## 4 Discussion
The recommended values were slightly expanded beyond the range of the previous results section as a conservative threshold because it may be dangerous to determine appropriate or inadequate values within a tight range setting. Additionally, the range was set in consideration of the radiation intensity and volume. For contrast index, the range of 15% for CR and 10% for SR were set considering our numerical results and regulation of tube current (mA) involved in CTDIvol or tube voltages (kVp). The allowed ranges of mAs and CTDIvol related to CR are \(\pm\) 15% and those of kVp related to SR are \(\pm\) 10% [15; 16]. Based on our results and the regulation of the QC, we presented the appropriate ranges making the standards more flexible. TA using a histogram is divided into several types such as first-order, and second-order methods [29; 30]. The first-order method, which is the design of this study, measures texture calculated using only a histogram. The second-order method could take into account the relative position between pixels along with the existing brightness distribution. It is called a gray-level co-occurrence matrix (GLCM), which creates a co-occurrence matrix based on the frequency count of one pixel and another neighboring pixel value in the original image [29; 30]. However, as a result of using GLCM when evaluating CR and SR using the AAPM phantom, there was no appearance of appropriate values to distinguish subtle differences or changes in phantom images. On the other hand, the first-order method could classify numerically between inappropriate images and normal images. Thus, the first-order statistical method was adopted since it was possible to find criteria for numerically common convergence in the various pieces of equipment. There are several limitations to this study. First, we did not perform the analysis with the receiver operating characteristic curve. In future research, the classification of suitable and inappropriate images and QC with TA will be performed by collaborating with radiologists. This step will contribute to conducting the exact assessment of the suitability of TA with the receiver operating characteristic curve. Second, the suggested criteria are only applicable to one dose level. Therefore, further studies should be performed at other lower radiation dose levels in order to confirm the maintenance of high quality at low doses. Third, TA analysis was only performed with images of the AAPM phantom. Hence, it will be necessary to evaluate the feasibility of other QC phantoms. Lastly, even if there is no statistically significant difference, there may be limitations in concluding that the proposed parameters are suitable for QC. Therefore, a verification of the statistical significance of the results is proposed through further research in the future. Despite these limitations, this was the first study conducted to present the TA method and the criteria for evaluating CR and SR for any CT scanner. Although additional verification may be required, our study showed the feasibility of TA for quantifying CR and SR evaluation with the AAPM phantom. Especially the first-order statistical TA method proved to be efficient in the quantification of CT QC.