# Continuous-Wave THz Imaging for Biomedical Samples
## 1 Introduction  
Terahertz (THz) occupies a frequency range from 0.1 to 10 THz in the electromagnetic spectrum between millimeter and infrared waves, and the corresponding wavelength range is between 30 \(\upmu\)m and 3 mm. Due to their properties, THz waves have far-reaching research value and broad application prospects in the field of biomedical imaging [1; 2]. Researchers have conducted a number of studies on THz imaging of in vivo biological tissues, including brain tumours, teeth, bone density testing, liver cancer, and burnt skin tissues [3]. The rapid development of THz imaging technology in biomedical applications is mainly due to four excellent properties. Firstly, THz waves have stronger penetrating ability than visible and infrared radiation, so that they can achieve perspective imaging for many opaque non-polar substances [4], such as ceramics, paper, wood, plastics, and non-polar liquids. THz imaging and detection technology is mostly used for the epidermis of tissues and some organs. It can also penetrate the epidermis of animals to the depth of hundreds of microns to image superficial tissues in vivo. THz imaging can serve as a novel technique that complements conventional medical imaging modalities, such as magnetic resonance imaging and X-ray imaging. Secondly, THz radiation has nondestructive properties [5] because their energy is one millionth of the X-ray wave energy, making them suitablefor the live detection of human and other organisms without ionising their component elements and breaking their structure. Thirdly, the vibrational and rotational energy levels of most biological macromolecules are located at the bandgap of THz, and every molecule has its own unique signature and fingerprint spectrum [6], allowing researchers to identify molecular structures and analyse substance components. Lastly, THz radiation have a strong absorption effect with water [7], which can be used to distinguish cancer tissues from normal tissues based on the difference of their water content. According to different detection and signal processing methods, THz detection technology can be divided into spectral detection technology and imaging technology. The principle of THz spectrum detection technology is that the spectrum of most low-frequency biomolecule motion (such as hydrogen bonds, molecular vibration and rotation, and van der Waals force) and the biomolecules have their own unique THz fingerprint spectrum characteristics so that researchers can identify biomolecular samples by analysing the difference of the absorption coefficient or reflection coefficient [6]. The THz Time Domain Spectroscopy (THz-TDS) system can distinguish diseased and normal tissues by analysing the peaks, troughs, absorption spectra, and specific principal component information of the time domain signal. THz-TDS has been widely studied with macromolecules, cells, cancer tissues and other biological structures [8; 9; 10; 11]. However, the THz-TDS detection area is associated with the size of the radiation spot, and the tissues are not completely uniform, which often affects the accuracy of the detection outcomes in clinical medicine. In contrast, CW THz imaging technology can be more accurate and visualised to identify the difference between lesions and normal tissues. Therefore, THz imaging for biological tissues samples detection has more advantages. Increased attention has been paid by researchers to THz biomedical imaging in recent years. The key application of THz imaging technology in the field of biomedicine is to extract information such as the absorption or refractive index distribution and morphological characteristics of biological samples from the THz intensity and phase images. In 1995, [PERSON] and [PERSON] demonstrated the first THz imaging system and successfully imaged a chip and leaf by transmission single-point scanning imaging [12]. According to the difference of the working mode of the THz sources, the THz imaging technology is divided into terahertz pulsed imaging (TPI) and continuous-wave (CW THz) THz imaging.