title: Optimal threshold of subtraction method for quantification of air-trapping on coregistered CT in COPD patients.
abstract: OBJECTIVE
  To investigate the optimal threshold of subtraction method for quantification of air trapping on co-registered CT in COPD patients in correlation with pulmonary function parameters.
  
  
  METHODS
  From June 2005 to October 2010, 174 patients were included in our study. Inspiration and expiration CT were performed followed by non-rigid registration using in-house software. The subtraction value per voxel between inspiration and registered expiration CT was obtained, and volume fraction of air trapping (air trapping index, ATI), using variable thresholds was calculated. ATI, expiration/inspiration ratio of mean lung density (E/I MLD) and the percentage of lung voxels below -856 HU on expiration CT (Exp-856) were correlated with FEF25-75% and RV/TLC.
  
  
  RESULTS
  The highest correlation coefficient with FEF25-75% was -0.656, using the threshold of 80 HU. As for RV/TLC, the highest correlation coefficient was 0.664, using the threshold of 30 HU. When plotting the relationship between subtraction thresholds and FEF25-75% and RV/TLC, the threshold of 60 HU was most suitable (r = -0.649 and 0.651). Those correlation coefficients were comparable to the results with E/I MLD (r = -0.670 and 0.657) and Exp-856 (r = -0.604 and 0.565).
  
  
  CONCLUSIONS
  The optimal threshold for quantification of air trapping was 60 HU and showed comparable correlations with pulmonary function parameters.
  
  
  CONCLUSIONS
  • The optimal CT threshold of subtraction method for air trapping was 60 HU. • ATI with 60 HU threshold was comparable to E/I MLD and Exp -856 . • Emphysema may substantially contribute to air trapping with statistical significance (P < 0.001).
authors.affiliation: Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, 88 Olympic-ro 43 Gil, Songpa-gu, Seoul, 138-736, Korea.
authors.forename: Sang Min
authors.initials: SM
authors.lastname: Lee
authors.affiliation: Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, 88 Olympic-ro 43 Gil, Songpa-gu, Seoul, 138-736, Korea. seojb@amc.seoul.kr.
authors.forename: Joon Beom
authors.initials: JB
authors.lastname: Seo
authors.affiliation: Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, 88 Olympic-ro 43 Gil, Songpa-gu, Seoul, 138-736, Korea.
authors.forename: Sang Min
authors.initials: SM
authors.lastname: Lee
authors.affiliation: Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, 88 Olympic-ro 43 Gil, Songpa-gu, Seoul, 138-736, Korea.
authors.forename: Namkug
authors.initials: N
authors.lastname: Kim
authors.affiliation: Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, 88 Olympic-ro 43 Gil, Songpa-gu, Seoul, 138-736, Korea.
authors.forename: Sang Young
authors.initials: SY
authors.lastname: Oh
authors.affiliation: Department of Pulmonary and Critical Care Medicine, and Clinical Research Center for Chronic Obstructive Airway Diseases, University of Ulsan College of Medicine, Asan Medical Center, Seoul, Korea.
authors.forename: Yeon-Mok
authors.initials: YM
authors.lastname: Oh
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D045853:Exhalation; D005260:Female; D005541:Forced Expiratory Volume; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008168:Lung / Q000000981:diagnostic imaging*; D008297:Male; D008450:Maximal Midexpiratory Flow Rate; D008875:Middle Aged; D011653:Pulmonary Diffusing Capacity; D029424:Pulmonary Disease, Chronic Obstructive / Q000000981:diagnostic imaging*; D011656:Pulmonary Emphysema / Q000000981:diagnostic imaging*; D011859:Radiography; D012115:Residual Volume; D012984:Software; D013382:Subtraction Technique; D014057:Tomography, X-Ray Computed / Q000379:methods; D014109:Total Lung Capacity; D014797:Vital Capacity
keywords: Airway obstruction; Chronic obstructive pulmonary disease; Computed tomography; Computer-assisted image processing; Pulmonary function test