## 4 Discussion  
In the field of radiology, CBCT has been widely used in medical imaging because of its advantages, including reduced examination time, 3D image reconstruction, and high-resolution anatomical information [31, 32]. Recently, considering the excessive radiation dose of CBCT, low-dose CBCT has been proposed, however, noise is generated because of the insufficient number of photons, and to solve this issue, reconstruction of CBCT has been actively conducted [33, 34, 35, 36]. One of the most widely used CBCT reconstruction algorithms, the FDK algorithm, has been shown, in many studies, to provide superior reconstruction results even when degraded by noise [37, 38, 39]. In addition, adjusting the parameters of the FDK algorithm can improve the quality of the reconstructed images. Therefore, this study focused on the FDK algorithm and evaluated the image quality of simulated reconstruction images by adjusting the parameters of the FDK algorithm. Although many previous studies have evaluated the image quality of the FDK algorithm, few have combined noise, similarity, and no-reference evaluations of reconstruction images, and few have evaluated the tendencies of different tissues or materials [17, 40, 41]. Therefore, in this simulation study, we aimed to quantitatively evaluate the image quality of the images reconstructed using the FDK algorithm according to the projection angle and filter applied through noise evaluation, similarity evaluation, and no-reference evaluation factors. From the visual comparative evaluation shown in Figure 3, we found that noise remained when the Ram-Lak filter was applied. Moreover, we observed that noise was significantly enhanced as the projection angle increased. Based on these visual evaluation results, we confirmed that when the CBCT images were reconstructed by applying the FDK algorithm, the low-dose protocol resulted in noisy reconstruction images owing to the low SNR in the projection image. In addition, we confirmed that streak artifacts were generated as the projection angle increased. Streak artifacts are dark bands or streaks that appear between dense objects  
Figure 7: Blind/referenceless image spatial quality evaluator (BRISQUE) at a projection angle of 1.2\({}^{\circ}\) for comparative evaluation of FDK reconstruction filters. in an image. Beam hardening is one of the most common causes of streak artifacts. An X-ray beam is composed of individual photons with a range of energies. In CBCT, the X-ray source is rotated around an object to acquire projections from different angles. Dense objects such as bone have a higher density than other surrounding tissues, and this high density causes the X-ray penetrating the object to be absorbed by the bone, thereby increasing the average energy of X-rays. When the emitted spectrum contains beams of relatively lower energy than those recorded by the detector (i.e., the beams are hardening), a nonlinear error (relatively high energy recorded in the beam path behind the highly absorbent material) is introduced into the recorded data spectrum. In 3D reconstruction, the error is backprojected onto the volume, resulting in dark streak artifacts. In addition, this artifact is more pronounced in CBCT images because the CBCT X-ray beam has a lower average kilovolt (peak) energy than conventional CT [42]. Moreover, we confirmed that the streak artifacts were enhanced as the projection angle increased. The sparse view resulted in undersampling of the streak artifacts. As the projection angle increases, the projection information for bones or objects in different views may overlap while acquiring projection data. This phenomenon, commonly known as view overlap, occurs in CBCT scans and causes streak artifacts around bones or dense objects. In the head CBCT simulation in this study, the streak artifacts were intensified by the petrous, a rock-shaped dense structure that is part of the body of the spinoid bone, and the greater wing temporal bone around the brain region [43, 44].