# Continuous-Wave THz Imaging for Biomedical Samples
## 2 CW THz Single-Point Scanning Imaging
### CW-THz Near-Field Microscopy Imaging  
The THz far-field lateral resolution is limited by the wavelength, but the near-field microscopy system can achieve more detailed information at a small distance (<lambda), which allows it to overcome the Abbe diffraction limit. In general, near-field microscopy imaging achieves sub-wavelength resolution at \(10^{-2}\lambda\)-\(10^{-3}\lambda\), such that CW-THz near-field microscopy imaging will be a potentially useful tool for contactless biological tissue testing [46,47]. In 2013, [PERSON] et al. completed a THz fiber-scanning near-field imaging system based on a Gunn oscillator at the frequency at 300 GHz as the source and a Schottky diode as the detector [48]. The THz beam was focused into a polyethylene fibre with a diameter of 240 \(\upmu\)m. A bull's-eye metallic spatial filter was integrated behind the fibre to improve the spatial resolution of 240 \(\upmu\)m (<\(\lambda\)/4) [49]. The scanning area was 10 \(\times\) 10 mm with a scanning speed of 100 pixel/min, and the time consumed was 30 min. According to the absorption distribution from the THz images, human liver cancer tissue and normal tissue were clearly identified and matched the investigation by pathological H&E staining inspection. At the same time, the group found that not all the THz images of the tissues could be matched with the pathological micrographs through the features of size and shape. This was because the fatty tissue was easily washed out by acetone during the H&E staining process, which caused the sample to be deformed or even lost. In order to break through the diffraction limit and achieve super-resolution imaging, it is necessary to record the evanescent signal within the sample surface (<\(\lambda\)). Therefore, the working distance between the sample and the detector should be very close to satisfy the requirements for near-field detection. THz waveguides are a good choice for near-field  
Figure 5: The setup of the CW THz ATR imaging scanning imaging system. imaging, not only improving the resolution of the reconstructed image but also providing low dispersion and low propagation loss in the THz message transmitting process. In 2015, [PERSON] et al. imaged the in vivo vessel inside the ears of mice by adopting wave-guided illumination and near-field scanning measurement based on a 340 GHz Gunn oscillator and a Schottky diode detector [50], the setup of which is shown in Figure 6. A rectangular metal waveguide with an aperture of 0.7 mm and length of 10 mm was used [51; 52]. The scanning step was 0.1 mm, and it took about 8 min to scan the 8 \(\times\) 8 mm image. The distance between the detector and skin surface was less than 0.2 mm (\(<\)\(\lambda\)/4). The blood vessels could be clearly distinguished from the THz images with a lateral resolution of 0.5 mm (\(<\)2/3\(\lambda\)). This system could monitor the quantitative changes of the absorption coefficient of blood and then detect the long-term changes of the physiological characteristics of blood in the body. The geometry of the CW THz near-field imaging system is complex. The transmission power through the sub-wavelength aperture suffers greatly from attenuation, so it needs a high sensitivity detector with single-point scanning measurement. One of the disadvantages of this system is the transmission power attenuation. Another way to improve the resolution is the CW THz solid immersion (SI) microscopy imaging method. In 2018, [PERSON] et al. developed CW-THz solid immersion (SI) microscopy reflection imaging with a spatial resolution of 0.15 \(\lambda\)[53; 54]. It is based on a 0.5 THz BWO source and a Golay cell detector. The system is composed of a large numerical aperture through an aspheric lens consisting of a high-density polyethylene (n\({}_{\text{HDPE}}\simeq 1.54\)) and a truncated sphere made of high-resistivity float-zone silicon (n\({}_{\text{HRFZ-Si}}\simeq 3.415\)). The THz images areas was 4 mm \(\times\) 4 mm of a leaf.