# Continuous-Wave THz Imaging for Biomedical Samples
## 2 CW THz Single-Point Scanning Imaging
### CW-THz Single-Point Phase Contrast Imaging  
The amplitude images can distinguish the area of the diseased tissue from the normal tissue, but it cannot obtain the information of the thickness and refractive index of the diseased tissue. This information can be interpreted through phase contrast imaging. CW THz single-point phase contrast imaging includes self-mixing detection and heterodyne detection [24; 55]. The electro-optic modulator generates the THz beam and detects the THz signal reflected from the sample surface, the THz detector uses a self-mixer on the receiver side and then transmits signals along the opposite direction of the external cavity module of the laser to the output port of the THz laser, where the self-mixing effect occurs in the cavity of the laser.  
Figure 6: Configuration of the CW-THz near-field imaging system based on THz waveguide.  
In 2014, [PERSON] et al. reported a CW-THz coherent imaging system based on the 2.59 THz QCL and self-coherent detector as shown in Figure 6. Using this system to image pork tissues consisting of muscle, fat, and skin, the amplitude and phase of the reflected signals can be obtained at the same time [56]. The penetration depth of this system exceeding 100 \(\upmu\)m is suitable for the detection of sub-surface features of skin tissues. In 2017, [PERSON] et al. developed a self-mixing detection technology at 323 GHz to achieve CW THz single-point phase contrast imaging, which they performed on a slice of bacon [55], and the schematic diagram of the self-mixing coherent imaging is shown in Figure 7. The phase contrast image showed the fatty and lean portions in the bacon slice. The different parts were observed clearly from the phase image due to their different reflectivity to THz waves. This phase contrast imaging system can also avoid instability of the geometric structure through the interference of two beams.  
Another single-point phase contrast imaging method is based on THz heterodyne detection. By mixing the radiofrequency signals with the low-noise local oscillator signals, the measured signal can be converted into an intermediate frequency signal of microwave radio frequency and can then be detected by a heterodyne detector. In 2011, [PERSON] et al. studied a CW THz single-point scanning phase contrast imaging system based on the frequency of 0.6 THz and a CMOS THz raster scanning heterodyne mixer detector instead of a Schottky diode. The THz beam diameter was 30 mm [24]. The system scanned 1000 points on a freshly cut leaf, each point required 30 ms, and the scanning speed was 30 frames/s.  
CW THz single-point scanning imaging can obtain high-quality THz images, but the temporal and spatial resolution of the imaging system are limited by the size of the scanning step and scanning speed. In addition, the mechanical errors caused by the translation platform in the scanning process will indirectly affect the imaging quality. With the increasing sensitivity of focal-plane array detectors, the full-field detection technology not only realises real-time imaging but also improves the imaging spatial resolution.