2.3 Lung Density Index
For each side of the lungs, separated inspiratory and expiratory lung volumes and densities were measured. We obtained the MLD during inspiration and expiration, and the ratio of end expiratory to inspiratory volume (E/I volume) and MLD (E/I MLD) was calculated. To investigate the threshold of pathologic expiratory airflow limitation, we examined the relative volume percentage between -850 and - 950 HU with an interval of 50 HU. Expiratory low attenuation areas (LAA) below -850 HU to -950 HU threshold (E850, E900, and E950) and high attenuation areas (HAA) above -600 HU to -650 HU threshold (E600 and E650) were measured. Figure 1 shows an example of CT results obtained from a representative subject.