# Quality Control in Dental Cone-Beam Computed Tomography
## 4 Discussions  
The traditional method of evaluating image quality is based on human observation. Besides the reasons mentioned in the Introduction section for the traditional methods being unsuitable for dental CBCT, another drawback is that they are highly subjective. Different observers, and even the same observer on different occasions, can give different results when they are presented with the same signals. Additionally, the results can be biased and difficult to reproduce.  
In this study, objective measures of quantitative image quality evaluation were used for image quality control of dental CBCT, such as CNR and MTF. These objective measures are reproducible, they are not dependent on the observer, and they can be conveniently assessed using computer software. Therefore, we used these measures to replace the evaluation and measurements based on contrast-detail objects and bar patterns, which have been in use for quality control for more than 20 years [31; 32; 33].  
The traditional dosimetry leads to three kinds of problems: (1) the formalisms use phantoms that are too small to properly represent radiation scatter from the large X-ray fields commonly used in CBCT applications. (2) The positioning of the phantom can be problematic, in particular for dental CBCT [23]. (3) The pencil ionization chamber is not  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{**No.**} & \multicolumn{1}{c}{**Site**} & \multicolumn{1}{c}{**Manufacturer**} & \multicolumn{1}{c}{**Model**} & \multicolumn{1}{c}{**Spatial Resolution/(Lp/mm)**} & \multicolumn{1}{c}{**Uniformity Index**} & \multicolumn{1}{c}{**CNI (\%)**} \\ \hline \multirow{2}{*}{1} & Shenyang Shenhe Dental Clinic & \multirow{2}{*}{Kavo} & \multirow{2}{*}{Kavo3 Dexam} & \multirow{2}{*}{1.52} & \multirow{2}{*}{23} & \multirow{2}{*}{6.718} \\ \hline \multirow{4}{*}{2} & China Medical University School\& Hospital of Stomatology & & & & \\ \cline{2-6}  & Stomatological Hospital & & & & & \\ \cline{2-6}  & & & & & & \\ \hline \multirow{2}{*}{3} & Stomatological Hospital, Jilin University & \multirow{2}{*}{Caerestream} & \multirow{2}{*}{CS9300} & \multirow{2}{*}{1.49} & \multirow{2}{*}{25} & \multirow{2}{*}{7.015} \\ \cline{2-6}  & Heilongjiang Oral Disease Prevention and Treatment Institute & & & & & \\ \hline \multirow{2}{*}{5} & Shanghai Xinhua Hospital & \multirow{2}{*}{Kavo} & \multirow{2}{*}{Kavo3 Dexam} & \multirow{2}{*}{1.57} & \multirow{2}{*}{26} & \multirow{2}{*}{6.810} \\ \cline{2-6}  & Shanghai Huashan Hospital & \multirow{2}{*}{Kavo} & \multirow{2}{*}{Kavo3 Dexam} & \multirow{2}{*}{1.61} & \multirow{2}{*}{23} & \multirow{2}{*}{6.975} \\ \cline{2-6}  & Beijing Jinsong Dental Clinic & & & & & \\ \hline \multirow{2}{*}{8} & Nanjing Stomatological Hospital & \multirow{2}{*}{Kavo} & \multirow{2}{*}{Kavo3 Dexam} & \multirow{2}{*}{1.53} & \multirow{2}{*}{21} & \multirow{2}{*}{7.112} \\ \cline{2-6}  & Jiangsu Provincial & & & & & \\ \cline{2-6}  & Stomatological Hospital & & & & & \\ \hline \multirow{2}{*}{10} & Shenyang Stomatological Hospital & \multirow{2}{*}{Kavo} & \multirow{2}{*}{Kavo3 Dexam} & \multirow{2}{*}{1.52} & \multirow{2}{*}{27} & \multirow{2}{*}{6.773} \\ \cline{2-6}  & & Action level & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental results of the image metric parameters.  
long enough, and the efficiency is low due to the large FOV. The KAP meter we used did not have these problems. It was directly attached to the z-ray outlet, covering almost all collimations.  
These new methods were carefully examined and discussed at the annual meeting of the China Ionizing Radiation Metrology Technical Committee and passed the review of experts. This study also had certain shortcomings. We did not find enough unqualified dental CBCT equipment to conduct a control experiment. We aim to complete this task in the next step to verify the feasibility of these methods further.