## 1 Introduction
Secondly, there is the digital radiography (DR) method, in which a flat panel detector (FPD) is used. After taking an X-ray, it can be transferred to a computer within a few seconds without a separate process, and additional image processing is possible. Furthermore, it is possible to obtain excellent images even with a lower radiation dose compared to the analog method. However, the equipment is expensive, and the digital FPD is sensitive to handling. Accordingly, there is a high risk of damage to the FPD when used as a target for mechanical devices. Thirdly, the computed radiography (CR) method is easily understood as a combination of analog and DR. The CR method is used an imaging plate (IP) as a detector. After taking an X-ray, when IP is inserted into the CR reader, the image is transmitted to the computer through the image processing process. CR transmits images faster than analog but slower than DR. Nevertheless, in the same way as DR, CR can also be transmitted to a computer for additional image processing. In addition, the CR is lower than DR in investment cost, and the IP detector is easier to manage than FPD, so it can be used for various purposes. For comparison, the characteristic comparison table of the X-ray detection methods is shown in Table 1, and the workflow of the X-ray detection methods is shown in Figure 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Analog Film** & **CR** & **DR** \\ \hline Image acquisition time & Slow & Normal & Fast \\ Image processing & Impossible & Possible & Possible \\ Dynamic range & Low & High & High \\ Cost (investment) & Low & Normal & High \\ Cost (using) & High (film, developer, etc.) & Normal (IP & Low \\ Number of reuses & 1 & 10,000 & 1,000,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The characteristic comparison table of the X-ray detection methods. Besides that, there are computed tomography (CT) X-ray methods. Since the CT X-ray method takes 360\({}^{\circ}\) of an object, it is possible to obtain an accurate image. Therefore, Waseda University and the National Institute of Advanced Industrial Science and Technology in Japan conducted a study on the transfer and accumulation of ash based on images taken using the CT X-ray imaging technique (Figure 2) [11; 12]. Furthermore, Korea University identified the length and shape of ash employing the CT X-ray imaging technique (Figure 3) [13]. However, CT X-ray equipment is expensive, and removal is inevitable when taking DPF. Accordingly, the CT X-ray method takes a lot of time to take a DPF. On the other hand, the DPF installed under vehicles was estimated to be able to take an image in a short period of time using portable CR X-ray equipment and flexible IP without its actual removal. Accordingly, a study was conducted to measure the damaged DPF using the CR X-ray imaging technique [14]. As the CR X-ray imaging technique photographs an object unidirectionally, there are no research studies applying the technique on vehicles, as it has limitations in obtaining a highly clear image required to identify characteristics of ash accumulation on each channel of the substrate. In this study, Image J software (capable of image processing and analysis) was used, and the accumulation of soot and ash was analyzed based on the pixel values obtained through the histogram function. Figure 1: Workflow of the X-ray detection methods. Figure 2: Ash transport during active regeneration (Waseda University with the National Institute of Advanced Industrial Science and Technology in Japan).