A size-based emphysema severity index: robust to the breath-hold-level variations and correlated with clinical parameters

Abstract

Objective: To determine the power-law exponents (D) of emphysema hole-size distributions as a competent emphysema index. Robustness to extreme breath-hold-level variations and correlations with clinical parameters for chronic obstructive pulmonary disease (COPD) were investigated and compared to a conventional emphysema index (EI%).

Patients and methods: A total of 100 patients with COPD (97 males and three females of mean age 67±7.9 years) underwent multidetector row computed tomography scanning at full inspiration and full expiration. The diameters of the emphysematous holes were estimated and quantified with a fully automated algorithm. Power-law exponents (D) of emphysematous hole-size distribution were evaluated.

Results: The diameters followed a power-law distribution in all cases, suggesting the scale-free nature of emphysema. D of inspiratory and expiratory computed tomography of patients showed intraclass correlation coefficients >0.8, indicating statistically absolute agreement of different breath-hold levels. By contrast, the EI% failed to agree. Bland-Altman analysis also revealed the superior robustness of D to EI%. D also significantly correlated with clinical parameters such as airflow limitation, diffusion capacity, exercise capacity, and quality of life.

Conclusion: The D of emphysematous hole-size distribution is robust to breath-hold-level variations and sensitive to the severity of emphysema. This measurement may help rule out the confounding effects of variations in breath-hold levels.

Keywords: breath-hold; chronic obstructive pulmonary disease; computed tomography; emphysema; power law; quantitative imaging.

Figure 1

Emphysematous holes identified and measured in volumetric CT of a subject during inspiration and expiration. Notes: (A) Inspiratory CT image of a subject, showing low-attenuation masks (>−950 HU; EI_ins% 13.79). (B) Low-attenuation masks of expiratory CT of the same subject (EI_exp% 1.57). (C) Three-dimensional visualization of emphysematous holes in A with size-based color coding. (D) Three-dimensional visualization of emphysematous holes in B with size-based color coding. (E) Distributions of emphysematous hole sizes in inspiratory and expiratory CT scans of the same subject as in A–D on cumulative log–log scale. The size distribution followed a power law with exponents D_ins =4.70 (R²=0.97) and D_exp =4.90 (R²=0.96). Abbreviations: CT, computed tomography; HU, Hounsfield units; EI, emphysema index; ins, inspiration; exp, expiration; D, power-law exponents.

Figure 2

Effects of inspiration and expiration on (A) EI%s and (B) Ds. Abbreviations: EI, emphysema index; D, power-law exponent.

Figure 3

Bland–Altman plots of the robustness of D and EI% to breath-hold levels. Notes: (A) EI% measured during inspiratory and expiratory CT scans; the mean difference was 9.8%. (B) D measured during inspiratory and expiratory CT scans; the mean difference was 0.022. Abbreviations: EI, emphysema index; CT, computed tomography; SD, standard deviation; D, power-law exponents; ins, inspiration; exp, expiration.