Verschakelen J A, Van fraeyenhoven L, Laureys G, Demedts M, Baert A L
Department of Radiology, University Hospitals, K. U. Leuven, Belgium.
AJR Am J Roentgenol. 1993 Oct;161(4):713-7. doi: 10.2214/ajr.161.4.8372744.
Lung tissue, blood, and air determine the physical density of the lung and hence the attenuation measured on CT scans. These components are not homogeneously distributed throughout the lungs, and their relative proportion changes continuously during respiration. The objective of this study was to measure densities in various areas of the normal lung with CT and to examine the influences of gravity and of the degree of lung inflation on these densities.
The subjects in the study were six healthy male volunteers. CT scans were obtained at the level of the aortic arch and 2 cm above the expiratory level of the diaphragm with the subjects supine and then with them prone. Scans were obtained at lung volumes of 10%, 50%, and 90% vital capacity by using a respiratory gating device connected to the CT scanner. The overall density of the lungs at these two levels and the densities in the dependent (posterior in supine, and anterior in prone body position) and nondependent (anterior in supine and posterior in prone body position) areas were measured by using a semiautomatic evaluation algorithm.
Changes in lung volume caused the same changes in lung density in the right and left lungs and in the upper and lower parts of the lungs. For both body positions (supine and prone), the difference between lung density at 10% vital capacity and that at 90% vital capacity was significantly larger (p < .05) in the dependent parts of the lung than in the nondependent parts. In both positions, differences in density between dependent and nondependent regions were significantly (p < .05) greater at 10% vital capacity than at 90% vital capacity, and the differences became small when the volume of the lung was near total lung capacity.
Changes in lung volume have different effects on changes in lung density in dependent and nondependent parts of the lung. The largest changes in lung density occur in the dependent regions. The difference in density between dependent and nondependent lung regions is smallest for lung volumes near total lung capacity. These findings may be useful as a baseline for interpreting CT measurements of regional lung density in suspected cases of lung disease, which would alter the physical density of lung tissue. Our results also suggest that measurements near total lung capacity should be included.
肺组织、血液和空气决定了肺的物理密度，进而决定了CT扫描所测量的衰减值。这些成分在整个肺内并非均匀分布，且它们的相对比例在呼吸过程中持续变化。本研究的目的是用CT测量正常肺各个区域的密度，并研究重力和肺膨胀程度对这些密度的影响。
本研究的受试者为6名健康男性志愿者。在主动脉弓水平以及膈肌呼气末水平上方2 cm处，分别在受试者仰卧位和俯卧位时进行CT扫描。通过连接到CT扫描仪的呼吸门控装置，在肺活量的10%、50%和90%时进行扫描。使用半自动评估算法测量这两个层面肺的总体密度以及下垂部位（仰卧位时为后部，俯卧位时为前部）和非下垂部位（仰卧位时为前部，俯卧位时为后部）的密度。
肺容积的变化在左右肺以及肺的上下部分引起相同的肺密度变化。对于两种体位（仰卧位和俯卧位），在肺的下垂部位，肺活量10%时的肺密度与90%时的肺密度之差显著大于（p < 0.05）非下垂部位。在两种体位下，下垂区域与非下垂区域之间的密度差异在肺活量10%时显著大于（p < 0.05）90%时，当肺容积接近肺总量时，差异变小。
肺容积的变化对肺下垂部位和非下垂部位的肺密度变化有不同影响。肺密度的最大变化发生在下坠区域。对于接近肺总量的肺容积，下垂和非下垂肺区域之间的密度差异最小。这些发现可能有助于作为解释疑似肺部疾病病例中区域肺密度CT测量的基线，因为肺部疾病会改变肺组织的物理密度。我们的结果还表明应包括接近肺总量时的测量值。
