Analysis of the reproducibility of the gray values and noise of a direct digital radiography system

The aim of this study was to evaluate the reproducibility of the gray values and noise of a direct digital radiography system (Visualix eHD) for various exposure times and analyzed regions. To obtain radiographic images in a standardized manner, the digital sensor of the system and a stepwedge were positioned in a phantom at a focus-film distance of 30 cm in a dental device at 70 kV, 7 mA and 2.2 mm filtration. Ten consecutive repetitions of X-ray imaging were performed at each exposure time (0.05, 0.07, 0.09 and 0.13 s). Gray values were analyzed using ImageJ software in five regions of interest (ROIs): alveolar bone (AB), soft tissue (ST) and three steps of the stepwedge (S1, S2 and S3). The results showed that both the variability of the gray values and the noise were statistically greater (p &lt; 0.05) in the most radiolucent region (ST). Only the noise was affected by the exposure time. In conclusion, the reproducibility of the gray values and the noise of the Visualix eHD system can vary in specific areas with different radiolucency.本研究旨在评估直接数字化放射成像系统（Visualix eHD）在不同曝光时长及分析区域下，其灰度值与噪声的可重复性。 为标准化获取放射影像，本实验将该系统的数字化传感器与阶梯楔形模体置于体模内，在牙科放射设备中设置70 kV管电压、7 mA管电流、2.2 mm滤过条件，焦片距为30 cm。 在各曝光时长（0.05、0.07、0.09及0.13 s）下，均连续进行10次X线成像重复实验。 采用ImageJ软件对5个感兴趣区域（regions of interest, ROIs）的灰度值进行分析：牙槽骨（alveolar bone, AB）、软组织（soft tissue, ST）以及阶梯楔形模体的三个阶梯（S1、S2、S3）。 结果表明，在射线透过度最高的区域（ST，软组织），灰度值的变异程度与噪声水平均显著更高，且差异具有统计学意义（p < 0.05）。 仅噪声水平受曝光时长的影响。 综上，Visualix eHD系统的灰度值与噪声的可重复性，在射线透过度不同的特定区域中存在差异。