confused with areas of either demineralization or hypomineralization. There is no decay evident on either the radiograph or in the NIR image.

The premolar shown in Fig. 7 best demonstrates the enormous potential of the NIR for imaging occlusal decay. This tooth contains a “hidden” lesion, i.e., a caries lesion that has penetrated through the enamel into the dentin requiring restoration that cannot be seen on a radiograph. Visual inspection (A) shows this tooth contains a suspicious looking dark pigmented fissure that may possibly contain decay beneath. The radiograph (D or F-speed) (B) shows no apparent lesion. However, the NIR image (C) shows a subsurface opacity suggesting a subsurface caries lesion similar in nature to that described in the diagram of Fig. 3b. The presence of this lesion was confirmed using PS-OCT and by histological sectioning of the tooth for direct examination. The tooth was cut in two along the dotted yellow-line shown in (C) normal to the long-axis of the lesion and the fissure. An image of the exposed half of the tooth is shown (D), indicating that there is a caries lesion with substantial enamel demineralization at the base of the pit and the decay has spread into the dentin causing demineralization and extensive staining. Therefore the clinician would be required to open up the lesion for restoration. It is interesting to note that although the fissure is heavily stained there is no demineralization on the walls of the lesion. The two orthogonal PS-OCT scans normal to the fissure (parallel to the cut) are shown in (E&F) taken before the tooth was sectioned in two. The slow-axis (parallel axis) b-scan (E), does not show the lesion, however the fast-axis (perpendicular axis) b-scan (F), highlighting depolarization from scattering matches the location of the demineralized area at the base of the fissure very well, with the red area of high reflectivity in (F) corresponding to the demineralized area of (D). There is no enhanced signal from the walls of the fissure since they are stained and not demineralized.

 

Fig. 5. (A) Reflected light image (visible) of molar with demineralization in the fissures, (B) radiograph (F-speed film), (C) NIR image of tooth showing localized areas of decay.

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Fig. 6. (A) Reflected light image (visible) of sound molar without demineralization, (B) Radiograph (F-speed film), (C) NIR image of tooth.

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Fig. 7. Premolar with a hidden subsurface lesion, the reflected light image (A) contains a deep, pigmented fissure, but it is not clear whether there is decay. (B) Radiograph (D-speed film) shows no decay. The NIR image (C) contains a large opacity in the yellow square. The tooth was cut in half along the dotted line in (C), and the tooth hemi-section (D) shows the lesion at the base of the pit (red circle). Note that there is no decay along the walls of the fissure. The decay does not show up very well in the PS-OCT parallel axis scan (E) taken transverse to the fissure while the PS-OCT perpendicular axis scan (F) clearly shows the decay at the base of the pit. Areas of high reflectivity are in red, moderate reflectivity in white, and low reflectivity in blue in the PS-OCT scans.

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In reflected visible light images the fissures can appear dark due to the lower reflectivity and it can be difficult to identify areas of demineralization. This difficulty can be compounded by the presence of stains and plaque. The molar shown in Fig. 8 has an extensive network of small fissures in which some are stained and demineralized and others are sound. It is much easier to identify the areas of demineralization in the NIR without the inteference of stains and plaque. Moreover, such stains and pigmentation fluoresce strongly interfering with fluorescence based caries detection methods.

Uniform illumination is valuable to avoid false positives due to shadowing effects caused by the illumination. Figure 9 shows two images of a molar with decay localized to a few fissures, one image was acquired with the collimator/cylindrical lens and the other with just the collimator. Although, it is not necessary to use the more uniform field of illumination to acquire images of the areas of occlusal decay, it is clearly important to have a uniform level of intensity across the tooth to maximize the contrast between lesion areas and the sound enamel.

4. Discussion

 

Fig. 8. Molar with an extensive network of fissures (A) the reflected light image shows dark areas that may be either stains or demineralization (B) NIR image shows that some fissures have demineralization and others are sound or normal.

