Szczykutowicz Timothy P, DuPlissis Andrew, Pickhardt Perry J
1 Department of Radiology, University of Wisconsin-Madison, 1111 Highland Ave, Rm 1005, Madison, WI 53705.
2 Department of Medical Physics, University of Wisconsin-Madison, Madison, WI.
AJR Am J Roentgenol. 2017 May;208(5):1064-1072. doi: 10.2214/AJR.16.17215. Epub 2017 Mar 7.
Many algorithms for clinical decision making rely on assessment of the CT number (expressed as Hounsfield units); however, to our knowledge, few, if any, studies have addressed how CT numbers change as a function of patient positioning within the scanner.
An anthropomorphic phantom underwent imaging with varying amounts of vertical orientation misalignment with respect to isocenter. CT number and noise were measured using ROIs in the upper thorax, mid thorax, and abdomen. The degree of noise nonuniformity and changes in the CT number were assessed by comparing values obtained in the anterior versus posterior ROIs. To add clinical relevance, data on vertical mispositioning were collected from 20,316 clinical abdominal CT scans. Box-and-whisker plot analysis was used to identify the range of patient positioning.
Absolute CT number changes of more than 20 HU were observed for some ROIs at phantom positions of 10 cm from isocenter, with important differences noted between the thoracic and abdominal regions. Noise uniformity varied by more than twofold for all regions at 10 cm below isocenter. On clinical CT examinations, off-centering of more than 1, 2, 4, and 6 cm occurred for 41%, 19%, 1.9%, and 0.3% of patients, respectively.
Radiologists should treat CT number measurements with caution when patients are grossly mispositioned in the scanner. The substantial changes in attenuation values shown in the present study are large enough to warrant further investigation.
许多临床决策算法依赖于CT值（以亨氏单位表示）的评估；然而，据我们所知，很少有研究（如果有的话）探讨CT值如何随患者在扫描仪内的位置变化而变化。
一个仿真人体模型在相对于等中心有不同程度垂直方向错位的情况下进行成像。使用位于上胸部、中胸部和腹部的感兴趣区（ROI）测量CT值和噪声。通过比较在前部和后部ROI中获得的值来评估噪声不均匀程度和CT值的变化。为增加临床相关性，从20316例临床腹部CT扫描中收集垂直错位的数据。采用箱线图分析来确定患者位置的范围。
在距等中心10 cm的模型位置，一些ROI观察到绝对CT值变化超过20 HU，胸部和腹部区域之间存在显著差异。在等中心以下10 cm处，所有区域的噪声均匀性变化超过两倍。在临床CT检查中，分别有41%、19%、1.9%和0.3%的患者出现了超过1 cm、2 cm、4 cm和6 cm的偏心。
当患者在扫描仪中严重错位时，放射科医生应谨慎对待CT值测量。本研究中显示的衰减值的显著变化大到足以值得进一步研究。
