Abstract —Dental caries is the infectious oral disease affecting people worldwide. Early dental caries detection helps to prevent caries progression by non-surgical methods and promote tooth remineralization. A simple method based on laser induced fluorescence spectrum with backscattered enhancement is proposed for the detection of initial caries on vitro teeth. Fluorescence spectra of carious erosion on different status were given. The radiation of backscattering, reflection and auto- fluorescence from teeth was registered simultaneously. By the analysis the characteristics of auto-fluorescence and anti-stokes fluorescence, both chemical variations and structural and morphological alterations can be evaluated. After the intensity of
fluorescence and backscattering light is normalized by the intensity of the reflection radiation, a measurable diagnostic criterion for various stages of carious lesion is put forward. For proving this opinion, contrastive study was given in healthy tooth, factitial caries, dental calculus and healthy enamel with surface breakdown. Preliminary results reveal that this new method has a high potential use to diagnose different levels of carious teeth and is reliable to the detection incipient carious erosion, calculus and filling teeth.
Keywords-dental caries; dental laser tips; laser induced fluorescence; backscattering light; detection
I. I NTRODUCTION Dental caries has high incidence and seriously damages people’s health, which is a kind of oral disease aroused by bacterial infection. It is a major aim in the preventive dentistry to prevent demineralization caused by carious teeth and promote remineralization of initial caries. But when the caries is only detected at an incipient stage, such aim could be reached. Investigation on diagnostic techniques of early caries has huge actual importance and commercial application. Traditional caries diagnostic methods like dental explorer or visual inspection can only find caries in moderate or cavitated stage which is irreversible and can be treated just by filling. Dental radiographs can only find the carious erosions which are very serious and have more than one third of the depth of tooth enamel. These conventional methods are not adequate to detect initial caries for their poor specificity and sensitivity. Furthermore, the pattern of dental caries has changed dramatically now because that the fluoride is added to the drinking water and the fluoride toothpastes are widely used. These new damage is found almost in the occlusal sunken surface, the adjacent touch places between teeth and cracks of posterior dentition ˈ that are difficult to examine. Therefore more sensitive and reliable diagnosis techniques are required to detect early non-cavitated caries and monitor its decay process . Over the past decades, many efforts have been made to develop optical methods for caries detection. These methods include fiber-optic trans-illumination (FOTI) [4-7], fluorescence , photo thermal radiometry [9-11], multi- photon imaging , optical coherent tomography , Raman spectroscopy [15-20]. All of these are based on character alterations of tooth configuration. Fluorescence techniques are always animate in terms of caries detection. Ongoing investigations with laser and light induced fluorescence have demonstrated different sensitivity and specificity on detecting artificial and natural lesions of teeth[21-24]. In past years we have put our attention on researching caries detecting method, which registered induced florescence occurring when probing laser radiation interacted with tooth. Now, by investigating the interaction principles between laser radiation and tooth and relevance of remission radiation with tooth optic properties, we presented a new method for caries detection which analyzed the fluorescence spectra inluding backscattering radiation, auto induced fluorescence and back- reflected light. This method is verified by theoretical analysis and experiment results. II. D IAGNOSTIC P RINCIPLES People commonly think that insanitary oral cavity leads to the occurrence of carious damage. The acid-base balance in tooth is broken by the increase of oral germs and food residue. Microbes produce acids, thus, the pH value in oral environment decreased to approximately 3.5. Under such situation, the speed of caries erosion is much over the speed of autogenously repair of dent. In the end a nonreversible lesion in tooth occurred . In the process of the radiation of laser on carious tooth, auto induced fluorescence, back-reflected light and backscattering light would all emerge. Scattering is the duration of interaction of photons with the substance, and altering the transmission direction. The morphology varieties of tooth tissue can be presented by scattering characters of light . Rayleigh scattering light could be selected as a contrast criterion in the same substance because the photon energy of Rayleigh scattering is same as excitation radiation. In the procedure of scatter, backscattered light, which can give status, mineralization level and teeth color, consist of much information about dental ingredients, composition, spatial configuration . Fluorophore absorb some photons of the excitation light, and produce fluorescence from the superficies of tissue. Induced fluorescence is the light radiated from living tissue with no extrinsic fluorescent materials. The germs in tooth are intrinsic fluorescent substance, which play a significant action during the procedure of carious erosion. The progress of carious damage in teeth is related with the continual spread of germs, and the induced fluorescent intensity would vary accordingly. Diagnostic fluorescence frequency of carious damage is relevant to diagnostic frequency of bacteria, such as corpoporphyrines, zink- porphyrines, protoporphyrines which have the peak intensity at 635, 620, 590 nm . Therefore using He-Ne laser the strong fluorescence by resonance excitation technique can be achieved.
