Iball Gareth R, Moore Alexis C, Crawford Elizabeth J
Leeds General Infirmary.
J Appl Clin Med Phys. 2016 Jul 8;17(4):291-306. doi: 10.1120/jacmp.v17i4.6165.
The study purpose was to develop and validate a quality assurance test for CT automatic exposure control (AEC) systems based on a set of nested polymethylmethacrylate CTDI phantoms. The test phantom was created by offsetting the 16 cm head phantom within the 32 cm body annulus, thus creating a three part phantom. This was scanned at all acceptance, routine, and some nonroutine quality assurance visits over a period of 45 months, resulting in 115 separate AEC tests on scanners from four manufacturers. For each scan the longitudinal mA modulation pattern was generated and measurements of image noise were made in two annular regions of interest. The scanner displayed CTDIvol and DLP were also recorded. The impact of a range of AEC configurations on dose and image quality were assessed at acceptance testing. For systems that were tested more than once, the percentage of CTDIvol values exceeding 5%, 10%, and 15% deviation from baseline was 23.4%, 12.6%, and 8.1% respectively. Similarly, for the image noise data, deviations greater than 2%, 5%, and 10% from baseline were 26.5%, 5.9%, and 2%, respectively. The majority of CTDIvol and noise deviations greater than 15% and 5%, respectively, could be explained by incorrect phantom setup or protocol selection. Barring these results, CTDIvol deviations of greater than 15% from baseline were found in 0.9% of tests and noise deviations greater than 5% from baseline were found in 1% of tests. The phantom was shown to be sensitive to changes in AEC setup, including the use of 3D, longitudinal or rotational tube current modulation. This test methodology allows for continuing performance assessment of CT AEC systems, and we recommend that this test should become part of routine CT quality assurance programs. Tolerances of ± 15% for CTDIvol and ± 5% for image noise relative to baseline values should be used.
本研究的目的是基于一组嵌套的聚甲基丙烯酸甲酯CTDI体模，开发并验证一种针对CT自动曝光控制（AEC）系统的质量保证测试。测试体模是通过将16 cm头部体模在32 cm体部环内偏移而创建的，从而形成一个三部分体模。在45个月的时间里，对所有验收、常规和一些非常规质量保证检查进行扫描，对来自四个制造商的扫描仪进行了115次单独的AEC测试。对于每次扫描，生成纵向毫安调制模式，并在两个感兴趣的环形区域进行图像噪声测量。还记录了扫描仪显示的CTDIvol和DLP。在验收测试中评估了一系列AEC配置对剂量和图像质量的影响。对于多次测试的系统，CTDIvol值超过基线偏差5%、10%和15%的百分比分别为23.4%、12.6%和8.1%。同样，对于图像噪声数据，偏离基线大于2%、5%和10%的分别为26.5%、5.9%和2%。CTDIvol和噪声偏差分别大于15%和5%的情况，大多数可归因于体模设置错误或协议选择不当。排除这些结果后，发现0.9%的测试中CTDIvol偏离基线大于15%，1%的测试中噪声偏离基线大于5%。该体模对AEC设置的变化很敏感，包括使用三维、纵向或旋转管电流调制。这种测试方法允许对CT AEC系统进行持续的性能评估，我们建议该测试应成为常规CT质量保证计划的一部分。相对于基线值，CTDIvol的公差应为±15%，图像噪声的公差应为±5%。
