# Continuous-Wave THz Imaging for Biomedical Samples
## 2 CW THz Single-Point Scanning Imaging
### CW-THz Transmission Single-Point Scanning Imaging  
The transmission single-point scanning imaging method requires the sample to be thin with weak absorption of the THz radiation; the sample is placed at the focal position of the incident beam and then scanned pixel-by-pixel by moving it along the X-Y direction using a two-dimensional translation stage. An example of the imaging arrangement is shown in Figure 1. Since the THz beam has normal incidence on a sample, the intensity information of the other side of the sample is obtained by single-point scanning detection [22]. The spatial resolution of this scanning imaging system is defined by the size of the focal beam-spot on the sample plane. Since a leaf can be approximated as a two-dimensional thin sample, it is suitable for verifying the effectiveness of the CW THz transmission-mode single-point scanning imaging system [23, 24, 25, 26, 27, 28, 29, 30, 31]. In 2017, [PERSON] et al. used the DET-900 GHz THz emission with a frequency of 900 GHz to scan a dry leaf point by point, and the leaf veins could also be seen clearly [28]. The transmission single-point scanning imaging system can be used to dist  
Figure 1: Continuous-wave (CW)-THz single-point scanning imaging system. PMs, off-axis parabolic mirrors; M\({}_{1}\), THz gold mirror. components in the tissues, with a beam diameter less than 6 mm. In 2017, [PERSON] et al. used the CW THz transmission imaging system to image a fresh rat brain tissue with a thickness of 40 \(\upmu\)m by raster scanning [31]; the THz source was a CW THz gas laser (FIRL100, Edinburgh Instruments, Edinburgh, UK) with the output frequency of 2.52 THz. The THz image size was 100 \(\times\) 60 pixels with the scanning step of 250 \(\upmu\)m. The lateral resolution of this imaging system is 260 \(\upmu\)m. Mathematical morphology was proposed for CW THz imaging to improve the peak signal-to-noise ratio (PSNR) and suppress the complex background of the reconstructed image. The edges of the rat's brain tissue could be clearly identified from the reconstructed image. In 2018, [PERSON] et al. directly distinguished the bacterial colonies of four bacterial species by using the transmission single-point scanning imaging system [32]. The source was an optically pumped CW THz gas laser (FIRL100, Edinburgh Instruments Ltd., Livingston, UK). The THz frequency was fixed at 2.52 THz with an output power of 40 mW. Golay cells (GC-1P, Tydex Ltd., St. Petersburg, Russia) were used as the detectors. The difference of THz absorption could be observed in the THz image, and each colony of the four bacteria could be directly distinguished. Confocal CW THz microscope imaging is extensively studied in order to improve the resolution and SNR of the system. The principle of the optical confocal microscope is that the positions of the pinholes in front of both the sample and the detector are mutually conjugated. The first pinhole filters out scattered light emerging from the laser. The second pinhole in front of the sample under investigation, and the last pinhole in front of the detector, filter out the central region of the image, eliminating the corrupted light scattered by the sample. Stray light can be filtered out by the confocal pinhole in front of the detector, and submillimeter resolution can be obtained, while the resolution of the imaging system is determined by the size of the scanning spot at the front of the sample [33]. In 2010, [PERSON] et al. designed the first confocal CW THz microscope scanning imaging system based on a far far-infrared gas laser at a frequency of 2.52 THz and a Golay cell detector [23]. The THz radiation passed through a pinhole with a diameter of 1 mm and imaged a single point of the sample onto the focus plane. The size of the pinhole located in front of the detector was 0.5 mm, and it was used to produce additional spatial filtering, which might lead to a better resolution of 0.46 mm, while the resolution along the optical axis of the imaging system was 0.4 mm. A fresh leaf was selected to test the spatial resolution.