## 4 Discussion  
In QC of CT scanners, CR and SR evaluation with the standard phantom has often been performed subjectively, by eye [13; 14; 16]. Qualitative evaluation in medical diagnostics has an important role to play, but subjective decisions increase the error rate. Different observers, and even the same observer on different occasions, can provide different results when presented with the same signals [35]. In this regard, [PERSON] et al. [36] revealed common mistakes made by individuals using a phantom when being tested for CT accreditation. Moreover, these analyses should be quantitative in nature, as there is already a subjective component to the assessment of clinical CT images [16; 18]. Additionally, the criteria used in the phantom evaluation should be consistent. Currently, in Europe and the United States, it is mainly recommended that CNR should be used for CR evaluation and modulation transfer function (MTF) or image sharpness assessment for SR analysis [16; 17]. CNR is also a useful index for distinguishing at low contrast, but it cannot reflect a whole CR image containing all sizes of holes by using a small size of ROI. Moreover, the acceptance criteria for CR include that circles of certain sizes should be shown (e.g., circles up to 6.4 mm in diameter must be distinguished from the background with the AAPM phantom [14; 18]), and all four cylinders (diameters: 2, 3, 4, 5, and 6 mm) are visualized by using the ACR phantom [17; 37]. For this reason, it would be better to quantify the entire CR image, as was performed in this study. For MTF measurement for SR analysis, line spread function or edge spread function should be acquired by specific images in the form of straight lines and then transformed into the spatial-frequency domain [38]. Although MTF is a useful method, the direct SR block images of the standard phantom could not be utilized (i.e., a slanted image or a point image with a clear difference in contrast is essential). In contrast, our method is easier to apply and has the advantage that a variety of images, including the AAPM phantom, can be used directly; this can simplify complex processes by only setting the ROI on the SR image. Depending on the equipment used, the values of CTDIVol were different even if the same tube currents (mAs) were applied in the AAPM phantom (Table 1). It is important to control for radiation dose when quantitatively comparing different CT scanners because the radiation dose is related to kVp and mA, which directly affects the signal-to-noise ratio, not the resolution. Therefore, in our study, a dose-based protocol was utilized (Table 1), similar to that used by [PERSON] et al. [28] and [PERSON] et al. [39]. As a result, the feasible TA descriptors were determined for CR and SR evaluation. Among the TA descriptors, brightness, uniformity, and randomness were inappropriate as useful criteria for CR and SR. Since the value of tube currents was different even at the same CTDIVol among all the scanners, there may have been a statistically significant  
Figure 6: Textural analysis of appropriate images per manufacturer and all distorted images combined, for spatial resolution (SR). (**a**) Mean values of the contrast index; (**b**) mean values of the softness index; (**c**) mean values of the skewness index. difference in brightness representing the number of X-rays. Uniformity represents a uniform image, whereas randomness represents a non-uniform image. In other words, these descriptors emphasize the characteristics whether they are uniform or not. Therefore, we assumed that these maybe did not fit the TA, the ability to distinguish two different objects. We recommend the following indices and ranges as suitable values: CR: contrast index, 29.5 \(\pm\) 15%; softness index, <0.015; SR: contrast index, 29 \(\pm\) 10%; softness index, <0.014; and skewness index, >\(-\)1.85.