Purpose: To determine whether measurement of the relative area of lung with attenuation coefficients lower than a certain threshold on thin-section computed tomographic (CT) scans obtained during expiration is a valuable method of quantifying the extent of pulmonary emphysema.
Materials and methods: Eighty-nine patients underwent CT (with 1-mm collimation) preoperatively during inspiration and expiration. Relative areas of lung with attenuation coefficients lower than various thresholds were calculated. These relative areas were compared with areas found macroscopically to have emphysema (59 patients [51 men, eight women; aged 40-77 years]) and with two microscopic indices (35 patients [29 men, six women; aged 42-77 years]) assessed on the resected specimens.
Results: The valid expiratory CT thresholds were found to be -820 and 910 HU for microscopic and macroscopic emphysema, respectively. However, results of stepwise multiple regression analyses showed that the inspiratory threshold of -950 HU was superior for both macroscopically and microscopically quantified emphysema. The correlation coefficients in expiratory CT were higher for the pulmonary volumes but similar for the diffusing capacity.
Conclusion: Expiratory quantitative CT is not as accurate as inspiratory CT for quantifying pulmonary emphysema and probably reflects air trapping more than reduction in the alveolar wall surface.