All-optical photoacoustic imaging and detection of early-stage dental caries

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.