Diagnosis And Evaluation Of Early Dental Caries And Cracked Tooth By Using Laser Surface Acoustic Waves

Dental caries and crack teeth’s early diagnosis technologies have always been the hot and difficult issues in oral clinical inspection field. Compared with the existing clinical diagnosis techniques for dental caries and cracked teeth, laser ultrasonic technique has many advantages, such as non-contact, high spatial resolution, fast detection speed, can adapt to complex topography and so on. It has enormous development potential and limitless application in clinical detection field. The present thesis studies the diagnosis and evaluation of early caries and cracked teeth by using laser ultrasound, theory and experiment. The feasibility of laser ultrasonics using for dental clinical detection is analyzed, and the effects of teeth’s morphology structural differences on the propagation properties of ultrasound are studied. And the nondestructive detection and evaluation of early caries and cracked teeth by using laser ultrasound are also studied.In this paper, on the basis of wave equation and thermoelasticity theory, a finite element model is accordingly developed to simulate ultrasound waves generated by a pulsed laser in human teeth. The temperature field and the displacement field generated in human teeth by laser with different wavelength and different power density are discussed. According to the theory calculations and the analysis of experiments, it demonstrates that the excitation efficiency of ultrasound is high when the generation laser’s wavelength is 266 nm and its power density is smaller than 100 MW/cm2, and in this case, the enamel and dentin will not be damaged at all, which make it possible to use laser ultrasound for clinical detection of teeth.By using a laser generating ultrasound on human teeth, according to the effects of dental lesions on the time and frequency domain characteristics of ultrasonic propagation, the early caries and cracked teeth can be diagnosed. Because of the diverse morphology and structure of human teeth, ultrasound’s propagation properties will also be affected. In this paper, an all-optics generation and detection system for ultrasound is built up firstly. The ultrasonic waves propagating in human incisor and molar with different surface curvature and structure are detected. Then the finite element models of incisor and molar are built up for calculating the time domain waveforms of surface acoustic waves (SAW), which propagate in incisor with different depth of enamel and molar with different surface curvature and enamel thickness. On the basis of time-frequency analysis of the experimental and calculational ultrasonic signals by using two-dimensional fast Fourier transform (2D-FFT) method, the dispersion curves of surface acoustic waves can be obtained. According to the research of SAW’s frequency domain characteristics, it demonstrates that the dispersion curves of SAW within 10 MHz will be greatly affected by the differences of enamel’s thickness and curvature, the bigger the curvature is, the bigger the phase velocity is. Therefore, according to the SAW’s dispersion curves within 10 MHz, the characteristics of teeth’s morphology and structure can be studied.For eliminating the influences of teeth’s morphology and structure on ultrasonic diagnosis, artificial caries with different progress are made on the same position of the same tooth. SAW signals are detected on sound enamel, early caries with 5 minutes demineralization and caries with 10 minutes demineralization, respectively. The results demonstrate that the phase velocity of SAW in high frequency region will greatly affected by the enamel’s demineralization. The early caries in incisor and molar can be successfully diagnosed. The finite element model of early caries is built up, and the lesion region’s elastic properties are characterized through the coefficient of demineralization ω. According to the quantitative calculation, the elastic modulus of sound enamel, early caries with 5 minutes demineralization and caries with 10 minutes demineralization can be inversed to ～92 GPa、 ～73 GPa、～56 GPa. The results demonstrate that early caries can be diagnosed and quantitative evaluation by using laser ultrasound.By using scanning laser line source technology, two kinds of small cracks in teeth can be detected according to the curves of the peak-to-peak amplitudes of SAW, which are changed with the scanning position. The finite element models of cracked teeth are built up for calculating the time domain waveforms of surface acoustic waves quantitatively, which propagate in incisor with different crack depth from 0.05 mm to 2.0 mm. According to the gain and attenuation coefficient (η and τ), the two cracks’depth can be evaluated to ～0.1mm and 0.8-1.0mm, respectively. The results demonstrate that cracked teeth can be diagnosed and quantitative evaluation by using laser ultrasound.In this paper, the study results will provide theoretical and experimental basis for diagnosis of early caries and cracked teeth by using laser ultrasound, and will help for the further development and application of laser ultrasonic technology in clinical testing.