Diagnosis of Dental Cavity and Osteoporosis Using Terahertz Transmission Images
We demonstrate potential medical applications of terahertz imaging system for diagnosis of dental cavity and osteoporosis. Electrical parameters of tooth and bone are acquired using the transmitted terahertz pulse measurement and genetic algorithm.
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1.
Introduction
Currently, x-ray is mostly used for diagnosis of dental cavity and osteoporosis[ 1][4]. Although the amount of the irradiated x-ray to human body is relatively small for the clinical diagnosis, the exposition of the x-ray to the human body should be minimized as much as possible, since the x-ray is an ionizing radiation. To investigate other possible diagnosis systems replacing x-ray, ultrasonic imaging and MRI(Magneto-Resonance-Imaging ) systems were studied. Unfortunately, an effective and safe diagnosis tool for detecting the dental cavity and the osteoporosis is not developed yet. So we present potential medical applications of THz transmission imaging system for the diagnosis of the dental cavity and the osteoporosis. In the proposed THz transmission image system, clinical information was successfully obtained including density of dentin, electrical properties of dentin, composition of tooth, and locations and size of the cavities[5]. Internal pulp cavity and external cavities were clearly observed in the THz images. Also, this technique has been demonstrated as potential medical applications, monitoring bone fracture, bone diseases such as, osteoporosis and arthritis, and diseases that originate in or affect bone marrow. Density distributions of the marrow were acquired. In addition, we have extracted electrical parameters(permittivity and conductivity) of a sliced tooth and a sliced bone up to 1THz.
2.
Diagnosis of Dental Cavity
Figure l(a) shows a THz transmission image of a dental pulp cavity. For comparison, photograph and x-ray image are shown together. Generally, clinical quality of a dental pulp cavity contains information for health and growth of a 265
T. Elsaesser et al., Ultrafast Phenomena XII © Springer-Verlag Berlin Heidelberg 2001
tooth. The dental pulp cavity is clearly shown as indicated by a circle. In the THz image, the cavity is detected as a brighter part, implying that the THz pulse is more transparent to the cavity. The THz pulse is absorbed by conductive loss of dentin. Figure l(b) shows vertical image of a decayed tooth. Both amalgam coverage over the cavity and cavity can be observed in the THz image. The amalgam is a kind of compound metal. The amalgam and cavity is found to be quite contrastive in the THz image comparable to x-ray image. The spatial resolution of the THz image is about SOOf..lm. Figure l(c) shows the extracted electrical parameters of a sliced tooth whose thickness is 1.1 mm in the dentin part. The conductance of the dentin was measured in the THz frequency spectrum region . The parameters can be further used for estimating the density of the dentin from the measured THz waveforms. During the parameter extraction, genetic algorithm was used for time-domain data fitting. Extracted model parameters were well matched to the THz measurement. The auto-correlation coefficient between the measured THz pulse waveform and the reconstructed THz pulse waveform is 0.97, which means that the extracted electrical parameters are accurate. TH.lma,e
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