## 4 Discussion
More research is needed to utilize these stem cells for bone regeneration procedures in donor sites of mandibular bone block [28]. Currently, CBCT is widely used in dentistry, offering 3D data on jaw bones and teeth with relatively low radiation doses and costs. Although volumetric measurements were performed using CBCT images, there are several known limitations to this approach. For instance, accurate measurement using 3D analysis software is difficult owing to the varied degrees of ossification at the bone boundaries [29; 30]. Moreover, the patient cannot be placed in the exact same position again for serial CBCT images. Therefore, the donor site images cannot be obtained by simple deduction of serial CBCT images. Lastly, reliable evaluation of the bone volume and quality is difficult to achieve owing to errors in the manual registration. To overcome these difficulties, we attempted both a confinement of the ROI and an intensity-based registration technique. Each CBCT image has a large variation in pixel values caused by an increased amount of scattered radiation [31], which prevents accurate registration. By the segmentation of mandible, the ROI used for registration was confined, which can result in smaller registration error. The registration was conducted using an intensity-based algorithm, which has been used for various multimodal registrations and is reported to be clinically acceptable when registering two CBCT images with less than 0.5 mm registration error [32; 33]. In this study, pixel value calculation was used to evaluate the quality of the newly generated bone, which is generally considered incorrect owing to the inaccuracy of CBCT images. Nonetheless, because this study implemented histogram matching, the images had to be transformed for the comparisons. The bone quality has been widely analyzed using the pixel value in microCT [34; 35]. A positive correlation between the structural indices of CBCT and micro-CT has been reported in several studies [36; 37]. In addition, several studies have compared CBCT pixel values with CT numbers using histogram matching [38]; however, few studies have compared CBCT with CBCT using histogram matching [17]. Analysis of bone quality is clinically important because bone volume alone is insufficient for the evaluation of the structural changes inside the donor site. The results of this study showed that the pixel value at T2 was significantly lower than that at T0 (\(p<0.05\)). It can be interpreted that a newly generated bone has poorer quality in general than the original bone. The technique demonstrated in the present study has yet to be proven clinically. Notwithstanding, it is still worth attempting in dentistry. For example, the pixel value of the insertion site and insertion torque can be compared to set up the relation of pixel value and bone quality in CBCT images. Two postoperative CBCT scans after a preoperative exam raise concerns about radiation protection, since there is no evidence that postoperative CBCT scans can be justified after bone graft. The postoperative CBCT scans in this study were taken for additional 3D information, which was crucial for treatment planning. In T1, CBCT was taken instead of panorama when evaluation of the initial fixation or of the nerve damage was required. In T2, CBCT was taken for three-dimensional evaluation of the implant placement site and for the production of surgical guide. This study had some limitations. Firstly, several factors contributed to the results. The pre-existing quality of the donor sites differed among patients. Additionally, the size of the harvested bone was not strictly standardized. Although all surgeries were performed by two clinicians using the same protocol, the differences observed might be in part due to skill differences and patient allocation. Secondly, the bone healing time (time between T1 and T2) was variable. Setting the same time points is necessary to control variations; even so, there are limitations in clinical practice.