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Serial NIR reflectance images and LUTs of the ablation process are shown in Fig. Samples were held in place using exchangeable magnetic mounts which aided in maintaining the sample's positioning between the ablation and imaging systems. (East Providence, RI) was used to provide a uniform spray of fine water mist onto the tooth surfaces. A pressure air-actuated fluid spray delivery system consisting of a 780S spray valve, a Valvemate 7040 controller, and a water reservoir from EFD, Inc. (Cambridge) were used to scan the laser beam over the sample surfaces.
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Computer-controlled XY galvanometers 6200HM series with Micromax Series 671 from Cambridge Technology, Inc. A razor blade was scanned across the beam to determine the diameter (1/e 2) of the laser beam. The laser beam was focused to a spot size of ~350-μm using a planoconvex ZnSe scanning lens of 125-mm focal length. This laser is capable of high repetition rates up to 500 Hz, and a fixed repetition rate of 100 Hz was used for the following experiments. The laser was custom modified to produce a Gaussian output beam (single spatial mode) with a pulse duration between 10–15-μs. In this study we focused on the algorithm used for the segmentation of NIR images used to guide an IR laser for the removal of natural occlusal lesions.Īn industrial marking laser, Impact 2500 from GSI Lumonics (Rugby, UK) operating at a wavelength of 9.3-μm was used to ablate lesion areas. There is also considerable interference from developmental defects which are almost indistinguishable from demineralization due to caries. Demineralization can encompass an entire occlusal surface of the tooth, be localized to a single spot, or hidden well below the surface. Teeth have highly convoluted topography particularly in the occlusal surfaces and caries lesions lack any distinguishable shape or contour to be processed using conventional pattern recognition algorithms. There are several challenges in creating maps of demineralization for guiding the laser from NIR reflectance images. New NIR reflectance imaging analysis protocols are required before an integrated caries removal system will be acceptable for clinical use.
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Previous NIR reflectance caries removal studies have relied on manual caries segmentation in each NIR image acquisition. A major factor contributing to the subjectivity of automated image-guided removal of caries heavily relies on the operator's caries assessment of the NIR image. CO 2 lasers (λ = 9.3-μm) operating with laser pulses 10–15-μs are well suited for caries removal. In an earlier study, we imaged carious teeth with NIR reflectance and used a CO 2 laser system to remove naturally occurring caries lesions on the occlusal surfaces of teeth in vitro. Lasers have been previously integrated with caries detection technologies including fluorescence and near-IR transillumination. Thus, an integrated system combining a rapid laser scanner with a caries lesion detection imaging modality, for instance NIR reflectance, is feasible for removing demineralization in a highly conservative manner at clinically applicable rates. Lasers can ablate tissue in a non-contact mode of operation and a pulsed laser beam does not interfere with the ability to image the tooth surface nor significantly distort the contrast between sound and demineralized dental hard tissues. Lasers can be operated at high pulse repetition rates and scanned at high speeds using micro-electromechanical system (MEMS) mirrors and miniature galvanometer based scanners. NIR imaging not only facilitates caries diagnosis, but could also be developed into a computer automation system for caries removal. Therefore, NIR reflectance can generate high contrast images that can delineate sound tissue from demineralization. Stains often interfere with visible reflectance and QLF assessment of demineralization, but are not visible in the NIR since pigmented organic molecules poorly absorb NIR light. NIR reflectance is a new caries detection technique that has considerable advantages over conventional caries detection methods, namely it is non-invasive (it uses harmless NIR light), has greater sensitivity in elucidating demineralization, and lacks interference from stain. Recent studies have shown that near-IR (NIR) reflectance imaging at 1450-nm and 1500–1700-nm can be used to acquire images of enamel demineralization with very high contrast.