title: Retrospective 3D registration of trabecular bone MR images for longitudinal studies.
abstract: OBJECTIVE
  To evaluate an automatic 3D registration algorithm for serial high-resolution images of trabecular bone (TB) in studies designed to evaluate the response of the trabecular architecture to intervention or disease progression.
  
  
  METHODS
  An efficient algorithm for registering high-resolution 3D images of TB is presented. The procedure identifies the six parameters of rigid displacement between two scans performed at different timepoints. By assuming a relatively small through-plane rotation, considerable time is saved by combining the results of a collection of regional 2D registrations throughout the TB region of interest (ROI). The algorithm was applied to 26 pairs of MR images acquired 6 months apart. Reproducibility of local TB structural parameters (plate, rod, and junction density) computed in manually selected regions were compared between baseline and registered follow-up images.
  
  
  RESULTS
  All 26 registrations were completed successfully in less than 30 seconds per image pair. The resampled follow-up images agreed with baseline to around one pixel throughout the volume at 137 x 137 x 410 microm(3) image resolution. Structural parameters in each region correlated well from baseline to follow-up with intraclass correlation coefficients ranging between 85%-97% for TB plate density. Interregional variations in the parameters were large as compared with intraregion reproducibility.
  
  
  CONCLUSIONS
  The proposed algorithm was successful in automatically registering baseline and follow-up TB images in a translational study, and may be useful in regional analyses in longitudinal MR studies of TB architecture.
authors.affiliation: Department of Radiology, University of Pennsylvania Health System, Philadelphia, Pennsylvania, USA. Jeremy.Magland@gmail.com
authors.forename: Jeremy F
authors.initials: JF
authors.lastname: Magland
authors.forename: Catherine E
authors.initials: CE
authors.lastname: Jones
authors.forename: Mary B
authors.initials: MB
authors.lastname: Leonard
authors.forename: Felix W
authors.initials: FW
authors.lastname: Wehrli
mesh_terms: D000465:Algorithms; D001185:Artificial Intelligence; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008137:Longitudinal Studies; D008279:Magnetic Resonance Imaging / Q000379:methods*; D010363:Pattern Recognition, Automated / Q000379:methods*; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D013382:Subtraction Technique; D013977:Tibia / Q000033:anatomy & histology*