## 4 Discussion  
Using the THz CSR beam at LEBRA, we studied transmission imaging for formalin fixed samples in which normal liver tissue and malignant tumor tissue coexisted. Although the transmission of the normal tissue had little dispersion among the samples, the transmission of the malignant tumor tissue varied widely between samples [30; 31; 32; 33]. We considered that the fibrous tissue and vacancies that developed surrounding the tumor tissue contributed to this variation. Imaging experiments confirmed that the fibrous tissue had a high transmission in the THz region by the imaging experiment. This experimental result indicated that fibrosis caused the tissue to lose water. Each sample showed progression of fibrosis around the tumor tissue. The tumor tissue with less advanced fibrosis had a lower transmission rate than the normal tissue. However, it was clarified that the transmission of tumor tissue averaged in the 2 mm\({}^{2}\) area became higher than that of the normal tissue as the proportion of the fibrous tissue to the malignant tumor tissue increased.  
In one sample, although the transmission of the malignant tumor tissue was lower than that of the normal tissue in the CSR THz imaging, there was no significant difference in the number of water molecules between the normal tissue and the tumor tissue in near infrared FEL imaging. THz waves are more easily absorbed by free water than by bound water in a living body [11; 12]. This experimental result suggested that the tumor tissue had more free water than the normal tissue. This is consistent with the well-known diagnostic principle of magnetic resonance imaging [34].  
The reason why the transmission of the tumor tissue was not stable for samples was that the influence of the fibrous tissue around the tumor tissue could not be ruled out in THz imaging. If the spatial resolution is improved to the order of 0.1 mm, the detection efficiency of the tumor tissue and the fibrous tissue will be further improved. It is necessary to increase the frequency used for imaging to 1 THz or more for high spatial resolution. To perform THz transmission imaging with a high spatial resolution, the power of the light source should be increased. [PERSON] et al. showed that the THz frequency of 2.52 THz with high resolution can detect hepatocellular carcinoma [35]. We have used lower THz frequency compared with [PERSON] et al., however, we have developed coherent edge radiation at the FEL undulator line that has an output 1000 times stronger than CSR and can be used in the frequency region of 0.1-2.5 THz. Our results suggested that lower THz frequency than past reports may be useful to discriminate between formalin fixed normal liver tissue, malignant tumor tissue, and malignant tumor tissues with advanced fibrosis. In the future, we plan to develop THz imaging for biological tissues using coherent edge radiation with an infrared FEL. However, an increase in the THz imaging resolution may have some problems. Firstly, we lose sensitivity to the tissue water content (to the intense low-frequency Debey relaxation band of water) with increasing frequency [14]. Secondly, the water vapor along the THz beam path can complicate (make unstable) collection and  
Figure 6: The visible image of formalin fixed liver (**a**,**b**) terahertz image of a liver tissue with underdeveloped fibrosis (sample #4). The yellow and green frames in (**a**) indicate the regions where the average transmission of the tumor tissue is evaluated.  
analysis of THz images at such high frequencies, while measurements of tissues in inert gas or even vacuum seem to be a daunting task [36; 37].