III. M ATERIALS AND D TECTION M ETHODS At this study, we used the He-Ne laser (632.8 nm) as a excitation radiation. The excitation power of laser was kept on 2 ± 0.1 mW. Fluorescence signal emitted from a researching sample go into SP500 monochromator and then enter into PMT (photoelectric multiple tube). The experimental devices are placed on a shockproof platform. As shown in Fig.1, there is not any addition of filters in detection structure. Before the teeth fluorescence spectra were collected and analyzed, the detection system should be calibrated.Extracted teeth were gathered by stomatology department, Hospital of WTU. And erosion degree of teeth were inspected by an experienced dentist. The intro teeth were scratched to remove the residual parenchyma on it and the specimen were completely washed by distillated water. Teeth were reserved in the 0.9% sodium chloride solution up to examination. The time interval between exelcymosis and test was no longer than seven days. When examination was carried on, the experimental temperature was set as 25 C.
A method using undulant nonlinear conversion of photons was put forward by doctor Masychev V. I., which is called PNC method . Rosslyn Medical of U.K. produced the detection system named as the Oliver-101 £ diagnosis system. A excitation light source is chosen as He-Ne laser for their study. The optic fiber equipment transfers the excitation radiation to the examined tooth and registers backscattering fluorescence and induced fluorescence of the tooth at the same time. A spectrometer installation examines the fluorescence of bacteria porphyrins. The excitation laser power was always maintained in a domain from 1 mW to 5 mW. They thought that the backscattered radiation comprised much messages relevant to caries ingredients, configuration, spatial pattern and recorded induced fluorescence signal can be employed to discriminate carious lesion. An optic filter was applied to decrease the intensity of excitation laser in their detection system. Meanwhile, most of light radiation with short wavelength and back reflection parts of the radiation from the tissue was restrained, but the fluorescence part of the signal would not be attenuated. In data treatment by computer, the attenuated backscattering radiation was utilized to normalize fluorescence intensity . In the forementioned two diverse techniques, a expert filter with long wavelength pass is chosen in their detection device. The laser excitation radiation is suppressed, and back reflection parts of the radiation from the tissue was restrained, but the fluorescence would not be attenuated. however, some most important information has been lost, for instance, changes of intensity in excitation line and radiation characteristic of short wavelength varied with tooth components, configuration, spatial pattern. In hospital application, calculus teeth and filling are often taken as carious lesion. Therefore a prospective research direction is to search a examine method that will surmount such shortcoming. For developing advanced optical techniques for caries erosion detection, it is important to understand the optic performances of tooth and the reason of them being produced. The changes of the optical performances is straightly associated with the level of mineral loss. Optical techniques are based on transmission, fluorescence and scattering characters of the human tooth, that is related to the demineralization extent. People also can achieve the radiation spectrum from filling tooth. There always are holes in filling teeth. In common cases, doctors will often get rid of the damage sections using drilling way before filling tooth. Thus tooth other section is an ocular appearance of intact tooth with enamel lesion, which were chosen to discriminate. The intensity of fluorescence emission of the external damaged enamel nearly has the identical amount as a healthy tooth. Due to the external breakdown, the spatial pattern of the broken tooth has varied dramatically, causing to that the intensity of anti-stokes scattering radiation is higher than intact tooth and has a maximum intensity near 626 nm, shorter wavelength. Thus, the spectra of filling teeth have clear difference with the intact teeth by this detection method. Irradiation to demineralization tooth leads to an accumulated absorption of tooth chromophores. The results has presented that variations in fluorescence spectra are straightly relevant to the degree of tooth mineral loss, especially when the excitation wavelength is accurately adjusted to that of emission light. Except light scattering, germs or their metabolins could affect the fluorescence intensity of the carious damages. In our experiments, the normalized coefficient of anti-stokes scattering C 1 can depict spatial pattern alterations in tooth tissue, and the normalized coefficient of stokes fluorescence C 2 , can represent chemical variations in dental tissue. Considering these normalized coefficients C 1, C 2 of special spectra of carious lesion, the various stages of dental caries were evaluated accurately. From fig.5, it can be seen that there is obvious distinction between the area of 5, 6 and 1, 2, 3, 4, corresponding to filling teeth, calculus and caries. The confusion of filling teeth, calculus and caries wouldn’t happen by our method.
The interaction of excitation radiation with tooth tissue and the association of remitted radiation with optical characters of caries is investigated, and spectra of excitation radiation, reflected and backscattering light are analyzed. Results present that the intensity of auto induced fluorescence is relevant to germs or their metabolites and the intensity of stokes and anti- stokes fluorescence radiation is related to tooth morphology. The most important aspect in our experiment, is the synchronous collection of the auto induced fluorescence, reflection radiation, backscattering light without filter and the analysis the data using a kind of algorithm. Discrimination of carious erosion was achieved by researching bacterial pathology variation in teeth and evaluating configuration and morphology alteration in caries. This method has a wide application perspective.