# Continuous-Wave THz Imaging for Biomedical Samples
## 2 CW THz Single-Point Scanning Imaging
### CW-THz Polarisation Single-Point Scanning Imaging  
Similar to the amplitude detection, high-performance and active polarisation detection can be realised with the polarisers. The polarisation of the beam will affect the imaging results of the detected samples. When the line polarised beam is incident on the sample, the two images can be obtained by co- and cross-polarised detection, and the scattering information of the surface of the tissue can be obtained by subtraction. Since the Fresnel signal comes from the interface plane between the glass and air with ordinary reflective single-point scanning imaging geometry, the cross-polarised detection can avoid the above situation.  
In 2013, [PERSON] et al. proposed a reflective CW-THz single-point scanning imaging system to acquire images of ex vivo fresh human colonic with a spatial resolution of 0.6 mm and 5 mm thick colorectal tissues sections, the schematic of which is depicted in Figure 3. A CO\({}_{2}\) optically pumped far-infrared gas laser operating at 584 GHz (513 \(\upmu\)m) was used as the THz laser, and a liquid helium-cooled silicon bolometer was used as the detector. The reflectivity of the cancerous region of colon tissue was significantly different from that of the normal tissue shown in the THz images [41]. The cross-polarised (CRP) reflective THz imaging system was able to show a good contrast between normal and cancerous colon tissue and eliminate the interference caused by specular reflections. In 2014, [PERSON] et al. promoted reflective polarised single-point scanning imaging geometry based on the same THz laser and the detector [40]. Rotating the polariser located before the sample by 90\({}^{\circ}\) could obtain CRP images. The results correctly showed the CRP THz intensity images of the location of the non-melanoma skin cancer tissues. The CRP and co-polarised (COP) images could be subtracted to show the morphological characteristics of the tissue. The full width at half max was 0.67 mm at the sample surface.  
Figure 3: Schematic of co- and cross-polarised CW-THz reflection imaging.  
They further modified a CW THz circular polarisation-sensitive detection technique for the edge of a tumour in 2016, adding a quarter-wave plate (QWP) to the original linear polarisation-sensitive detection system before the sample at 584 GHz [42]. Fresh ex vivo human non-melanoma skin cancer sections with 5 um thickness were imaged to demonstrate the ability of the system. The scanning speed was 100 ms/point. The sample was illuminated respectively with linearly or circularly polarised THz radiation and collected by the system in the cross- and co-polarisation states. The schematic is shown in Figure 4. The beam waist of the sample surface was 0.42 mm and 0.45 mm in the incident linearly polarised (LP) and circularly polarised (CP) cases, respectively. Both the CRP LP and COP CP THz reflection images contained no Fresnel reflections from the glass-air interface; the information mainly came from the sample. The intensity of the CRP LP images was higher than in the COP CP images for the reason that the CRP CP THz radiation penetrated the tissue more deeply than the CRP LP THz radiation, and the attenuation of the reflected signal obtained after CRP CP THz radiation illuminated the sample was greater than that used by CRP LP THz radiation.