Applications of laser-based ultrasonics to the characterization of the internal structure of teeth.

Dental health care and research workers require a means of imaging the structures within teeth in vivo. For example, there is a need to image the margins of a restoration for the detection of poor bonding or voids between the restorative material and the dentin. In addition, a high-resolution imaging modality is needed to detect tooth decay in its early stages. If decay can be detected early enough, the process can be monitored and interventional procedures, such as fluoride washes and controlled diet, can be initiated to help remineralize the tooth. Currently employed x-ray imaging is limited in its ability to visualize interfaces and incapable of detecting decay at a stage early enough to avoid invasive cavity preparation followed by a restoration. To this end, nondestructive and noncontact in vitro measurements on "as-is" extracted sections of human incisors and molars using laser-based ultrasonics are presented. Broadband ultrasonic waves are excited in the extracted sections by using a pulsed carbon-dioxide (CO2) laser operating in a region of high optical absorption in the dental hard tissues. Optical interferometric detection of the ultrasonic wave surface displacements is accomplished with a path-stabilized Michelson-type interferometer. Laser ultrasonics is found effective in characterizing the anisotropic and inhomogeneous nature of dentin. In addition, time-of-flight analysis of the measured bulk transmission waveforms allows for detection of dentino-enamel and carious dentin-dentin junctions. These results are compared to those obtained for specially prepared tooth phantoms that mimic the mechanical properties of dental hard tissues.