COPD is a common heterogenous disease in which spirometric results are not sufficient to explain these heterogeneities. Han et al.  reported that CT could be a valuable modality to differentiate between patients of COPD especially those with the same spirometric results. Temizoz et al.  found that MDCT and measurement of CT pixel attenuation values were good methods for quantifying emphysema. Bakker et al.  also reported the importance of quantitative CT measurement in assessment of degree and distribution of emphysema.
Muller et al. were the first to describe and verify pathologically the density mask technique, in which CT pixels with attenuation below a certain threshold value [initially – 910 HU] were identified as emphysema . Madani et al.  revealed that the most appropriate HU to be used in the density mask method is − 960 or – 970 HU.
The threshold value – 950 HU showed the best correlation to demonstrate degree of emphysema . Gevenois et al. reported that the strongest pathologic correlation with emphysema at macroscopic and microscopic level had been established at a threshold of [− 950 HU] in a 1-mm non-contrast enhanced high-resolution CT images . According to the study done by Wang et al., the threshold of − 950 HU is optimal for CT densitometry analysis of emphysema when the CT examinations are obtained at full inspiration . In the current study, the density mask method was applied to the entire lung as well as to each individual lobe, and we used [− 950 HU] as % LAA for quantifying emphysema.
In the present study, the CT examination was done without the use of intravenous contrast material administration because non-enhanced volumetric CT study is a standard technique for COPD imaging as intravenous contrast material affects the attenuation values of the imaged organs . In our study, all examinations were acquired in full inspiration. No expiratory CT examination was performed; it is consistent with Gevenois et al. who showed that expiratory quantitative CT is not as precise as inspiratory CT in measuring lung emphysema. Moreover, the patients having difficulty breathing can tolerate inspiratory CT more than expiratory CT .
Pulmonary function tests (PFT) parameters although, commonly used in assessment and classification of COPD cases, could not detect the heterogeneous changes of the disease process or the early abnormalities . In the present study, two cases with normal PFT showed decreased mean lung density [MLD] which was correlated with their clinical signs.
In the present study, we found significant relationship between the pulmonary function tests (FEV1 and FEV1/FVC ratio), and all parameters of quantitative assessment with – 950 HU (%LAA) agreed with different studies [21, 24]. Highly significant negative correlation was detected between FEV1/FVC ratio, and all lung density parameters (p < 0.01 and r ≥ 0.4) also, between FEV1 and right, left, and both upper, left, and both lower lung density index (p < 0.01 and r ≥ 0.4). There was significant negative correlation between FEV1 and all lung density parameters (p < 0.05 and r < 0.4). While FVC was not significantly correlated with all lung density parameters except for that of right, left, and both upper lung density index being significant negative correlation (p < 0.05 and r < 0.4).
When studying the correlation between emphysema extent and pulmonary function, Saitoh et al. found a strong correlation when the upper lobes are predominantly affected, Matsuo et al. showed that a strong correlation between predominant lower lobes involvement and FVC, FEV1 [27, 28]. However, in the current study, we could not observe obvious difference in correlation coefficients between the upper lobes and the lower lobes. Nevertheless, we observed that the mean LAA% values of the right and left upper lungs were slightly higher than those of the right and left lower lung lobes.
The CT lung analysis adds extra information for diagnosis particularly in debatable circumstances . The use of quantitative MDCT analysis provides more accurate estimates of the COPD severity and disease distribution . An alternative method to quantification of emphysema, based on the frequency histogram of lung attenuation, estimates the CT attenuation at a given percentile along the histogram. The 15th percentile threshold is used to assess the variations of emphysema cases rather than changes in the lung volume [13, 30]. It was evident that the percentile method is more powerful for follow-up of emphysema .
In our study, we found that there was highly significant positive correlation between pulmonary function tests (FEV1, FEV1/FVC ratio) and right, left and both lungs PD15% (p < 0.01 and r ≥ 0.4) while FVC showed significant positive correlation with left lung PD15% only (p < 0.05 and r < 0.4).
The lung density analysis is superior to expiratory CT methods because it provides information about the whole lung. In addition, it appears that lung density analysis will allow for new horizons in providing information about lung parenchyma, lung density and density distribution, and additionally, the subthreshold values’ percentage in emphysematous patients. Yet, lung density analysis has its own limitations: one of them is that its effectiveness is based on co-operation of the patient. For example, incomplete inspiration results in minimal decrease in lung density and misinterpretation .