A Microwave Coaxial Sensor for Non-Destructive Detection and Analysis of Cracked Teeth

A new approach for the detection of cracks in a tooth is presented using a microwave coaxial sensor. In the test, the probe tip is in close contact with the tooth surface, and the fringe electromagnetic fields interact with the tooth. The presence of any crack affects the effective permittivity, causing changes in the signal received. Cracks in two representative teeth are evaluated. Low signal power was applied during the scan, and no heating effect was produced. From the scanning of the positions right on and away from the cracks, significant differences are seen in the magnitude and phase of the reflection coefficient. By comparison, it is found that better performance can be achieved using the phase at a high frequency. The probe is also used for permittivity determination. The permittivity of the crown is computed using the error correction technique, and the data are used in further modelling. In the analysis, the phase greatly changes when the thickness of the dielectric layer is below the aperture diameter. Thus, the characteristic of the limited signal penetration is revealed, enabling local inspection and accurate identification. With a simplified geometric model, it is shown that the air gap between the probe tip and the tooth surface does not contribute much to the whole sensing volume. In addition, there is close correlation between the extent of the crack and the phase, suggesting the potential for quantitative characterisation. The methodology proposed here could provide an alternative solution to efficient non-destructive detection of cracked teeth.