Table 1.
Summary of Quantitative Metrics for Parenchymal and Airway Disease Based on Computed Tomography Used in the COPDGene Study
| Quantitative Metric | Definition | Comments |
|---|---|---|
| Bronchial measures | ||
| Segmental or subsegmental bronchial wall thickness | Average thickness of a given airway segment, measured on curved multiplanar reformat | Correlates with increased risk of COPD exacerbation |
| WA% (34, 35) | Total area of airway − [(area of airway lumen)/total area of airway] × 100 | Correlates with physiologic airflow obstruction and with impaired respiratory quality of life |
| Pi10 (34) | The square root of the wall area of a theoretical airway with internal perimeter 10 mm, derived by plotting the internal perimeters of all measured airways against the square root of the wall area of airways | A useful summary measure of airway wall thickness, adjusted for airway size |
| Measures of emphysema | ||
| LAA-950 | Percentage of lung voxels with CT attenuation less than or equal to −950 HU, measured on inspiratory CT | Correlates with physiologic airflow obstruction and with impaired respiratory quality of life. Several genetic markers have been identified for these measures. May be disproportionately elevated in current cigarette smokers, presumably owing to smoking-related lung inflammation |
| Perc 15 | CT attenuation at the 15th percentile of the lung CT histogram | |
| Lung mass (27) | Lung mass (g) = [(HU + 1,024)/1,024] × voxel volume × number of voxels | — |
| Lung density at 15th percentile, corrected for lung volume | Perc 15 converted to lung density in g/L and corrected for baseline total lung volume | Can be used to evaluate progression of emphysema on sequential scans |
| Local histogram measures | Can be used to estimate the severity of different patterns of emphysema | — |
| Measures of airway disease | ||
| Excessive central airway collapse (46) | >50% decrease in cross-sectional area of the trachea between inspiration and end expiration | Associated with increased incidence of respiratory exacerbations |
| LAA-856exp | Percentage of lung voxels with CT attenuation less than or equal to −856 HU measured on expiratory CT scan | Measures of gas trapping; correlate well with physiologic airflow obstruction but do not discriminate between emphysematous and nonemphysematous gas trapping |
| E/I attenuation ratio (47) | Ratio of mean lung attenuation on expiratory and inspiratory scans | |
| E/I volume ratio (47) | Ratio of lung volume on expiratory and inspiratory scans | |
| PRM (50) | Coregistration of inspiratory and expiratory images on a voxel-by-voxel basis to examine the change in density between inspiratory and expiratory images | This method is used to separate gas trapping due to emphysema from gas trapping due to small airways obstruction |
| PRMfSAD (50) | Percentage of lung voxels with CT attenuation greater than −950 HU on inspiration but less than or equal to −856 HU on expiration on coregistered scans | Measure of nonemphysematous gas trapping, presumed due to small airway obstruction. This is an independent predictor of lung function decline |
| ND-E/I (49) | Ratio of mean lung density at end expiration to end inspiration in lung voxels with normal density (inspiratory attenuation greater than −910 HU and expiratory attenuation greater than −856 HU) | A potential measure of early small airway disease in lung that would have been classified as normal by traditional metrics |
| Jacobian determinant (55) | Derived from the deformation of lung volumes at end of respiratory phases to map local volume change between inspiration and expiration | Provides a measure of the level of lung expansion and contraction with respiration |
| MAL2 (57) | The percentage of normal voxels within 2-mm distance from emphysematous voxels | Mean Jacobian determinant of these voxels is associated with subsequent FEV1 decline |
| Vascular measures | ||
| PA/A ratio (67) | Ratio of the diameter of the pulmonary artery to the diameter of the aorta | An indirect measure of pulmonary hypertension. Correlates with increased frequency of exacerbations of COPD |
| BV5 (74) | The aggregate blood volume in the small pulmonary vessels <5 mm2, a measure of pulmonary vascular disease | Correlates with pulmonary vascular pressure |
| Extrapulmonary measures | ||
| Coronary artery calcification (79) | Agatston-based scoring system | Coronary calcification scoring on nongated scans correlates with gated scores |
| Osteoporosis (88) | Measurement of bone mineral density on thoracic CT scans | Bone density is substantially reduced in cigarette smokers |
| Chest wall composition (100) | Automated measurement of pectoralis muscle and subcutaneous fat | Pectoralis area is associated with several indices of COPD severity, including mortality |
Definition of abbreviations: COPD = chronic obstructive pulmonary disease; COPDGene = Genetic Epidemiology of Chronic Obstructive Pulmonary Disease; CT = computed tomography; E/I = expiratory/inspiratory; HU = Hounsfield units; LAA = low-attenuation area; MAL2 = mechanically affected lung within 2 mm of emphysematous voxels; ND-E/I = normal-density E/I; PA/A = pulmonary artery/aorta; Perc 15 = lung attenuation at 15th percentile; PRM = parametric response mapping; PRMfSAD = PRM measure of functional small airway disease; WA% = wall area percentage.