Rationale and Objectives
To evaluate the relationship between measurements of lung volume (LV) on inspiratory/expiratory computed tomography (CT) scans, pulmonary function tests (PFT), and CT measurements of emphysema in individuals with chronic obstructive pulmonary disease.
Materials and Methods
Forty-six smokers (20 females and 26 males; age range 46–81 years), enrolled in the Lung Tissue Research Consortium, underwent PFT and chest CT at full inspiration and expiration. Inspiratory and expiratory LV values were automatically measured by open-source software, and the expiratory/inspiratory (E/I) ratio of LV was calculated. Mean lung density (MLD) and low attenuation area percent (<−950 HU) were also measured. Correlations of LV measurements with lung function and other CT indices were evaluated by the Spearman rank correlation test.
Results
LV E/I ratio significantly correlated with the following: the percentage of predicted value of forced expiratory volume in the first second (FEV 1 ), the ratio of FEV 1 to forced vital capacity (FVC), and the ratio of residual volume (RV) to total lung capacity (TLC) (FEV 1 %P, R = −0.56, P < .0001; FEV 1 /FVC, r = −0.59, P < .0001; RV/TLC, r = 0.57, P < .0001, respectively). A higher correlation coefficient was observed between expiratory LV and expiratory MLD ( r = −0.73, P < .0001) than between inspiratory LV and inspiratory MLD ( r = −0.46, P < .01). LV E/I ratio showed a very strong correlation to MLD E/I ratio ( r = 0.95, P < .0001).
Conclusions
LV E/I ratio can be considered to be equivalent to MLD E/I ratio and to reflect airflow limitation and air-trapping. Higher collapsibility of lung volume, observed by inspiratory/expiratory CT, indicates less severe conditions in chronic obstructive pulmonary disease.
With the development of imaging analysis using computed tomography (CT) in individuals with chronic obstructive pulmonary disease (COPD), several quantitative CT indices have been advocated and proved to be significant for predicting lung function. The more commonly used indices are the percentage of low attenuation area (LAA%) and mean lung density (MLD). Further, these indices on expiratory CT scans have been often reported to be stronger predictors for lung function than those on inspiratory scans . Regarding MLD, it is also known that the expiratory/inspiratory (E/I) ratio of MLD demonstrates significant correlations with pulmonary function tests (PFTs) .
In contrast, lung volume (LV) measured by CT has not been as rigorously assessed in subjects with COPD. Although inspiratory/expiratory LV and plethysmographic measures of total lung capacity (TLC) and residual volume (RV) , the relationship between CT-based LV measurements, including LV E/I ratio, and airflow limitation or air-trapping is still undefined.
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Materials and methods
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Subjects
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Table 1
Clinical Characteristics of 46 LTRC Subjects
Mean ± SD Range Age (y) 67.7 ± 7.9 46–81 Smoking index (pack-years) 50.8 ± 38.4 5–180 FEV 1 (%predicted) 57.9 ± 24.6 15–114 FEV 1 /FVC 0.55 ± 0.14 0.25–0.81 RV/TLC 0.50 ± 0.12 0.32–0.73 DL co (%predicted) 61.6 ± 22.2 22–103
DLco, diffusing capacity for carbon monoxide; FEV 1 , forced expiratory volume in the first second; FVC, forced vital capacity; LTRC, Lung Tissue Research Consortium; TLC, total lung capacity; RV, ratio of residual volume.
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Pulmonary Function Tests
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Thin-section CT
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Measurements of Lung Volume and other CT Indices
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![Figure 1, A 69-year-old man with chronic obstructive pulmonary disease (Global Initiative for Chronic Obstructive Lung Disease [GOLD] stage 2) Reconstructed coronal computed tomography (CT) images, which are made by the software, at full inspiration (a) and full expiration (b) are shown. Lung volume (LV) expiratory/inspiratory (E/I) ratio is 0.70, and mean lung density (MLD) E/I ratio is 0.95. Note that LV is calculated based on all axial images, not on the coronal images.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/CollapsibilityofLungVolumebyPairedInspiratoryandExpiratoryCTScans/0_1s20S1076633209006291.jpg)
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Statistical Analysis
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Results
CT Measurements and Correlations with Lung Function
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Table 2
CT Measurements and Correlations with Lung Function
CT Measurements Correlations with Pulmonary Function Tests Mean ± SD Range FEV 1 %P FEV 1 /FVC RV/TLC DLco%P Insp-LAA% (%) 14.1 ± 11.9 1.2 to 49.7 −0.625 ∗ −0.713 ∗ 0.532 † −0.606 ∗ Exp-LAA% (%) 9.3 ± 11.2 0.6 to 46.8 −0.637 ∗ −0.729 ∗ 0.574 ∗ −0.633 ∗ Insp-MLD (HU) −848.8 ± 35.7 −906.3 to −755.3 0.494 † 0.562 ∗ −0.397 ‡ 0.284 Exp-MLD (HU) −796.1 ± 60.7 −893.4 to −660.5 0.661 ∗ 0.743 ∗ −0.607 ∗ 0.411 ‡ MLD E/I ratio 0.94 ± 0.05 0.78 to 0.99 −0.583 ∗ −0.648 ∗ 0.537 † 0.366 § Insp-LV (L) 5.12 ± 1.27 3.08 to 9.09 −0.010 −0.198 −0.168 0.149 Exp-LV (L) 3.74 ± 1.07 1.76 to 6.31 −0.406 ‡ −0.588 ∗ 0.252 −0.160 LV E/I ratio 0.73 ± 0.14 0.39 to 0.95 −0.563 ∗ −0.594 ∗ 0.571 ∗ −0.358 §
FEV 1 , forced expiratory volume in the first second; FVC, forced vital capacity; RV, ratio of residual volume; TLC, total lung capacity; insp, inspiratory; exp, expiratory; LAA%, low attenuation area percent (<−950 HU); MLD, mean lung density; E/I, expiratory/inspiratory; LV, lung volume.
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Correlations of LV Measurements with MLD and LAA%
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Table 3
Correlations between CT Lung Volume and Other Indices
Insp-LAA% Exp-LAA% Insp-MLD Exp-MLD MLD E/I ratio Insp-LV 0.175 0.096 −0.463 ‡ −0.233 0.030 Exp-LV 0.449 ‡ 0.482 † −0.565 ∗ −0.725 ∗ 0.628 ∗ LV E/I ratio 0.411 ‡ 0.554 ∗ −0.256 −0.767 ∗ 0.952 ∗
insp, inspiratory; exp, expiratory; LAA%, low attenuation area percent (<−950 HU); MLD, mean lung density; E/I, expiratory/inspiratory; LV, lung volume.
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Table 4
Multivariate Analysis Using LV and LAA% to Predict MLD
LV LAA% R 2 Std B_P_ Std B_P_ Inspiratory phase 0.721 −0.336 <.001 −0.723 <.0001 Expiratory phase 0.760 −0.458 <.0001 −0.553 <.0001
MLD, mean lung density; LV, lung volume; LAA%, low attenuation area percent (<−950 HU); Std B, standardized coefficient B.
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Discussion
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Acknowledgment
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