## 4 Discussion

Table 4: Correlation coefficient \(r\) values of \(R_{M}{}^{\alpha/Na}\)(_average_) with \(BMD^{ls}\) and \(BMD^{lf}\), respectively. Figure 6: \(BMD^{ls}\) and \(BMD^{lf}\) and are correlated with \(R_{M}{}^{\alpha/Na}\)(_average_). The correlation coefficients are \(r\) = 0.647 and \(r\) = 0.792, respectively. Note that the correlation of \(BMD^{f}\) corresponds to \(r\) = 0.857. In our study, we targeted the entire range of a forearm, and found the best position providing the highest correlation value. In addition, since the absolute value of bone size or volume may affect the BMD estimation, we considered the ratio value of cortical and total bone areas of the forearm. For further analysis, we investigated the correlation values between femoral neck BMD and other 12 parameters: volumes and areas in different areas: cortical volume in radius, total volume in radius, cortical volume in ulna, total volume in radius, average cortical volume in radius and ulna, average total volume in radius and ulna, cortical area in radius, total area in radius, cortical area in ulna, total area in ulna, average cortical area in radius and ulna, and average total area in radius and ulna, where the area is the midshaft. Figure 7 summarizes the comparison of the correlation coefficient values according to the volumes and area in different sites. It shows that the correlation coefficients from the volumes and areas in different areas were lower than the values from our proposed method. In addition, one of the advantages in our method is that a forearm, especially the midshaft area, needs to be exposed via X-ray. On the other hand, using DXA and QCT, the entire axial skeleton near femur neck is exposed via X-ray. Because the cancer occurrence by X-ray exposure is one of the main factors, the X-ray exposure of the forearm midshaft area is better than that of the axial skeleton near femur neck. Obviously, we expect the deviated results between two groups having a job requiring upper body labor and a job requiring lower body labor. For the future research, the job-matching-based research needs to performed for elaborating the relationship. Furthermore, this pilot study (\(N=23\)) needs to be rigorously further validated by considering more extensive subjects. We further validated our proposed method with five healthy subjects with age of 27.6 \(\pm\) 2.71. We calculated the ratio of the cortical to the total bone area in the forearm and estimated the femoral BMD based. The RMSE and MAPE were 0.028 g/cm\({}^{2}\) and 2.86%, respectively. Even though the validation subject number is small, we found the additional data were fitted in our proposed method. We hope more related studies are performed in the near future. The CBCT technique was recently introduced two decades ago and currently widely used for clinical diagnosis, especially for peripheral and dental use [28; 29; 30; 31]. It provides two-dimensional and three-dimensional images for the radiographed area with a relatively low cost as compared to conventional CT. The radiation dose is comparable to that of panoramic imaging in the case of small field of views (FOVs). Due to the low cost and low radiation dose, it became popular, its use widespread. However, the gray values from CBCT are considered approximate values, which are not accurately expressed as HU comparing with conventional CT. There is a report that the gray values obtained by CBCT are higher than the HU values obtained by conventional CT of the same region [32]. The inaccuracy of gray values by CBCT may result in the incorrect boundary of cortical bone. As the density becomes weaker, the boundary extraction for the cortical bone becomes inaccurate. Figure 7: Comparison of correlation coefficient values according to volumes and areas in different sites. In our experiments, the more severe the osteoporosis was, the lower the accuracy for the boundary extraction was. For the case of the inaccurate boundary extraction, three radiologists in Wonkwang University Hospital confirmed and manually corrected the boundary when possible.