Kemerink G J, Kruize H H, Lamers R J, van Engelshoven J M
Department of Radiology, University Hospital Maastricht, The Netherlands.
Med Phys. 1996 Oct;23(10):1697-708. doi: 10.1118/1.597757.
This study was performed to assess density resolution in quantitative computed tomography (CT) of foam and lung. Density resolution, a measure for the ability to discriminate materials of different density in a CT number histogram, is normally determined by quantum noise. In a cellular solid, variations in mass in the volumes sampled by CT cause an additional degradation of density resolution by the linear partial volume effect. The sample volume, which is directly related to spatial resolution, can be varied by choosing different section thicknesses and reconstruction filters. Several polyethene (PE) foams, as simple models of lung tissue, and five patients were investigated using various sample volumes. For the uniform PE foams, density resolution could be directly determined as the full width at half maximum of CT number histograms. Density resolution for foams with cell sizes of 0.8-1.5 mm was dominated by effects caused by the limited sample size, not by quantum noise. The relative magnitudes of density resolution could roughly be explained with a model for a hypothetic random cellular solid. Since lungs are not of uniform density, analysis of patient data was more complicated. A combined convolution least-squares fit procedure, together with information obtained in the studies of foam, were used to determine density resolution in lung studies. Density resolution, both for foams and lung, was strongly dependent on sample volume, and was quite poor for thin sections and sharp filters. Consequently, histogram-shape related parameters are sensitive to the spatial resolution chosen on CT. Thin section densitometry, using a 1-mm section with a standard or high resolution filter, is not recommended except in determining average density. When using thicker sections, an in-plane spatial resolution similar to section thickness is advised.
本研究旨在评估泡沫和肺部定量计算机断层扫描（CT）中的密度分辨率。密度分辨率是衡量CT数值直方图中区分不同密度物质能力的指标，通常由量子噪声决定。在多孔固体中，CT采样体积内质量的变化会因线性部分容积效应导致密度分辨率进一步下降。与空间分辨率直接相关的采样体积可通过选择不同的层厚和重建滤波器来改变。我们使用不同的采样体积对几种作为肺组织简单模型的聚乙烯（PE）泡沫以及五名患者进行了研究。对于均匀的PE泡沫，密度分辨率可直接确定为CT数值直方图半高宽。对于细胞尺寸为0.8 - 1.5毫米的泡沫，密度分辨率主要受限于采样尺寸，而非量子噪声。密度分辨率的相对大小可用假设的随机多孔固体模型大致解释。由于肺部密度不均一，患者数据的分析更为复杂。我们采用卷积最小二乘法拟合程序，并结合泡沫研究中获得的信息来确定肺部研究中的密度分辨率。泡沫和肺部的密度分辨率都强烈依赖于采样体积，对于薄层和锐化滤波器而言相当差。因此，与直方图形状相关的参数对CT上选择的空间分辨率很敏感。除了确定平均密度外，不建议使用1毫米层厚并搭配标准或高分辨率滤波器的薄层密度测定法。使用较厚层厚时，建议平面内空间分辨率与层厚相似。
