Bullous Parametric Response Map for Functional Localization of COPD

Abstract

Advanced bronchoscopic lung volume reduction treatment (BLVR) is now a routine care option for treating patients with severe emphysema. Patterns of low attenuation clusters indicating emphysema and functional small airway disease (fSAD) on paired CT, which may provide additional insights to the target selection of the segmental or subsegmental lobe of the treatments, require further investigation. The low attenuation clusters (LACS) were segmented to identify the scalar and spatial distribution of the lung destructions, in terms of 10 fractions scales of low attenuation density (LAD) located in upper lobes and lower lobes. The LACs of functional small airway disease (fSAD) were delineated by applying the technique of parametric response map (PRM) on the co-registered CT image data. Both emphysematous LACs of inspiratory CT and fSAD LACs on expiratory CT were used to derive the coefficients of the predictive model for estimating the airflow limitation. The voxel-wise severity is then predicted using the regional LACs on the co-registered CT to indicate the functional localization, namely, the bullous parametric response map (BPRM). A total of 100 subjects, 88 patients with mild to very severe COPD and 12 control participants with normal lung functions (FEV₁/FVC % > 70%), were evaluated. Pearson's correlations between FEV₁/FVC% and LAV%_HU-950 of severe emphysema are - 0.55 comparing to - 0.67 and - 0.62 of LAV%_HU-856 of air-trapping and LAV%_fSAD respectively. Pearson's correlation between FEV₁/FVC% and FEV₁/FVC% predicted by the proposed model using LAD% of HU-950 and fSAD on BPRM is 0.82 (p < 0.01). The result of the Bullous Parametric Response Map (BPRM) is capable of identifying the less functional area of the lung, where the BLVR treatment is aimed at removing from a hyperinflated area of emphysematous regions.

Keywords: BLVR; Functional localization; PFT; PRM; Predictive model.

Fig. 1

Distribution of LAV% at a different stage of GOLD (n = 100); low is the measurements of lower lobes and up is the measurements of upper lobes

Fig. 2

Pearson’s correlation between whole lung LAV% and lung function at different GOLD stage. The results of the plot A and B are based on the measurement given in Table 3. Plot A compares the average LAV% with FEV1% pred, whereas plot B compares with FEV1/FVC%

Fig. 3

Cluster analysis of low attenuation densities (LADs) on inspiratory CT and expiratory CT. Comparison of 10 scales of LAD% clustered by univariate K-means clustering (inspiratory CT on A, expiratory CT on B), and comparison of FEV₁/FVC% distributions of LADs (inspiratory CT on C and expiratory CT on D)

Fig. 4

First row: left is inspiratory CT, the center is the original expiratory CT, right is the co-registered result of expiratory CT. Second row: left is the vascular tree rendering of inspiratory CT, and center is the expiratory CT. The visual inspection shows extensive pruning in lower left lobes and relatively mild pruning in lower right lobes. Third row: Red color marks the emphysema regions (HU-950), green color marks the normal regions, yellow marks the air-trapping of expiratory CT, distinguished from emphysema of inspiratory CT using technique of parametric response map (PRM). The fourth row is the result of second-row using low attenuation clusters. Fifth row: left is the visualization result of PRM in a three-dimensional model, the center is the mapping of the PRM on HU values of inspiratory CT, right is the result of regional functional prediction mark on the grids, which has blue color as the most severe candidates for the target lobes

Fig. 5

Scatter plot of LAV%_AirT using HU-856 threshold on expiratory CT and LAV%_{Emph950 + fSAD} using classification results of PRM. The result has shown a linear relationship in LAV%