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

Abstract

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 out of the probe aperture interact with the tooth material. The presence of any crack affects the effective permittivity, causing changes in the signal energy received. Cracks in two representative teeth are evaluated, and they cannot be readily identified in X-ray images. 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 good performance can be achieved using the phase at a high frequency. The probe is also used for permittivity determination. From the measurement of standard liquids, high accuracy is achieved. The permittivity of the crown is computed using the error correction technique, and the data are used in further analytical modelling.In the analysis, by varying the thickness of the dielectric layer, the phase greatly changes when the thickness 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, from the numerical simulation, there is close correlation between the extent of the crack and the phase value, suggesting the potential for quantitative characterisation.The methodology proposed here could provide an alternative solution to efficient non-destructive detection of cracked teeth.

Non-destructive testing of human teeth using microwaves: a state-of-the-art review

“…Li et al [26] used an open coaxial line probe to detect cracks in two teeth over 1-40 GHz. One tooth was extracted due to splitting, and two cracks were barely visible when the two split parts were held together.…”

“…2, two cases are compared: one with the coupling liquid only (a mixture of glycerine and water, case A) and the other one with the liquid between the probe and the tooth (case B). Glycerine was chosen, [24] 10 GHz: 7.00-j0.29 (enamel) 6.65-j0.74 (dentin) Enamel [25] 0.1-8.5 GHz Not available 2.4-4.6 with an average of 4.0 Enamel [26] 1-20 GHz as the mixture of glycerine and water did not evaporate at room temperature. The intersection point of the two permittivity spectra indicates a more accurate permittivity value of the tooth at a specific frequency, ie.…”

Dielectric Characterization of Healthy Human Teeth from 0.5 to 18 GHz with an Open-Ended Coaxial Probe

“…The type of preservative solution used in the study and its possible contribution to the measured dielectric properties have not been reported. Studies conducted by Meaney et al [ 41 ] and Li et al [ 42 ] also applied the OECP technique to characterize ex vivo teeth, but for different dental purposes. For the purpose of biodosimetry, Meaney et al [ 41 ] compared the relative permittivity of the enamel of five incisor and dental resin samples in the frequency range from 0.1 to 8.5 GHz.…”

“…The authors reported that given the irregular tooth surface, the measurement results contained inevitable air contributions, which was responsible for adding variability to the measurements. Finally, Li et al [ 42 ] applied OECP with a small aperture for the detection of cracked teeth between 1 and 20 GHz. Although measurements of dielectric properties conducted at multiple locations on the permanent molar crown showed consistency, the authors also assumed the presence of small air gaps between the probe tip and the tooth surface.…”