title: Dependent lung opacity at thin-section CT: evaluation by spirometrically-gated CT of the influence of lung volume.
abstract: OBJECTIVE
  To evaluate the influence of lung volume on dependent lung opacity seen at thin-section CT.
  
  
  METHODS
  In thirteen healthy volunteers, thin-section CT scans were performed at three levels (upper, mid, and lower portion of the lung) and at different lung volumes (10, 30, 50, and 100% vital capacity), using spirometric gated CT. Using a three-point scale, two radiologists determined whether dependent opacity was present, and estimated its degree. Regional lung attenuation at a level 2 cm above the diaphragm was determined using semiautomatic segmentation, and the diameter of a branch of the right lower posterior basal segmental artery was measured at each different vital capacity.
  
  
  RESULTS
  At all three anatomic levels, dependent opacity occurred significantly more often at lower vital capacities (10, 30%) than at 100% vital capacity (p = 0.001). Visually estimated dependent opacity was significantly related to regional lung attenuation (p < 0.0001), which in dependent areas progressively increased as vital capacity decreased (p < 0.0001). The presence of dependent opacity and regional lung attenuation of a dependent area correlated significantly with increased diameter of a segmental arterial branch (r = 0.493 and p = 0.0002; r = 0.486 and p = 0.0003, respectively).
  
  
  CONCLUSIONS
  Visual estimation and CT measurements of dependent opacity obtained by semiautomatic segmentation are significantly influenced by lung volume and are related to vascular diameter.
authors.affiliation: Department of Diagnostic Radiology, Dong-A University College of Medicine, 3-1 Ga, Dong daesin-dong, Seo-gu, Pusan 602-103, Korea. kinamlee@chollian.net
authors.forename: Ki Nam
authors.initials: KN
authors.lastname: Lee
authors.forename: Seong Kuk
authors.initials: SK
authors.lastname: Yoon
authors.forename: Choon Hee
authors.initials: CH
authors.lastname: Sohn
authors.forename: Pil Jo
authors.initials: PJ
authors.lastname: Choi
authors.forename: W Richard
authors.initials: WR
authors.lastname: Webb
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging*; D008176:Lung Volume Measurements; D008297:Male; D013147:Spirometry; D014057:Tomography, X-Ray Computed; D014797:Vital Capacity