As a consequence of extensive research over the last decade, the Terahertz(THz) technology has advanced considerably. Significant amount of research is conducting on application of THz imaging in biological problems as unique properties of THz technology can address many unresolved issues in this area. Compared to X-ray imaging, THz wave can realize a non-invasive and non-ionizing imaging for biological tissues due to its low photon energy. The scattering effect is also reduced considerably as the size of the scattering particles in biological tissues is much smaller than THz wavelength range (3-100um). This effect along with the inherent small wavelength of THz waves makes the spatial resolution of 250um laterally and 20um axially easily achievable. Additionally, coherent detection technique (which measures the amplitude and the phase of THz signal) in THz imaging provides precise information on the material properties such as refractive index and absorption coefficient over the THz range frequency. Since these properties depend on the chemical content of the biological cells, monitoring them can be used for medical diagnosis and examine the condition of biological cells.
Two main applications of THz imaging are dental carries detection and cancer detection. Early erosion in tooth through radiography is hard to diagnose, but THz imaging can be applied to detect the early carries through characterizing the properties of different tooth layers (enamel and dentine). The indication of early carries is the change in refractive index of enamel which is resulted from mineral loss in this layer. THz imaging has shown a great potential to be an alternative to x-ray for dental imaging. However, as a consequence of substantial attenuation of THz wave in an actual tooth, most of the researches have been conducted on characterization of a thin slice or on the surface of the tooth. This issue originates from irregular shape of the tooth, thick tooth layers (few millimeters in total), and high absorption coefficient of composed materials (varies from 2dB/mm to 4dB/mm). Therefore, to develop a THz dental imaging system, a high power, compact and affordable THz source is essential.
Skin cancer and breast cancer detection can be considered as other applications of THz imaging. Since skin is a superficial layer, it can be a suitable target for THz imaging in a reflection measurement configuration. In skin structure, the basal layer lies against the dermis, innermost layer. Studies have shown that too much UV exposure to this layer can transform it to cancerous cells. Compared to the normal cells, the cancerous cells show an increase in the interstitial water or a change in the vibrational modes of their water molecules; therefore, THz imaging can be utilized to evaluate the boundary and infiltration depth of skin cancer. Breast cancer can also be a suitable target for THz imaging as the breast is mostly composed of low water content fat tissues. In breast cancer cases, there is an essential need to identify accurately tumor margin to avoid second breast-conserving surgery. The available technologies are incapable of accurate detection and up to 55% of the cases need the second operation. The high sensitivity of THz absorption to water density can be exploited for breast cancer detection as the cancerous cells contain more water compared with the normal fat tissues.
Although THz medical imaging has demonstrated a great potential and some commercial THz systems have been developed for medical applications, the THz medical imaging is still in its early stages and more research should be conducted to enable this technology to compete with other alternative imaging techniques.
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