Photoacoustic imaging system using a bundle of hollow-optical fibers to detect hidden dental caries is proposed. Firstly, we fabricated a hidden caries model with a brown pigment simulating a common color of caries lesion. It was found that high frequency ultrasonic waves are generated from hidden carious part when radiating Nd:YAG laser light with a 532 nm wavelength to occlusal surface of model tooth. We calculated by Fourier transform and found that the waveform from the carious part provides frequency components of approximately from 0.5 to 1.2 MHz. Then a photoacoustic imaging system using a bundle of hollow optical fiber was fabricated for clinical applications. From intensity map of frequency components in 0.5-1.2 MHz, photoacoustic images of hidden caries in the simulated samples were successfully obtained.
Photoacoustic method to detect hidden dental caries is proposed. It was found that high frequency ultrasonic waves are generated from hidden carious part when radiating laser light to occlusal surface of model tooth. By making a map of intensity of these high frequency components, photoacoustic images of hidden caries were successfully obtained. A photoacoustic imaging system using a bundle of hollow optical fiber was fabricated for using clinical application, and clear photoacoustic image of hidden caries was also obtained by this system.
A method based on photoacoustic analysis is proposed to diagnose dental pulp vitality. Photoacoustic analysis enables to get signal from deeper tissues than other optical analyses and therefore, signal detection from root canal of thick dental tissues such as molar teeth is expected. As a light source for excitation of photoacoustic waves, a microchip Q-switched YAG laser with a wavelength of 1064 nm was used and owing to large penetration depth of the near infrared laser, photoacoustic signals from dental root were successfully obtained. It was found that the photoacoustic signals from the teeth containing hemoglobin solution in the pulp cavity provide vibration in high frequency region. It was also shown that the intensities of the high frequency component have correlation with the hemoglobin concentration of solution. We applied short-time Fourier transform for evaluation of photoacoustic signals and this analysis clearly showed photoacoustic signals from dental root.
To improve sensitivity of dental caries detection by laser-induced breakdown spectroscopy (LIBS) analysis, it is proposed to utilize emission peaks in the ultraviolet. We newly focused on zinc whose emission peaks exist in ultraviolet because zinc exists at high concentration in the outer layer of enamel. It was shown that by using ratios between heights of an emission peak of Zn and that of Ca, the detection sensitivity and stability are largely improved. It was also shown that early caries are differentiated from healthy part by properly setting a threshold in the detected ratios. The proposed caries detection system can be applied to dental laser systems such as ones based on Er:YAG-lasers. When ablating early caries part by laser light, the system notices the dentist that the ablation of caries part is finished. We also show the intensity of emission peaks of zinc decreased with ablation with Er:YAG laser light.
A laser-induced breakdown spectroscopy (LIBS) system targeting for the in vivo analysis of tooth enamel is described. The system is planned to enable real-time analysis of teeth during laser dental treatment by utilizing a hollow optical fiber that transmits both Q-switched Nd:YAG laser light for LIBS and infrared Er:YAG laser light for tooth ablation. The sensitivity of caries detection was substantially improved by expanding the spectral region under analysis to ultraviolet (UV) light and by focusing on emission peaks of Zn in the UV region. Subsequently, early caries were distinguished from healthy teeth with accuracy rates above 80% in vitro.
After confirming that the gingival circulation had little effect on transmitted light plethysmography measurement in the upper central incisor in both in vivo experiments and numerical Monte Carlo simulation studies, a three-layer model comprising of a pulp chamber sandwiched between two dentin layers has been introduced to quantify the pulp chamber hematocrit (Hctp) from the measured optical density. Two-flux theory was utilized to derive a mathematical equation for transmitted intensity in terms of tooth dimensions, Hctp, and light-source wavelength. Each layer was assumed homogeneous so as to represent its optical properties by the bulk absorption and scattering constants. The mean error between the Hctp estimate based on the three-layer-model equation and the Hctp actual in the extracted model tooth was −0.00115 with standard deviation (SD) of 0.00733 at 522 nm wavelength, while for 810 nm +0.09157 and 0.02493. The Hctp estimate of the upper central incisor in 10 young volunteers at 522 nm using the three-layer model ranged from 0.002 to 0.061 with the mean of 0.032. The Hctp change reflects blood volume shift in the pulp microcirculation to possibly indicate dental pulp vitality.
A new multi-wavelength optical-plethysmograph has been designed to study the relation between the transmitted
optical density (OD) of the tooth vs. hemoglobin (Hb) content and oxygen saturation (SO2) of the pulpal blood using the
467, 506, 522 and 810 nm light emitting diodes (LEDs). The experimental model utilized the extracted human upper
incisor where the pulp cavity was filled with the blood having various values of Hb and SO2. A resin cap was made to
fit the tooth crown and optical fibers for transmission measurement. The LEDs were pulsed sequentially at 520 Hz with
the pulse duration of 240 μs. The OD as a function of Hb for the isosbestic wavelengths of 506 and 522 nm increased
almost linearly from 8.0 to 11.0 for Hb changing from 0.0 (saline control) to 2.5 g/dL, but beyond 2.5 g/dL no change
was observed. At 810 nm, the OD increased linearly till Hb of 13.4 g/dL, but its change was much smaller with 1.0 OD
per 13.4 g/dL. As for SO2, the OD at 467 nm with Hb of 1.0 g/dL that simulated the mean pulpal Hb content in vivo
varied by about 1.0 for SO2 changing from 100 to 40%. The OD change with respect to Hb change at 506 and 522 nm
showed better sensitivity than that at 810 nm. The combination of 467 and 506 or 522 nm wavelengths can provide a
noninvasive measurement of both pulpal Hb content and SO2 to diagnose pulp vitality of teeth in vivo.
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