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Fig. 9. Comparison of illumination with a 20-mm in diameter gaussian beam (A) from the right side of the tooth and a (B) thin elliptical beam (20-mm major axis) generated with the cylindrical lens.

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NIR images of occlusal caries demonstrate that NIR imaging can also be used for the detection and imaging of caries lesions in occlusal surfaces where most new decay develops. This novel and straightforward method, exploits the high transparency of dental enamel and the strong scattering and weak absorption of the underlying dentin to deliver a uniform distribution of diffuse NIR light underneath the transparent enamel of the crowns to facilitate high contrast NIR imaging of the occlusal decay and detect hidden lesions beneath the surface. We have shown by imaging teeth with various natural occlusal caries lesions that this NIR technology can be used to acquire images of dental decay that is not detectable by conventional means either radiographically or by visual/tactile examination. Moreover, our initial NIR images suggest that fluorosis (hypomineralization), pigmentation and staining do not significantly interfere with imaging demineralization in the NIR. Such confounding variables interfere significantly with visual diagnosis of dental decay and cause false positives in fluorescent caries detection methods.

The bench-top setup of Fig. 2 can be easily converted to a clinical imaging system by connecting a endoscope attachment to the InGaAs FPA with a 90° mirror and introducing a small hand held fiber-optic probe to place against the side of the tooth to deliver the NIR light. These highly promising images of occlusal caries lesions and the images presented previously of interproximal lesions [1] suggest that NIR imaging has considerable potential as a tool for routine caries screening of the entire dentition. Another major advantage of this method is that it enables the dentist to treat early dental decay effectively in a non-surgical manner. Early lesions confined to the outer enamel layer such as those in Figs. 5, 8, and 9 can be treated with fluoride therapy. The dentist can acquire multiple NIR images of them during subsequent visits to determine if fluoride therapy is effective in arresting the lesion or whether the lesion has expanded, requiring more aggressive intervention. Such an approach is not practical with radiographic methods due to repeated x-ray exposure.

We can envision using PS-OCT in tandem with NIR imaging to acquire specific depth resolved images of the lesion depth and severity in suspect areas defined in the near-IR image and both of these NIR imaging systems can potentially share similar broadband light sources. This approach was demonstrated for the premolar tooth examined in Fig. 7, where the NIR image suggested a hidden lesion beneath the surface and the PS-OCT images confirmed the presence of the lesion and better established it’s specific location and severity.

It is interesting to compare NIR imaging with the existing optical methods for occlusal caries detection that have received FDA approval in the U.S. The Diagnodent^™ is designed to detect lesions such as those shown in Figs. 4 & 7 in which decay has penetrated to the dentin, but does not work well on early lesions confined to the enamel such as those of Figs. 5, 8, 9, that have not accumulated a sufficient density of bacterial porphyrin byproducts to fluoresce or for interproximal lesions that are not accessible to the probe. Moreover, the Diagnodent^™ does not provide an image of the decay and is easily confounded by stains and plaque[13]. QLF^™ is designed for very early lesions such as those of Figs. 5, 8, 9 in which the demineralization is localized at the surface [14]. The mechanism of QLF is of a similar nature to the NIR, i.e., light exciting the occlusal surface is attenuated by light-scattering in the caries lesion. However, the imaging light is yellow or red fluorescence generated below the enamel in dentin via excitation with incident blue-green light from the surface[15]. Thus, QLF will not be useful for imaging deep lesions such as those similar to the lesions of Figs. 4 & 7.

In summary, to the best of our knowledge, these are the first NIR images of occlusal caries lesions and they demonstrate the potential of the NIR for imaging dental decay and for overcoming some of the limitations of conventional methods of caries detection, namely, visual, tactile and radiographic. At the present time, the principal limitation of this method is the high cost of InGaAs imaging technology. However, it is likely that lower cost NIR technology will become available in the near future or alternative systems can be developed operating at 830-nm or 1550-nm, balancing cost, sensitivity and performance.