平面成像中的图像质量和辐射剂量 - CDRAD 体模的图像质量优劣图。
Image quality and radiation dose in planar imaging - Image quality figure of merits from the CDRAD phantom.
机构信息
Department of Radiology, Vestfold Hospital Trust, Tønsberg, Norway.
Oslo Center for Biostatistics and Epidemiology, Research Support Services, Oslo University Hospital, Oslo, Norway.
出版信息
J Appl Clin Med Phys. 2019 Jul;20(7):151-159. doi: 10.1002/acm2.12649. Epub 2019 Jun 1.
PURPOSE
A contrast-detail phantom such as CDRAD is frequently used for quality assurance, optimization of image quality, and several other purposes. However, it is often used without considering the uncertainty of the results. The aim of this study was to assess two figure of merits (FOM) originating from CDRAD regarding the variations of the FOMs by dose utilized to create the x-ray image. The probability of overlapping (assessing an image acquired at a lower dose as better than an image acquired at a higher dose) was determined.
METHODS
The CDRAD phantom located underneath 12, 20, and 26 cm PMMA was imaged 16 times at five dose levels using an x-ray system with a flat-panel detector. All images were analyzed by CDRAD Analyser, version 1.1, which calculated the FOM inverse image quality figure (IQF ) and gave contrast detail curves for each image. Inherent properties of the CDRAD phantom were used to derive a new FOM h, which describes the size of the hole with the same diameter and depth that is just visible. Data were analyzed using heteroscedastic regression of mean and variance by dose. To ease interpretation, probabilities for overlaps were calculated assuming normal distribution, with associated bootstrap confidence intervals.
RESULTS
The proportion of total variability in IQF , explained by the dose (R ), was 91%, 85%, and 93% for 12, 20, and 26 cm PMMA. Corresponding results for h were 91%, 89%, and 95%. The overlap probability for different mAs levels was 1% for 0.8 vs 1.2 mAs, 5% for 1.2 vs 1.6 mAs, 10% for 1.6 vs 2.0 mAs, and 10% for 2.0 mAs vs 2.5 mAs for 12 cm PMMA. For 20 cm PMMA, it was 0.5% for 10 vs 16 mAs, 13% for 16 vs 20 mAs, 14% for 20 vs 25 mAs, and 14% for 25 vs 32 mAs. For 26 cm PMMA, the probability varied from 0% to 6% for various mAs levels. Even though the estimated probability for overlap was small, the 95% confidence interval (CI) showed relatively large uncertainties. For 12 cm PMMA, the associated CI for 0.8 vs 1.2 mAs was 0.1-3.2%, and the CI for 1.2 vs 1.6 mAs was 2.1-7.8%.
CONCLUSIONS
Inverse image quality figure and h are about equally related to dose level. The FOM h, which describes the size of a hole that should be seen in the image, may be a more intuitive FOM than IQF . However, considering the probabilities for overlap and their confidence intervals, the FOMs deduced from the CDRAD phantom are not sensitive to dose. Hence, CDRAD may not be an optimal phantom to differentiate between images acquired at different dose levels.
目的
CDRAD 等对比度细节体模常用于质量保证、图像质量优化和其他一些目的。然而，它经常在不考虑结果不确定性的情况下使用。本研究的目的是评估源自 CDRAD 的两个优点（FOM），以及利用创建 X 射线图像的剂量变化对 FOM 的影响。确定了重叠的概率（评估较低剂量获得的图像优于较高剂量获得的图像）。
方法
在 12、20 和 26cmPMMA 下方的 CDRAD 体模使用配备平板探测器的 X 射线系统在五个剂量水平下进行了 16 次成像。所有图像均由 CDRAD Analyser 版本 1.1 进行分析，该版本计算了优点反向图像质量因子（IQF），并为每张图像提供了对比度细节曲线。利用 CDRAD 体模的固有特性，推导出了一个新的优点 h，它描述了直径和深度相同但刚好可见的孔的大小。使用剂量的异方差回归分析数据。为了便于解释，假设正态分布并使用相关的自举置信区间计算重叠概率。
结果
在 IQF 中，由剂量（R）解释的总变异性的比例分别为 91%、85%和 93%，对于 12、20 和 26cmPMMA 分别为 91%、89%和 95%。对于 h 的相应结果分别为 91%、89%和 95%。不同 mAs 水平的重叠概率对于 0.8 与 1.2 mAs 为 1%，对于 1.2 与 1.6 mAs 为 5%，对于 1.6 与 2.0 mAs 为 10%，对于 2.0 mAs 与 2.5 mAs 为 10%，对于 12cmPMMA 为 1%。对于 20cmPMMA，对于 10 与 16 mAs 为 0.5%，对于 16 与 20 mAs 为 13%，对于 20 与 25 mAs 为 14%，对于 25 与 32 mAs 为 14%。对于 26cmPMMA，各种 mAs 水平的概率从 0%到 6%不等。尽管估计的重叠概率很小，但 95%置信区间（CI）显示出相对较大的不确定性。对于 12cmPMMA，0.8 与 1.2 mAs 的相关 CI 为 0.1-3.2%，1.2 与 1.6 mAs 的 CI 为 2.1-7.8%。
结论
反向图像质量因子和 h 与剂量水平大致相等相关。描述图像中应出现的孔的大小的优点 h 可能比 IQF 更直观。然而，考虑到重叠的概率及其置信区间，从 CDRAD 体模推导出的优点对剂量不敏感。因此，CDRAD 可能不是区分不同剂量水平下获得的图像的最佳体模。
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