Guo Junfeng, Wang Chao, Chan Kung-Sik, Jin Dakai, Saha Punam K, Sieren Jered P, Barr R G, Han MeiLan K, Kazerooni Ella, Cooper Christopher B, Couper David, Newell John D, Hoffman Eric A
Departments of Radiology and Biomedical Engineering, University of Iowa, Iowa City, Iowa 52242.
Department of Statistics and Actuarial Science, University of Iowa, Iowa City, Iowa 52242.
Med Phys. 2016 May;43(5):2598. doi: 10.1118/1.4947303.
A test object (phantom) is an important tool to evaluate comparability and stability of CT scanners used in multicenter and longitudinal studies. However, there are many sources of error that can interfere with the test object-derived quantitative measurements. Here the authors investigated three major possible sources of operator error in the use of a test object employed to assess pulmonary density-related as well as airway-related metrics.
Two kinds of experiments were carried out to assess measurement variability caused by imperfect scanning status. The first one consisted of three experiments. A COPDGene test object was scanned using a dual source multidetector computed tomographic scanner (Siemens Somatom Flash) with the Subpopulations and Intermediate Outcome Measures in COPD Study (SPIROMICS) inspiration protocol (120 kV, 110 mAs, pitch = 1, slice thickness = 0.75 mm, slice spacing = 0.5 mm) to evaluate the effects of tilt angle, water bottle offset, and air bubble size. After analysis of these results, a guideline was reached in order to achieve more reliable results for this test object. Next the authors applied the above findings to 2272 test object scans collected over 4 years as part of the SPIROMICS study. The authors compared changes of the data consistency before and after excluding the scans that failed to pass the guideline.
This study established the following limits for the test object: tilt index ≤0.3, water bottle offset limits of [-6.6 mm, 7.4 mm], and no air bubble within the water bottle, where tilt index is a measure incorporating two tilt angles around x- and y-axis. With 95% confidence, the density measurement variation for all five interested materials in the test object (acrylic, water, lung, inside air, and outside air) resulting from all three error sources can be limited to ±0.9 HU (summed in quadrature), when all the requirements are satisfied. The authors applied these criteria to 2272 SPIROMICS scans and demonstrated a significant reduction in measurement variation associated with the test object.
Three operator errors were identified which significantly affected the usability of the acquired scan images of the test object used for monitoring scanner stability in a multicenter study. The authors' results demonstrated that at the time of test object scan receipt at a radiology core laboratory, quality control procedures should include an assessment of tilt index, water bottle offset, and air bubble size within the water bottle. Application of this methodology to 2272 SPIROMICS scans indicated that their findings were not limited to the scanner make and model used for the initial test but was generalizable to both Siemens and GE scanners which comprise the scanner types used within the SPIROMICS study.
测试对象（体模）是评估多中心和纵向研究中使用的CT扫描仪的可比性和稳定性的重要工具。然而，有许多误差来源会干扰源自测试对象的定量测量。在此，作者调查了在使用用于评估肺密度相关以及气道相关指标的测试对象时，操作人员误差的三个主要可能来源。
进行了两种实验以评估由不完美扫描状态导致的测量变异性。第一个实验由三个子实验组成。使用双源多探测器计算机断层扫描仪（西门子Somatom Flash），按照慢性阻塞性肺疾病研究中的亚组和中间结果测量（SPIROMICS）吸气协议（120 kV，110 mAs，螺距 = 1，层厚 = 0.75 mm，层间距 = 0.5 mm）对一个慢性阻塞性肺疾病基因（COPDGene）测试对象进行扫描，以评估倾斜角度、水瓶偏移和气泡大小的影响。在分析这些结果后，得出了一项指南，以便为该测试对象获得更可靠的结果。接下来，作者将上述发现应用于作为SPIROMICS研究一部分在4年期间收集的2272次测试对象扫描。作者比较了在排除未通过该指南的扫描前后数据一致性的变化。
本研究确定了该测试对象的以下限度：倾斜指数≤0.3，水瓶偏移限度为[-6.6 mm, 7.4 mm]，且水瓶内无气泡，其中倾斜指数是一种结合了围绕x轴和y轴的两个倾斜角度的度量。在满足所有要求的情况下，对于测试对象中的所有五种感兴趣材料（丙烯酸、水、肺、内部空气和外部空气），由所有三个误差来源导致的密度测量变化在95%置信度下可限制在±0.9 HU（正交相加）。作者将这些标准应用于2272次SPIROMICS扫描，并证明与测试对象相关的测量变化显著减少。
识别出了三种操作人员误差，它们显著影响了在多中心研究中用于监测扫描仪稳定性的测试对象所获取扫描图像的可用性。作者的结果表明，在放射学核心实验室收到测试对象扫描时，质量控制程序应包括对倾斜指数、水瓶偏移和水瓶内气泡大小的评估。将此方法应用于2272次SPIROMICS扫描表明，他们的发现不仅限于用于初始测试的扫描仪品牌和型号，而是可推广到SPIROMICS研究中使用的包括西门子和通用电气（GE）扫描仪在内的两种扫描仪类型。
