title: Paired inspiratory-expiratory chest CT scans to assess for small airways disease in COPD.
abstract: BACKGROUND
  Gas trapping quantified on chest CT scans has been proposed as a surrogate for small airway disease in COPD. We sought to determine if measurements using paired inspiratory and expiratory CT scans may be better able to separate gas trapping due to emphysema from gas trapping due to small airway disease.
  
  
  METHODS
  Smokers with and without COPD from the COPDGene Study underwent inspiratory and expiratory chest CT scans. Emphysema was quantified by the percent of lung with attenuation < -950HU on inspiratory CT. Four gas trapping measures were defined: (1) Exp(-856), the percent of lung < -856HU on expiratory imaging; (2) E/I MLA, the ratio of expiratory to inspiratory mean lung attenuation; (3) RVC(856-950), the difference between expiratory and inspiratory lung volumes with attenuation between -856 and -950 HU; and (4) Residuals from the regression of Exp(-856) on percent emphysema.
  
  
  RESULTS
  In 8517 subjects with complete data, Exp(-856) was highly correlated with emphysema. The measures based on paired inspiratory and expiratory CT scans were less strongly correlated with emphysema. Exp(-856), E/I MLA and RVC(856-950) were predictive of spirometry, exercise capacity and quality of life in all subjects and in subjects without emphysema. In subjects with severe emphysema, E/I MLA and RVC(856-950) showed the highest correlations with clinical variables.
  
  
  CONCLUSIONS
  Quantitative measures based on paired inspiratory and expiratory chest CT scans can be used as markers of small airway disease in smokers with and without COPD, but this will require that future studies acquire both inspiratory and expiratory CT scans.
authors.affiliation: Channing Division of Network Medicine, Brigham and Women's Hospital and Harvard Medical School, 181 Longwood Avenue, Boston, MA 02115, USA. craig.hersh@channing.harvard.edu
authors.forename: Craig P
authors.initials: CP
authors.lastname: Hersh
authors.forename: George R
authors.initials: GR
authors.lastname: Washko
authors.forename: Raúl San José
authors.initials: RS
authors.lastname: Estépar
authors.forename: Sharon
authors.initials: S
authors.lastname: Lutz
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Friedman
authors.forename: MeiLan K
authors.initials: MK
authors.lastname: Han
authors.forename: John E
authors.initials: JE
authors.lastname: Hokanson
authors.forename: Philip F
authors.initials: PF
authors.lastname: Judy
authors.forename: David A
authors.initials: DA
authors.lastname: Lynch
authors.forename: Barry J
authors.initials: BJ
authors.lastname: Make
authors.forename: Nathaniel
authors.initials: N
authors.lastname: Marchetti
authors.forename: John D
authors.initials: JD
authors.lastname: Newell
authors.forename: Frank C
authors.initials: FC
authors.lastname: Sciurba
authors.forename: James D
authors.initials: JD
authors.lastname: Crapo
authors.forename: Edwin K
authors.initials: EK
authors.lastname: Silverman
mesh_terms: D015897:Comorbidity; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D011650:Pulmonary Alveoli / Q000000981:diagnostic imaging / Q000503:physiopathology; D029424:Pulmonary Disease, Chronic Obstructive / Q000000981:diagnostic imaging* / Q000453:epidemiology* / Q000503:physiopathology; D011656:Pulmonary Emphysema / Q000000981:diagnostic imaging* / Q000453:epidemiology* / Q000503:physiopathology; D011856:Radiographic Image Enhancement / Q000379:methods; D013902:Radiography, Thoracic; D015203:Reproducibility of Results; D015656:Respiratory Mechanics; D012680:Sensitivity and Specificity; D012907:Smoking / Q000453:epidemiology*; D014057:Tomography, X-Ray Computed / Q000379:methods / Q000706:statistics & numerical data*; D014481:United States / Q000453:epidemiology