# Continuous-Wave THz Imaging for Biomedical Samples
## 6 Conclusions and Perspectives  
This paper mainly reviews the methods of CW THz imaging as applied in biological imaging in recent research. We first introduce the characteristics of THz radiation, which can be used in biomedical detection. Compared with the TPI system, the geometry of the CW THz imaging system is simpler and more stable, and most such systems enable real-time detection at room temperature. The size of the optical elements in the imaging system will make it easier to achieve the development of miniaturisation and portability in the future. Secondly, we introduce many methods of CW THz single-point amplitude imaging, including full-field imaging and three-dimensional imaging. The CW THz transmission and reflection single-point scanning imaging systems are the most frequently used amplitude imaging method according to an analysis of various imaging systems. The internal structure of tissues can be accurately distinguished by measuring the distribution of the absorption and reflectivity in the THz radiation. Most of the information on the biological  
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tissue obtained by CW THz phase contrast imaging and CW THz-CT imaging, such as the refractive index and thickness information of the tissue, can be converted into the complex refractive index, and all of this information contributes to the diagnosis of early-stage cancer. Sub-wavelength imaging resolution can be obtained by the imaging method of CW THz confocal scanning microscopy and near-field microscopy. CW THz differential detection can improve the SNR of reconstructed imaging, while polarisation THz imaging obtains accurate amplitude images of the samples without the strong Fresnel reflections from the sample-holder interfaces. The algorithms of the extrapolation, sub-pixel matching, and synthetic aperture are all used to enlarge the size of the detector to improve the resolution of the reconstructed images. Finally, the status of these imaging methods in biomedical sample detection is introduced. Based on all these imaging methods, the CW THz full-field phase contrast imaging and 3D imaging should be further explored. The former can not only realize the real-time imaging but also calculate the refractive index distribution of the measured object through the phase information. This has a very great significance for recognizing the structure and properties of some biological slices. The 3D imaging can observe the defects and injuries of the internal structure of biological tissues. This intuitive imaging method can be helpful to doctors to make a more accurate clinical diagnosis. Fresh biological samples retain the water content in tissues, but THz radiation is sensitive to water content, which has both pros and cons for experimental research. On the one hand, the water content enables the distinction between cancerous and normal tissues in the THz images. On the other hand, the detection results are affected by the distribution of the THz absorption by the water, which may be high enough to limit the THz penetration of the tissues. In this case, sample pretreatment such as freezing, formalin fixation, paraffin embedding, etc. may be helpful. In addition, if the thermal radiation effect of a sample is detected for too long in transmissive imaging, the soft tissue structure will be broken due to the strong THz radiation absorption by water. It is necessary to ensure the fixation of tissues with the above methods or use reflective imaging and THz-ATR imaging. A biopsy is a common method to diagnose tumours, but it is tedious and time-consuming as well as traumatising for the boundary of the tumour area. In addition, while X-ray CT imaging and magnetic resonance imaging (MRI) are also available for tumour diseases, the patient needs to be subjected to higher radiation doses. Therefore, because THz imaging has the advantages of a short imaging time, free labelling, and smaller radiation doses for patients, it has become a significant research topic in recent years.