We present an advanced technique of polarized light microscopy (PLM) for the assessment of carious lesions, wherein the degree of polarization (DOP) is evaluated. Through modifications of a conventional PLM setup, images were generated by calculating the DOP from an image series of different linear polarization images acquired with a polarization camera. Demineralization is reliably displayed by the DOP in accordance with the common imaging methods. Evaluating the DOP by PLM allows the characterization of the different pathohistological zones of caries with a new quantitative approach.
Polarized light microscopy (PLM) is an established technique for the inspection of thin tooth sections in dental research. However, conventional PLM is mainly based on the qualitative evaluation of color-dependent birefringence, depolarization and transmittance but lacks a quantitative interpretation and, thus, the comparison with other polarimetric imaging methods. Here, we describe an easy to implement extension of PLM that enables measuring the degree of polarization (DOP). By replacing the analyzer and RGB camera in PLM with a monochrome polarization camera, linear polarization states can be directly determined from the transmitted light. Additionally rotating the polarizer in the illumination path and, by that, the state incident into the sample facilitates reconstructing a DOP image from multiple linear measurements. The resulting depolarization measurements are compared with conventional PLM images as well as X-ray micro-computed tomography (μCT) data of the intact teeth. Our results show that caries and demineralization appear to be directly related to depolarization in enamel. However, the interpretation is more complicated for dentin, which shows a reduced DOP also in sound tissue. We assume that these insights support the development and analysis of future dental polarimetry techniques in vivo, such as intra-oral polarization-sensitive optical coherence tomography (PS-OCT)
Caries of the interdental space are usually difficult to detect with conventional bitewing radiography. We have developed an interdental probe for polarization-sensitive optical coherence tomography (PSOCT) that can distinguish between healthy and carious tooth structures. The development is based on a fiber optic probe with a compact motor design that allows volumetric imaging. In a clinical study, demineralization was detected using proximal PSOCT scans of premolars and molars, and compared with blind diagnoses from four dentists. The results show that PSOCT can detect both, early demineralization and advanced proximal caries, some of which could not be diagnosed with bitewing radiography.
Optical coherence tomography (OCT) is a non-invasive, radiation-free optical technique for caries diagnosis. The presented study dealt with the correlation of conventional caries detection methods and using polarisation-sensitive OCT (PS-OCT). Human extracted carious teeth were assessed by dentists with visual and radiographic examination and PS-OCT images were gained. Proximal lesions could be detected early in the intensity-based OCT cross-sectional images. The degree of polarization can clearly delineate proximal caries from surrounding healthy tooth structure. The obtained results pave the way to reduce invasive filling therapy through early possibilities of intervention.
The prerequisite for a successful caries management is the early and precise detection of lesions. We have recently shown that depolarization imaging based on polarization-sensitive optical coherence tomography (PS-OCT) is a promising tool to detect demineralized enamel. To validate this contrast mechanism, we acquired data sets of tooth samples with occlusal caries lesions using PS-OCT and x-ray micro-computed tomography (µCT). Spatial alignment of the PS-OCT and µCT volumes was achieved by creating point clouds from surface segmentations, and subsequent registration. PS-OCT depolarization signals are in good agreement with the µCT results, showing the potential of this approach for caries diagnostics.
While dental caries represents the major chronic disease of humans, visual and tactile inspection are the methods of choice in daily dental practice. Several optical technologies have been developed in recent years for the purpose of dental examination, including near-infrared light transillumination as a promising tool for the supplementation and partial replacement of radiography. In case of occlusal alterations, the incidence of surface discolorations impedes the visual assessment, whereas tactile inspection appears to yield little diagnostic information or might be detrimental. Optical coherence tomography (OCT) facilitates depth-resolved imaging with μm resolution, utilizing near-infrared light, and has already shown its potential for various dental applications. We have recently demonstrated that depolarization imaging utilizing the degree of polarization uniformity (DOPU) extends those abilities by the detection of early carious lesions, as it provides an unambiguous demineralization contrast. Here, we show that this approach also enables the assessment of occlusal enamel lesions in the presence of stains, and compare PS-OCT cross sections with polarized light microscopy (PLM) images of thin sections. For tooth samples with discoloration or demineralization, respectively, PS-OCT and PLM results are in good agreement.
