All-optical photoacoustic imaging and detection of early-stage dental caries Ashwin Sampathkumar Riverside Research New York, NY, USA ashwin@riversideresearch.org David A. Hughes, Katherine J. Kirk University of the West of Scotland Paisley, Scotland, UK Wilfred Otten University of Abertay Dundee, Scotland, UK Chris Longbottom Kings College London, UK Abstract—This paper describes a non-contact optical technique for imaging and detection of early-stage dental caries. Tooth decay, at its earliest stages, manifests itself as small, white, subsurface lesions in the enamel. Current detection methods including visual and tactile investigations and bite-wing X-ray radiographs suffer from poor sensitivity and specificity at the earliest (and reversible) stages of the disease due to the small size (<100 microns) of the lesion. We have developed a fine-resolution (500-nm) ultra-broadband (GHz) all-optical photoacoustic imaging (AOPAI) system to image the early stages of tooth decay. Photoacoustic (PA) signals are generated using a Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) laser operating at 532 nm with a 5-ns pulse duration. The light-induced broadband ultra-sound wave is detected at the surface of the tooth with a path-stabilized Michelson interferometer. 2D images are generated from PA signals using k-wave reconstruction methods. Ex-vivo tooth samples exhibiting white-spot lesions were scanned and were found to generate a larger PA signal in the lesion regions compared to healthy enamel. This high contrast potentially allows lesions to be imaged and measured at a much earlier stage compared to current clinical techniques. PA images were cross referenced with histological and micro-CT images to validate our experimental results. Our AOPAI system provides a non-contact method for early detection of white-spot lesions with a high detection bandwidth that offers advantages over previously demonstrated ultrasound methods. The technique provides the ample sensing depth afforded by an ultrasound system combined with the fine spatial resolution of an optical system. Keywords—Dental caries; white spot lesions; all-optical; photoacoustics; GHz bandwidth I. INTRODUCTION Dental caries remain as one of the most-common oral diseases in the world [1]. Caries result from demineralization of the hard tissues of the teeth (enamel and dentin) caused by bacterial fermentation of sugar remnants of food accumulated on the tooth surface [2]. Under normal oral hygienic conditions, the hard tissues of the teeth (enamel and dentin) undergo a constant cycle of demineralization and remineralization (highlighted by ‘a’ and ‘b’ in Fig. 1). However, if the environmental pH of the tooth drops below 5.5, the balance of the cycle is disrupted and demineralization proceeds faster than remineralization. Improper oral hygiene combined with bacterial fermentation of food debris produces waste products that increase the acidity of the environment, which further accelerates the demineralization process [2]. At their early stages, caries manifest as small (<100 μ m) sub-surface lesions (Fig. 1:c), containing exogenous organic material, appearing as white 'spots' on the tooth surface. If undiagnosed, the lesion grows in size, spreading through the dentin layer (Fig. 1:e-1:g). When the surface of the lesion is disrupted, the lesion is known as a cavity (Fig. 1:h). The process accelerates, eventually producing an infection that can extend to the pulp chamber (which contains nerves and blood vessels) causing inflammation and pain. At this point, the tooth would either be extracted, or a filling put in place (Fig. 2). If the carie lesion is detected at a sufficiently early stage (Fig 1:c- d), treatment regimes such as increased brushing and flossing or fluoride treatments, can be prescribed, which restores the tooth back in to the normal mineralization cycle and thereby halt and possibly reverse disease progression. Current methods for detecting dental caries employing the dental explorer and X-ray radiography are subjective and not quantitative and Fig. 1. Illustration of the constant cycle of demineralization and remineralization in the tooth. Progression of cavity formation is shown through steps c-g. 1269 978-1-4799-7049-0/14/$31.00 ©2014 IEEE 2014 IEEE International Ultrasonics Symposium Proceedings 10.1109/ULTSYM.2014.0313