## 1 Introduction  
Hidden caries are dentin lesions that cannot be easily detected via visual inspection because the occlusal enamel appears normal or only minimally demineralized [1]. Even though hidden caries may be radiographically detected, X-ray radiation in routine examinations may sometimes cause side effects [1; 2; 3]. Therefore, a method substituting for or assisting radiography and enabling lesions inside teeth to be diagnosed from the occlusal surface is needed.  
Photoacoustic imaging is one imaging method used to noninvasively observe the subsurface morphology of living bodies [4]. It detects ultrasonic waves generated by optical absorbing substances when they thermally expand after absorbing irradiated optical pulses [4]. The penetration depth of photoacoustic imaging is greater than that of purely optical imaging techniques, which are usually affected by the strong optical scattering of tissue. Furthermore, photoacoustic imaging retains the high spatial resolution of optical methods, which cannot be achieved by conventional ultrasound imaging.  
Photoacoustic imaging has been used in a variety of applications, including the imaging of blood vessels [5], brain functions [6], and other uses [7; 8; 9]. Several groups have reportedly applied the photoacoustic method to the diagnosis of dental caries. [PERSON] et al. used an ultraviolet laser light that is strongly absorbed by enamel to generate surface acoustic waves and optically measured the elasticity of the enamel layer [10]. [PERSON] used pulsed CO\({}_{2}\) and Nd:YAG lasers for excitation and detected photoacoustic waves using an optical interferometer [11]. [PERSON] et al. used 1064-nm and 532-nm Nd:YAG lasers for excitation and an interferometer based on a 532-nm laser light for detection and successfully obtained photoacoustic signals [12; 13]. These photoacoustic systems have primarily been used for the detection of early caries formed on the enamel surface. Conversely, [PERSON] et al. applied photoacoustic tomography to detect caries lesions [14]. In their experiments, a 532-nm Nd:YAG laser irradiated the occlusal surface of an extracted tooth sample and an ultrasonic transducer rotated around the sample. Even though they successfully obtained tomographic images via reconstruction calculations, the scanning of the transducer may be difficult for in vivo applications. [PERSON] et al. performed simulations to estimate changes in the temperature and pressure when dental tissues were irradiated with laser light. The results showed that those changes were in a margin of safety when applying photoacoustic method to clinical applications [15]. [PERSON] et al. reported that they differentiated optically-induced surface acoustic waves from caries and normal parts by using a system based on Michelson interferometer [16; 17]. [PERSON] et al. found that the intensities of photoacoustic waves induced by visible and near-infrared lasers were different in the degree of progress of dental caries [18]. These reports have shown the feasibility of the photoacoustic detection of dental caries; however, these methods have not been applied to the detection of hidden caries. Optical coherence tomography is also emerging as an optical method for detection of early dental caries. However, the method has a limitation in the imaging depth for detection of hidden caries lesions [19; 20; 21].  
The aim of the work reported in this article is to test the feasibility of a photoacoustic method for the detection of hidden caries. For this purpose, we propose a photoacoustic imaging system for in vivo applications.