## 3 Results  
Figure 2 shows the low-dose CT image scanned from the 610-01-02 part of the CIRS phantom and the images after filter processing. Figure 2a shows a low-dose CT image obtained, including the diameter values of the six holes and ROIs set for the CNR and COV evaluation. The images obtained by applying the TV, NL-TV, and proposed adaptive NL-TV filters to the acquired low-dose CT images are shown in Figure 2b, 2c, and 2d, respectively. By visually evaluating the resulting CT images, we confirmed that the noise levels of the images to which the TV and adaptive NL-TV filters were applied were reduced. In particular, when the proposed adaptive NL-TV filter was used in the hole areas with the smallest diameters of 6.4 and 3.2 mm, areas that were not observed because of noise clearly appeared (as can be seen from the red arrows in Figure 2d). Figure 3 shows a graph of the results of the CNR and COV evaluated in the ROI in Figure 2. Figure 3a shows a graph of the CNR results evaluated on the low-dose CT images and the images acquired by applying each filter. The average value of CNR calculated in the area of holes with six diameters was derived as 8.62, 20.59, 17.38, and 22.40 in low-dose CT images using none, TV, NL-TV, and adaptive NL-TV filters, respectively. The average CNR value was the highest when adaptive NL-TV was used, and the second highest value was obtained when a TV filter was applied to the image. The CNR value in the low-dose CT image using the adaptive NL-TV filter was improved by approximately 2.60, 1.09, and 1.29 times compared to none, TV filter, and NL-TV filter, respectively. Figure 3b shows a graph of the COV results evaluated on the low-dose CT images and the images acquired by applying each filter. The average value of COV calculated in the area of holes with six diameters was derived as 1.16, 1.53, 1.06, and 82.49 in low-dose CT images processed using TV, NL-TV, and adaptive NL-TV filters and in unprocessed low-dose CT images, respectively. Using the same tendency as in the CNR results, the best average COV value was calculated when an adaptive NL-TV filter was used. The COV value in the low-dose CT image using the adaptive NL-TV filter improved by approximately 1.10, 1.45, and 2.36 times compared with the low-dose CT images processed using TV and NL-TV filters and unprocessed low-dose CT images, respectively. Figure 2d is the result of applying the proposed method to visually identify the smallest holes and have the least noise, while (b) and (c) are the results of scaling the parameters to a line where the discriminability of the small holes is maintained. It can be seen that the proposed method can remove the noise more effectively while maintaining the discrimination  
Figure 4 shows the low-dose CT image acquired using the 610-03 part of the CIRS phantom and the filtered image. Line AB, shown in Figure 4a, is where the ESF and normalized LSF are obtained. The 610-03 part of the phantom comprises holes with eight different diameters that can be used to evaluate the spatial resolution. Overall, we visually confirmed that the noise level was significantly reduced when the proposed adaptive NL-TV filter was applied to the low-dose CT images (in Figure 4d). When the eight-hole areas were enlarged and observed (yellow box), an edge similar to that of the low-dose CT image was displayed using an adaptive NL-TV filter. In particular, we were able to confirm that the hole area, which was not distinguished owing to the excessive blurring introduced by the application of the TV and NL-TV filters, was well distinguished when adaptive NL-TV was applied to the low-dose CT image (as can be seen from the red arrows in Figure 4d). This was the result of removing the noise while maintaining the resolution of the holes as much as possible; other methods tried to maintain the discrimination of the small holes, but it was difficult to achieve an optimal result between noise reduction and resolution.