## 4 Discussion  
In this study, based on the conceptual design of the FDB proposed by [PERSON] et al. [27], more comprehensive evaluations of dose reduction and image quality of the head, thorax and abdomen scans were carried out through theoretical studies. In calculating the radiation dose, the Monte Carlo method, which is considered the gold standard for dose calculation, was used [40; 41; 42; 43], and the energy distribution of X-rays was also considered. The results show that the energy spectra of the X-rays change with the type of attenuation filter (bowties). Therefore, the relative dose calculated in this study was considered to be more accurate. Furthermore, we investigated the radiation dose and image quality using different scanning protocols based on the same specified image noise. Therefore, the comparisons are considered to be more reasonable.  
According to the preliminary results of this study, it is confirmed that FDB has a greater potential to reduce the radiation dose to the patient and improve the image quality than SB, regardless of the scanning parts of the patients. Especially for the organs or tissues in the surface layer of patients, the absorbed doses can be significantly reduced using FDB. The main reason for this is that the WB in the FDB is three-dimensional and dynamic, and it is easy adjust to match the scanned subject.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Scanning Protocol**} & \multicolumn{2}{c}{**Water Phantom**} & \multicolumn{3}{c}{**An Adult**} \\ \cline{2-6}  & **Head** & **Body** & **Head** & **Thorax** & **Abdomen** \\ \hline NoAtt\_tcm0 & 0.49 & 0.31 & 0.40 & 0.35 & 0.45 \\ SB\_tcm0 & 0.59 & 0.39 & 0.43 & 0.35 & 0.45 \\ FDB\_tcm0 & 0.67 & 0.51 & 0.47 & 0.45 & 0.59 \\ NoAtt\_tcm1 & 0.59 & 0.35 & 0.46 & 0.38 & 0.51 \\ SB\_tcm1 & 0.65 & 0.45 & 0.46 & 0.42 & 0.55 \\ FDB\_tcm1 & 0.67 & 0.51 & 0.51 & 0.46 & 0.64 \\ \hline \hline \end{tabular}
\end{table}
Table 6: NPS isotropy of reconstructed images using different scanning protocols.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Protocol**} & \multicolumn{2}{c}{**Head**} & \multicolumn{2}{c}{**Thorax**} & \multicolumn{2}{c}{**Abdomen**} \\ \cline{2-7}  & **Whole (a)** & **Brain** & **Eyes** & **Whole (a)** & **Lungs** & **Whole (b)** & **Intestine** & **Stomach** \\ \hline NoAtt\_tcm0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\ SB\_tcm0 & 0.69 & 0.80 & 0.71 & 0.85 & 0.94 & 0.78 & 0.84 & 0.85 \\ FDB\_tcm0 & 0.33 & 0.41 & 0.43 & 0.68 & 0.87 & 0.43 & 0.55 & 0.58 \\ NoAtt\_tcm1 & 0.91 & 0.91 & 0.91 & 0.96 & 0.97 & 0.94 & 0.94 & 0.94 \\ SB\_tcm1 & 0.66 & 0.76 & 0.67 & 0.84 & 0.92 & 0.74 & 0.81 & 0.80 \\ FDB\_tcm1 & 0.30 & 0.37 & 0.38 & 0.66 & 0.85 & 0.40 & 0.52 & 0.54 \\ \hline \hline \end{tabular}
\end{table}
Table 5: The relative dose absorbed in the head, thorax and abdomen using different scanning protocols.  
However, more studies and developments are needed to put FDB into clinical use. First, a technique for controlling the changes in the weakly attenuating bowtie (WB) in volume, shape and position to match the scanning parts of individual patients must be developed. The optimal liquid component used as the HB is also worth investigating. Furthermore, only the CBCT was studied in this work, and further studies on fan-beam CT might be more useful.