title: Detection of emphysema progression in alpha 1-antitrypsin deficiency using CT densitometry; methodological advances.
abstract: BACKGROUND
  Computer tomography (CT) densitometry is a potential tool for detecting the progression of emphysema but the optimum methodology is uncertain. The level of inspiration affects reproducibility but the ability to adjust for this variable is facilitated by whole lung scanning methods. However, emphysema is frequently localised to sub-regions of the lung and targeted densitometric sampling may be more informative than whole lung assessment.
  
  
  METHODS
  Emphysema progression over a 2-year interval was assessed in 71 patients (alpha 1-antitrypsin deficiency with PiZ phenotype) with CT densitometry, using the 15th percentile point (Perc15) and voxel index (VI) -950 Hounsfield Units (HU) and -910 HU (VI -950 and -910) on whole lung, limited single slices, and apical, central and basal thirds. The relationship between whole lung densitometric progression (DeltaCT) and change in CT-derived lung volume (DeltaCTVol) was characterised, and adjustment for lung volume using statistical modelling was evaluated.
  
  
  RESULTS
  CT densitometric progression was statistically significant for all methods. DeltaCT correlated with DeltaCTVol and linear regression indicated that nearly one half of lung density loss was secondary to apparent hyperinflation. The most accurate measure was obtained using a random coefficient model to adjust for lung volume and the greatest progression was detected by targeted sampling of the middle third of the lung.
  
  
  CONCLUSIONS
  Progressive hyperinflation may contribute significantly to loss of lung density, but volume effects and absolute tissue loss can be identified by statistical modelling. Targeted sampling of the middle lung region using Perc15 appears to be the most robust measure of emphysema progression.
authors.affiliation: Department of Respiratory Medicine, University Hospitals of Coventry and Warwickshire, Clifford Bridge Road, Coventry, CV2 2DX, UK. david.parr@uhcw.nhs.uk
authors.forename: David G
authors.initials: DG
authors.lastname: Parr
authors.forename: Martin
authors.initials: M
authors.lastname: Sevenoaks
authors.forename: Chunqin
authors.initials: C
authors.lastname: Deng
authors.forename: Berend C
authors.initials: BC
authors.lastname: Stoel
authors.forename: Robert A
authors.initials: RA
authors.lastname: Stockley
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods*; D018450:Disease Progression; D004646:Emphysema / Q000150:complications / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D019896:alpha 1-Antitrypsin Deficiency / Q000150:complications / Q000000981:diagnostic imaging* / Q000503:physiopathology*