# Fully Automated Electronic Cleansing Using CycleGAN in Computed Tomography Colonography
## 4 Discussion  
### Images Generated by CycleGAN  
The CycleGAN-generated image (Figure 7) of the proposed method shows that the cleansing process did not damage the intestinal tract, and only barium was removed. In the conventional method, there were cases in which barium remained in the image, such as false-positive lesions, or barium remained in the image without being removed at all. If barium is retained, it is necessary to confirm whether it is barium or a lesion from the original image to avoid pointing out a false-positive lesion, which adds time and burden to the preparation of the CTC imaging reports. The proposed method can remove more barium than the conventional method, and lesions remain unremoved, making it easier to detect.  
To confirm that an excess area was not removed when the barium was removed, an outline of the intestinal tract wall surface was drawn and overlapped with the original CTC image and the CycleGAN-generated image, as shown in Figure 12. In cases with good barium pretreatment (Figure 12a), poor pretreatment (Figure 12b), and lesions (Figure 12c), the intestinal wall, and lesions were not removed, and only barium was selectively removed, indicating that this technique correctly recognized and removed barium.  
Figure 11: Cleansing rates in cases with good and bad pretreatment. (**a**) Cleansing rates in cases with good pretreatment and (**b**) cleansing rates in cases with bad pretreatment.  
However, even with the images generated using the proposed method, barium was not removed in some cases where there was no air space in the intestinal tract and the colon was completely filled with barium. This may be due to the fact that few of the original CTC images used in the study were filled with barium. CTC is also used to identify extraintesinal lesions as they are included in the range of imaging from the upper to lower abdomen. The Extracolonic Findings Reporting and Data System was used to determine extraintesinal organs. It has been reported that tumor lesions in abdominal organs other than the colon and lesions, such as aortic aneurysms, can be detected [25; 26]. Therefore, lesions and organs outside the intestinal tract must be depicted in CycleGAN-generated images without defects. As shown in Figure 9, the proposed method showed no degradation of the lesions outside of the intestinal tract, indicating that it can be used to diagnose these lesions.  
Figure 12: Preservation of intestinal information by barium removal (**Left**: Input image and **Right**: output image.) (**a**) Good pretreatment; (**b**) bad pretreatment; and (**c**) image with lesion.  
The CycleGAN introduced in this study consists of a network that performs two types of transformations and converts MC images into pseudo-CTC images. The observation of the results obtained from converting MC images to CTC images using CycleGAN showed that many good pretreatment patterns with uniform barium were generated, and barium images with nonuniform CT values that were poorly pretreated were not reproduced. In this situation, CycleGAN training showed a greater loss in the images of patients with poor barium generation. However, the results of the image quality evaluation and barium removal rate were acceptable, and we believe that this is not a problem for the purposes of this study.