The detection of incipient carious lesions is crucial for the preservation of dental hard tissue with preventive therapies. Optical coherence tomography (OCT) based imaging has been proofed to be a valuable methodology for the depth-resolved detection of alterations at dental hard tissue. Due to changes of birefringent properties at carious lesions, the measurement of polarization properties with polarization-sensitive OCT (PS-OCT) provides an additional contrast that could enhance the detection of incipient carious lesions. The aim of the present study was to monitor changes of polarization properties during the initial demineralization process by means of PS-OCT. Therefore, 18 bovine enamel palates were demineralized up to 7 weeks in an artificial demineralization model using lactate acid and buffer solution to mimic natural caries progression. The palates were stored at pH 5.5 and 35°C with constant fluid movement. In order to detect changes of depolarization at different stages of demineralization, the degree of polarization uniformity (DOPU) was calculated from PS-OCT measurements, performed under wet and dry conditions. In addition, polarization microscopy and micro-computed tomography of thin sections were analyzed to assess lesion depth and mineral loss. Our main finding is that consecutive stages of demineralization are correlated to an increasing DOPU contrast between sound enamel and the lesion area, where the lesion area is connected to decreasing DOPU values. Our results show that PS-OCT based imaging of birefringence changes is sensitive to very early stages of the demineralization process and adds a valuable contrast for the detection and monitoring of incipient enamel lesions.
The common way to diagnose hard and soft tissue irregularities in the oral cavity is initially the visual inspection by an experienced dentist followed by further medical examinations, such as radiological imaging and/or histopathological investigation. For the diagnosis of oral hard and soft tissues, the detection of early transformations is mostly hampered by poor visual access, low specificity of the diagnosis techniques, and/or limited feasibility of frequent screenings. Therefore, optical noninvasive diagnosis of oral tissue is promising to improve the accuracy of oral screening. Considering this demand, a rigid handheld endoscopic scanner was developed for optical coherence tomography (OCT). The novelty is the usage of a commercially near-infrared endoscope with fitting optics in combination with an established spectral-domain OCT system of our workgroup. By reaching a high spatial resolution, in vivo images of anterior and especially posterior dental and mucosal tissues were obtained from the oral cavity of two volunteers. The convincing image quality of the endoscopic OCT device is particularly obvious for the imaging of different regions of the human soft palate with highly scattering fibrous layer and capillary network within the lamina propria.
As dental caries is one of the most common diseases, the early and noninvasive detection of carious lesions plays an important role in public health care. Optical coherence tomography (OCT) with its ability of depth-resolved, high-resolution, noninvasive, fast imaging has been previously recognized as a promising tool in dentistry. Additionally, polarization sensitive imaging provides quantitative measures on the birefringent tissue properties and can be utilized for imaging dental tissue, especially enamel and dentin. By imaging three exemplary tooth samples ex vivo with proximal white spot, brown spot, and cavity, we show that the combination of polarization sensitive OCT and the degree of polarization uniformity (DOPU) algorithm is a promising approach for the detection of proximal carious lesions due to the depolarization contrast of demineralized tissue. Furthermore, we investigate different sizes of the DOPU evaluation kernel on the resulting contrast and conclude a suitable value for this application. We propose that DOPU provides an easy to interpret image representation and appropriate contrast for possible future screening applications in early caries diagnostics.
Since optical coherence tomography (OCT) provides three-dimensional high-resolution images of biological tissue, the benefit of polarization contrast in the field of dentistry is highlighted in this study. Polarization-sensitive OCT (PS OCT) with phase-sensitive recording is used for imaging dental and mucosal tissues in the human oral cavity in vivo. An enhanced polarization contrast of oral structures is reached by analyzing the signals of the co- and crosspolarized channels of the swept source PS OCT system quantitatively with respect to reflectivity, retardation, optic axis orientation, and depolarization. The calculation of these polarization parameters enables a high tissue-specific contrast imaging for the detailed physical interpretation of human oral hard and soft tissues. For the proof-of-principle, imaging of composite restorations and mineralization defects at premolars as well as gingival, lingual, and labial oral mucosa was performed in vivo within the anterior oral cavity. The achieved contrast-enhanced results of the investigated human oral tissues by means of polarization-sensitive imaging are evaluated by the comparison with conventional intensity-based OCT.
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