## 4 Discussion  
From the above results of the experiments which we performed to show the feasibility of photoacoustic method for detection of hidden dental caries, we found that the photoacoustic method can be applied for detection of hidden dental caries in principle. However, in the current state of the imaging system and method which we proposed, only relatively large caries lesions with sizes larger than around 1 mm could be detected. In clinical practice, it is sometimes necessary to find lesions before they have reached this size so that one can use a preventive technique. To apply the proposed method for clinical applications, therefore, the sensitivity and resolution of the detected images should be drastically improved. This may be possible by using an image reconstruction technique such as circular back-propagation method [24] because the current system uses only a simple B-mode imaging method. In the photoacoustic methods for detecting dental caries reported so far, an optical interferometer or a piezoelectric transducer has been used to detect the ultrasonic waves. However, optical interferometers are usually too sensitive to fluctuations in the distance from the sample surface to be used in clinical applications. In addition, when piezoelectric transducers are used, usually the sample needs to be dipped in water to match the acoustic impedance, and this is difficult for in vivo applications. In our proposed method, in contrast, a composite ultrasonic probe with a soft polymer coating was used. The soft probe can be attached to the surface of the tooth crown, as shown in Figure 2b, without water or gel and this makes the imaging system more feasible for clinical applications. However, we also found that this kind of composite probes have nonlinearity in the  
Figure 11: (**a**) Appearance of the occlusal surface, (**b**) Computed tomography (CT) cross section along the line A–B, and (**c**) CT cross section along the line C–D. Figure 12: Photoacoustic image of an extracted tooth containing a caries lesion. frequency response as shown in Figure 8. Therefore, one should take this nonlinearity into account when performing photoacoustic imaging. Even though beam scanning of the laser light is necessary to obtain photoacoustic images, typical systems based on a scanning mirror cannot be easily applied to clinical applications for dental caries detection because such systems are usually bulky and do not fit into oral cavities. To solve this problem, we developed a laser scanning system using a bundle of flexible hollow-optical fibers shown in Figure 4. By using this probe, one can scan the laser beam using a simple linear stage at the input end and this makes the system simple and low in the cost. The analysis of frequency spectra of photoacoustic waves from the extracted tooth samples with simulated caries inside showed that hidden caries lesions with a brown color may be detected by focusing the spectral differences. The difference between the spectral intensities from pigmented part and the normal part shown in Figure 5b indicates that the intensities of the photoacoustic waves generated in the pigmented part are higher in the frequency range of 0.5 MHz to 1.5 MHz than those of waves generated in normal parts of a tooth. This may be caused by a resonance vibration in the cavity generated by the absorption of the laser pulse at the simulated caries lesion. We performed the same experiments with different caries model teeth and the majority of the model teeth showed similar results. Considering this and the spectral difference between the pigmented part and the stained occlusal surface shown in Figure 7b, we found that we could detect simulated hidden caries lesions by focusing on the frequency range of 0.5-1.2 MHz. Then we could differentiate between photoacoustic waves generated from pigmented regions inside teeth from those generated from normal parts or stained occlusal surfaces.