Kemerink G J, Lamers R J, Pellis B J, Kruize H H, van Engelshoven J M
Department of Radiology, University Hospital Maastricht, The Netherlands.
Med Phys. 1998 Dec;25(12):2432-9. doi: 10.1118/1.598454.
Our purpose in this study was to investigate the influence of segmentation threshold and number of erosions on parameters used in quantitative computed tomography (CT) of the lung (erosions are shrink operations on the segmented area). Parameters assessed were mean lung density, area of the segmented lung, two percentiles, and the pixel index, which is the relative area of the histogram below -905 Hounsfield Units (HU). We analyzed images of ten emphysematous and ten nonemphysematous patients, that had been scanned at carina level in inspiration and expiration, using sections of 1, 2, 3, 5, and 10 mm in combination with a standard, a smooth, and an ultrasmooth reconstruction kernel. The lungs were segmented using pixel tracing at thresholds of -200, -400, and -600 HU with 0-4 erosions, followed by histogram analysis. The area of the segmented lungs decreased with 0.9%-3.2% per 100 HU decrease in threshold and with 2.2%-3.1% per erosion, dependent on patient group and respiratory status. Estimated mean lung density changed up to 30% by changing the threshold and the number of erosions. The pixel index and the 10th percentile depended only slightly on threshold and number of erosions, but the 90th percentile showed a strong dependence of up to 40%. It is concluded that the segmentation protocol can have a large impact on densitometric parameters and that standardization is mandatory for obtaining comparable results. Ideally a threshold equal to the average of the densities of lung and soft tissue should be used, but -400 HU will do in a limited but common density range (-910 to -790 HU). For densitometry two erosions are recommended, for volumetry zero erosions should be used.
本研究的目的是调查分割阈值和腐蚀次数对肺部定量计算机断层扫描（CT）中使用的参数的影响（腐蚀是对分割区域进行的收缩操作）。评估的参数包括平均肺密度、分割肺的面积、两个百分位数以及像素指数，像素指数是低于-905亨氏单位（HU）的直方图的相对面积。我们分析了10例肺气肿患者和10例非肺气肿患者的图像，这些患者在吸气和呼气时于隆突水平进行了扫描，使用了1、2、3、5和10毫米的层厚，并结合标准、平滑和超平滑重建核。使用像素追踪法在-200、-400和-600 HU的阈值下对肺部进行分割，并进行0至4次腐蚀，随后进行直方图分析。分割肺的面积随阈值每降低100 HU减少0.9%-3.2%，随每次腐蚀减少2.2%-3.1%，这取决于患者组和呼吸状态。通过改变阈值和腐蚀次数，估计的平均肺密度变化高达30%。像素指数和第10百分位数仅略微依赖于阈值和腐蚀次数，但第90百分位数显示出高达40%的强烈依赖性。得出的结论是，分割方案可能对密度测定参数有很大影响，为获得可比结果，标准化是必不可少的。理想情况下，应使用等于肺和软组织密度平均值的阈值，但在有限但常见的密度范围（-910至-790 HU）内，-400 HU也可以。对于密度测定，建议进行两次腐蚀；对于容积测定，应使用零次腐蚀。
