Department of Medical Informatics, University Medical Center Hamburg-Eppendorf 20246 Hamburg, Germany.
Med Phys. 2010 Feb;37(2):615-9. doi: 10.1118/1.3284282.
Spirometry exhibits baseline drift and frequent measurement errors so it cannot be used by itself to provide tidal volume-based image sorting or breathing motion modeling. Other breathing surrogates, in this study an abdominal bellows system, are drift free but do not measure tidal volume. Simultaneously using spirometry and the bellows system allows the user to convert the recorded bellows signal to tidal volume but still relies on spirometry measurements. The authors therefore propose to use CT-based air content, rather than a spirometer, to convert the bellows signal to tidal volume.
41 4D CT data sets are acquired, while the breathing cycle is simultaneously measured using spirometry and an abdominal pressure bellows system. The assumptions underlying the conversion of the bellows measurement to tidal volume by CT-based air content are analyzed. This comprises of detailed correlation studies of the spirometry-measured tidal volume, the bellows signal, and CT-based air content.
For 15/41 patients, the spirometry signals are not consistently acquired during the 4D CT session, so correlating spirometry to bellows measurements and CT-based air content leads to erroneous conversion coefficients. After introducing a minimum correlation threshold to remove these data, good correlations are obtained between the remaining breathing signals. The ratio of CT-based air content to tidal volume is measured to be 1.11 +/- 0.08; the expected value is 1.11 because room air is 11% more dense than air in the lungs.
The observed problems of spirometry recording illustrate the challenges encountered when using spirometers as breathing surrogate for 4D CT acquisition. The high correlation between spirometry and bellows breathing signals and the verified factor of 1.11 between CT-based air content and tidal volume mean that the bellows measurement (or other equivalent surrogates) can be reliably converted to tidal volume using the CT-based air content, avoiding the need for a spirometer.
由于肺量计存在基线漂移和频繁的测量误差，因此不能单独使用它来提供基于潮气量的图像分类或呼吸运动建模。在本研究中，其他呼吸替代物，如腹部风箱系统，没有漂移，但不测量潮气量。同时使用肺量计和风箱系统可以使用户将记录的风箱信号转换为潮气量，但仍然依赖于肺量计的测量。因此，作者建议使用基于 CT 的空气含量，而不是肺量计，将风箱信号转换为潮气量。
采集了 41 组 4D CT 数据集，同时使用肺量计和腹部压力风箱系统同步测量呼吸周期。分析了基于 CT 空气含量将风箱测量值转换为潮气量的假设。这包括对肺量计测量的潮气量、风箱信号和基于 CT 的空气含量进行详细的相关性研究。
在 41 名患者中有 15 名/41 名患者的肺量计信号在 4D CT 扫描过程中没有持续采集，因此将肺量计与风箱测量值和基于 CT 的空气含量相关联会导致转换系数错误。在引入最小相关阈值去除这些数据后，剩余的呼吸信号之间获得了良好的相关性。基于 CT 的空气含量与潮气量的比值测量为 1.11 +/- 0.08；预期值为 1.11，因为室内空气比肺部空气密度高 11%。
肺量计记录中观察到的问题说明了在使用肺量计作为 4D CT 采集的呼吸替代物时遇到的挑战。肺量计和风箱呼吸信号之间的高度相关性以及基于 CT 的空气含量与潮气量之间的验证因子 1.11 意味着可以使用基于 CT 的空气含量可靠地将风箱测量值（或其他等效替代物）转换为潮气量，而无需使用肺量计。
