# Continuous-Wave THz Imaging for Biomedical Samples
## 2 CW THz Single-Point Scanning Imaging
### CW THz Reflection Single-Point Scanning Imaging  
The high sensitivity of THz waves to water content in fresh tissues often makes reflection mode imaging more suitable. The CW THz reflection single-point scanning imaging optical geometries are generally set into two types [36; 37; 38; 39; 40; 41; 42]. The incident beam and the reflected beam are set with a certain angle of the first type as shown in Figure 2a. The sample is illuminated by THz radiation through normal incidence and detected by the Golay cell detector of the second type, as shown in Figure 2b. For the oblique imaging system in Figure 2a, the shape of the focal spot was elliptic. The two off-axis parabolic mirrors would be binding effects mutually due to the intersection angle between the optic axis of the illumination and the optic axis of the collection, which evidently decreased the focal spot on the surface of the sample than the normal incidence reflector imaging device. Thus, the oblique illumination imaging system will acquire higher spatial and axial resolution than the normal incidence system. When the THz beams under the same output power irradiated the thicker samples, the penetration depth of the sample under the oblique incidence mode was less than that under the normal incidence mode. For the normal incidence mode in Figure 2b, the intensity of the beam was attenuated about 50% after the incident beam passed through the beam splitter, and the one part that continued along its path was illuminated on the surface of the sample and aborted by the sample; then, the reflected beam carrying the information of the sample was received by the detector after passing through the same beam splitter again. The intensity would be lost about 50% again, so the efficiency for the intensity of the beam utilisation ratio for the normal incidence reflector imaging setup was very low. The sample is placed on a quartz slide to increase the intensity of the reflected beam, and it could more effectively avoid the interference of other experimental conditions except for the sample in the detection plane. Another slide of the same type is placed on top of the sample to put pressure on the sample and keep its surface flat [22]. The sample is located at the focal plane. In order to avoid errors of non-uniformity caused by the reflection window, the image without the sample as the background image is recorded first. In 2014, [PERSON] et al. design a CW THz reflective high-speed point by point scanning imaging system with the frequency of 0.21 THz and a Schottky diode detector [36]. The high scan speed was 15 s/frame and was improved by an aspheric f-theta scanning lens, the scan area was 100 \(\times\) 150 mm, and the spatial resolution was nearly 4 mm. Peanut shells were used as samples to demonstrate this THz imaging system for food quality inspection. The THz images showed that all the peanuts in their shells could be clearly  
Figure 2: CW-THz reflection single-point scanning imaging system: (**a**) Obliquely incident; (**b**) Normal incidence. Here, BS stands for beam-splitter. The direction of the incident beam was marked with black arrows, and the direction of the reflected beam was marked with orange arrows. identified. In 2015, [PERSON] et al. obtained the reflectivity maps of the central corneal thickness of rabbits in vivo based on the amplifier-multiplier chain (Virginia Diodes, Charlottesville, VA, USA) at 650 GHz [37]. The curved surface of the cornea was scanned at normal incidence. The intensity image could measure the change of water content in the corneal tissue, thus predicting many pathologies. In 2020, [PERSON] et al. performed the THz reflection imaging of an ex vivo rat brain tissue sliced into frozen sections of 10 \(\upmu\)m [39]. The THz source was a CW THz laser (Edinburgh Instruments, FIRL100). The scanning area of the sample was 51 \(\times\) 59 pixels, the scanning speed was 13 pixels/s, and the scanning step was 250 \(\upmu\)m. The reflected information was collected by the Golay cell detector (TYDEX, GC-1P).