Quantitative computed tomography measurements to evaluate airway disease in chronic obstructive pulmonary disease: Relationship to physiological measurements, clinical index and visual assessment of airway disease

Abstract

Purpose: To correlate currently available quantitative CT measurements for airway disease with physiological indices and the body-mass index, airflow obstruction, dyspnea, and exercise capacity (BODE) index in patients with chronic obstructive pulmonary disease (COPD).

Materials and methods: This study was approved by our institutional review board (IRB number 2778). Written informed consent was obtained from all subjects. The subjects included 188 current and former cigarette smokers from the COPDGene cohort who underwent inspiratory and expiratory CT and also had physiological measurements for the evaluation of airflow limitation, including FEF25-75%, airway resistance (Raw), and specific airway conductance (sGaw). The BODE index was used as the index of clinical symptoms. Quantitative CT measures included % low attenuation areas [% voxels≤950 Hounsfield unit (HU) on inspiratory CT, %LAA_-950ins], percent gas trapping (% voxels≤-856HU on expiratory CT, %LAA _-856exp), relative inspiratory to expiratory volume change of voxels with attenuation values from -856 to -950HU [Relative Volume Change (RVC)_{-856 to -950}], expiratory to inspiratory ratio of mean lung density (E/I-ratio _MLD), Pi10, and airway wall thickness (WT), luminal diameter (LD) and airway wall area percent (WA%) in the segmental, subsegmental and subsubsegmental bronchi on inspiratory CT. Correlation coefficients were calculated between the QCT measurements and physiological measurements in all subjects and in the subjects with mild emphysema (%LAA_-950ins <10%). Univariate and multiple variable analysis for the BODE index were also performed. Adjustments were made for age, gender, smoking pack years, FEF25-75%, Raw, and sGaw.

Results: Quantitative CT measurements had significant correlations with physiological indices. Among them, E/I-ratio _MLD had the strongest correlations with FEF25-75% (r=-0.648, <0.001) and sGaw (r=-0.624, <0.001) while in the subjects with mild emphysema subsegmental WA% and segmental WA% had the strongest correlation with FEF25-75% (r=-0.669, <0.001) and sGaw (r=-0.638, <0.001), respectively. The multiple variable analyses showed that RVC_{-856 to -950} was an independent predictor of the BODE index showing the highest R² (0.468) as an independent variable among the QCT measurements.

Conclusion: Quantitative CT measurements of gas trapping such as E/I-ratio _MLD, correlate better with physiological indices for airway disease than those of airway such as WA% or LD. In mild emphysema, however, quantitative CT measurements of airway correlate better with the physiological indices. RVC_{-856 to -950} is a predictor of the BODE index.

Keywords: Air trapping; Airway disease; CT; Chronic obstructive lung disease (COPD); Quantitative CT.

Fig. 1. Quantification of emphysema and airway dimensions

(a) Quantitative CT of emphysema. All lung voxels ≤ −950 HU in CT value are color-coded by lobe. (b) Quantitative CT of airway dimensions. Volumetric acquisition permits segmentation of the airway tree, and curved reformatting. (c) Orthogonal cross-section of a subsegmental airway, derived from curved reformat, shows delineation of outer and inner bronchial wall, permitting calculation of airway dimensions.

Fig. 2. Representative cases of the 4–point grading system for the visual assessment of airway disease

(a) Grade 1 (absence of bronchial wall thickening). A 63-year-old man with a %LAA_−856exp of 44.67%. (b) Grade 2 (borderline bronchial wall thickening). A 63-year-old woman with a %LAA_−856exp of 46.56%. (c) Grade 3 (definitive bronchial wall thickening). A 74-year-old woman with a %LAA_−856exp of 61.71%. (d) Grade 4 (severe bronchial wall thickening). A 56-year-old man with a %LAA_−856exp of 65.65%.

Fig. 3. Representative scatter plots showing the relationships between the airway physiological indices and quantitative CT measurements

(a) FEF25–75% – %LAA-_856expin all subjects. (b) FEF25–75% – Mean airway wall area% in the subsegmental bronchi in all subjects. (c) FEF25–75% – %LAA-_856exp in the subjects with mild emphysema. (d) FEF25–75% – Mean airway wall area% in the subsegmental bronchi in the subjects with mild emphysema. Note that dots are more scattered in the areas of lower %LAA-_856exp on (a) and (c), which may partly be due to physiological gas trapping.