# Continuous-Wave THz Imaging for Biomedical Samples
## 1 Introduction
The photoconductive method and the optical rectification method are commonly used to generate THz pulses in the THz-TDS systems. The photoconductive method uses high-speed photoconductor materials as the transient current source to generate THz radiation by a self-mode-locked Ti sapphire laser. The optical rectification method utilises the reverse process of the electro-optic effect of ZeTe or LiTaO\({}_{3}\) to generate THz pulses, and the power of the radiation is related to the incident radiation power and energy conversion efficiency. The measurement of the TPI is a coherent measurement method, which can obtain both the amplitude and phase information. To detect THz waveforms with the time-domain pulse THz systems, both the electro-optic sampling method and the photoconductive antenna technology are commonly adopted. The electro-optical sampling method has a wide detected bandwidth, so it is suitable for full-field imaging [13; 14]. The photoconductive receiver, by using the coherent delay scanning detection method, can obtain the real-time power of the THz waveform. This method can improve the signal-to-noise ratio (SNR) of the system, but the structures of the TPI systems are complex. The CW THz imaging systems work in a narrow frequency band and utilise the scattering effect of the THz wave on the edge of the internal defect of the samples by detecting the distribution of the intensity of the scattering effect. The average output power of CW THz is higher than that of the THz pulse sources, although the peak output of the pulse sources can be higher than 14 mW [8]. Furthermore, the geometry of the CW THz imaging systems is compact and real time without additional pump optical detection elements. Due to the above advantages, the CW THz imaging technology is more suitable for biomedical detection. The CW THz sources include optically pumped THz gas lasers [15], backward-wave oscillators [16], quantum cascade lasers (QCL) [17],and Gunn diodes. The detector can be divided into point scanning and area array detection according to the working principle. The former includes the Schottky diode [18] and Golay cell [19], and the latter includes microbolometer arrays and pyroelectric cameras [20,21]. Correspondingly, the CW THz imaging technology mainly includes CW THz single-point scanning imaging, full-field imaging, and CW THz three-dimensional imaging. Each has its different imaging system and advantages that are suitable for specific application conditions. The following sections will introduce the characteristics of these methods and their developments in the biomedical field. This paper gives an overview of the current state of CW imaging methods and applications with biomedical samples. The principle and application conditions of the CW THz point-by-point scanning imaging methods are presented in Section 2. The features and application situations of the CW THz full-field imaging methods are detailed in Section 3. The characteristics of the CW THz three-dimensional imaging methods and its biomedical applications are outlined in Section 4. In Section 5, recent research on CW THz imaging technology for various biological tissues applications is discussed. Finally, we summarise the current research status and challenge of CW imaging technology in the field of biomedical applications and offer an outlook on future development.