Objective: Evaluation of the dentin- or enamel-layer thickness is essentially important for dental practices, which is commonly diagnosed by radiographic dental profile. An ultrasonic technique for evaluating the layer thickness between a tooth surface and an enamel-dentin or a dentin-pulp junction is proposed using an acoustic delay line composed of two pairs of arch-shaped interdigital transducers (IDTs) on a piezoelectric ceramic plate, via a liquid layer as an ultrasonic coupler. Method: In constructing a measurement system for the above purpose, two leaky Lamb wave IDTs are mounted on one surface of a 155 mm-thick piezoelectric ceramic plate (TOKIN, NEPEC-6), under the condition that the opposite surface is in contact with a water layer. Both of the IDTs have an interdigital periodicity of 410 mm. An ultrasound beam generated in the water layer, by applying a pulsed rf electric signal to one of the two arch-shaped IDTs, propagates in the water layer and then in a tested tooth-sample. Results: The delay times of the reflected echo-signals detected at another IDT with an aperture angle of 90 degrees, correspond to the respective propagation times. Leaky Lamb wave propagation is described in relation to the transducer performances. The estimated values of the dentin- and enamel-layer thickness, from the observed propagation times and the published values of the longitudinal wave velocities of the two layers in human teeth, are in good agreement with the optically measured data. Conclusion: This method is also available for testing a tooth with a wedge-shaped defect. The remarkable time difference between two reflected echoes in a very narrow region arises from the very slow velocity of the ultrasound and the elastic property. The present non-destructive technique is promising for human-tooth evaluation, which has the advantages of beam focusing and electrically adjustable focal point.

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