The design, fabrication and characterization of an ultrasonic crack detection system for human teeth.

摘要

This dissertation entails the design, fabrication, and characterization of a novel ultrasonic imaging system for human teeth. The system development required research in three separate technical areas: (1) a novel thickness-mode piezoelectric transducer, (2) analog transceiver electronics, and (3) digital signal processing (DSP) tailored for crack detection.; The research started with design and simulation of an innovative thickness-mode piezoelectric transducer tailored for human teeth. The goal of the transducer design was to optimize its performance with respect to electro-acoustic power generation efficiency, short acoustic pulse generation, and efficient acoustic energy coupling into the hard tissues of the human tooth. The final transducer is based on a single, novel piezoelectric (PLZT-98(TM)) capacitor for the active element, and operates resonantly at a center frequency of 20 MHz with an instantaneous bandwidth of 30%.; The second segment of the research concentrated on the design and implementation of the analog transceiver electronics of the system. The primary design issue was to maintain a high signal-to-noise ratio (SNR) in monostatic operation. The transmit pulse, a gated-cw waveform, provides an efficient use of energy in conjunction with the piezoelectric transducer. The receiver front-end design is based on traditional, analog superheterodyne techniques to extract the down-converted baseband (envelopes) of the return pulses.; The third part of the research, DSP algorithms were developed for processing the digitized output from the superheterodyne receiver. The envelopes are cross-correlated against a pulse echo signature from a control crack to maximize the SNR and probability-of-detection specifically for cracks. The DSP algorithms in conjuction with the analog superheterodyne receiver combine to form a matched filter processor.; Finally, two key results are presented to exemplify the overall system performance, a proof-of-concept for dental crack detection using a simulated tooth with a synthetic 25-um crack and improved dento-enamel junction (DEJ) imaging in an extracted human tooth. The results show unequivocally the detection of the crack and a great improvement in DEJ detection. In summary, this dissertation reports a significant step forward the imaging of human teeth using ultrasound and provides a design basis for a future ultrasonic dental imaging system.