Anode heel effects are known to cause non-uniform image quality, but no method has been proposed to evaluate the non-uniform image quality caused by the heel effect. Therefore, the purpose of this study was to evaluate non-uniform image quality in digital radiographs using a novel circular step-wedge (CSW) phantom and normalized mutual information (nMI). All X-ray images were acquired from a digital radiography system equipped with a CsI flat panel detector. A new acrylic CSW phantom was imaged ten times at various kVp and mAs to evaluate overall and non-uniform image quality with nMI metrics. For comparisons, a conventional contrast-detail resolution phantom was imaged ten times at identical exposure parameters to evaluate overall image quality with visible ratio (VR) metrics, and the phantom was placed in different orientations to assess non-uniform image quality. In addition, heel effect correction (HEC) was executed to elucidate the impact of its effect on image quality. The results showed that both nMI and VR metrics significantly changed with kVp and mAs, and had a significant positive correlation. The positive correlation is suggestive that the nMI metrics have a similar performance to the VR metrics in assessing the overall image quality of digital radiographs. The nMI metrics significantly changed with orientations and also significantly increased after HEC in the anode direction. However, the VR metrics did not change significantly with orientations or with HEC. The results indicate that the nMI metrics were more sensitive than the VR metrics with regards to non-uniform image quality caused by the anode heel effect. In conclusion, the proposed nMI metrics with a CSW phantom outperformed the conventional VR metrics in detecting non-uniform image quality caused by the heel effect, and thus are suitable for quantitatively evaluating non-uniform image quality in digital radiographs with and without HEC.

中文翻译：

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利用互信息评估数字射线照相中阳极跟效应引起的图像质量不均匀

已知阳极后跟效应会导致图像质量不均匀，但是尚未提出任何方法来评估由后跟效应引起的图像质量不均匀。因此，本研究的目的是评估使用新型圆形阶梯楔形（CSW）体模和归一化互信息（nMI）的数字X线照片中的不均匀图像质量。所有X射线图像均从配备有CsI平板探测器的数字射线照相系统中获取。在不同的kVp和mAs下，对新的丙烯酸CSW幻像进行了十次成像，以使用nMI指标评估整体和不均匀的图像质量。为了进行比较，以相同的曝光参数对传统的对比度细节分辨率模型进行了十次成像，以评估可见比（VR）指标的整体图像质量，并且将体模放置在不同的方向上，以评估不均匀的图像质量。此外，还执行了脚跟效应校正（HEC），以阐明其效应对图像质量的影响。结果表明，nMI和VR指标均随kVp和mAs显着变化，并具有显着正相关。这种正相关关系表明，在评估数字X射线照片的总体图像质量时，nMI指标与VR指标的性能相似。nMI度量随方向显着变化，并且在阳极方向上进行HEC后也显着增加。但是，VR指标不会随方向或HEC发生显着变化。结果表明，就阳极后跟效应引起的图像质量不均匀而言，nMI指标比VR指标更为敏感。总之，在检测由脚跟效应引起的图像质量不均匀时，建议的带有CSW体模的nMI度量优于传统的VR度量，因此适用于定量评估带有或不带有HEC的数字X线照片中的图像不均匀。