title: Determining collagen distribution in articular cartilage using contrast-enhanced micro-computed tomography.
abstract: OBJECTIVE
  Collagen distribution within articular cartilage (AC) is typically evaluated from histological sections, e.g., using collagen staining and light microscopy (LM). Unfortunately, all techniques based on histological sections are time-consuming, destructive, and without extraordinary effort, limited to two dimensions. This study investigates whether phosphotungstic acid (PTA) and phosphomolybdic acid (PMA), two collagen-specific markers and X-ray absorbers, could (1) produce contrast for AC X-ray imaging or (2) be used to detect collagen distribution within AC.
  
  
  METHODS
  We labeled equine AC samples with PTA or PMA and imaged them with micro-computed tomography (micro-CT) at pre-defined time points 0, 18, 36, 54, 72, 90, 180, 270 h during staining. The micro-CT image intensity was compared with collagen distributions obtained with a reference technique, i.e., Fourier-transform infrared imaging (FTIRI). The labeling time and contrast agent producing highest association (Pearson correlation, Bland-Altman analysis) between FTIRI collagen distribution and micro-CT -determined PTA distribution was selected for human AC.
  
  
  RESULTS
  Both, PTA and PMA labeling permitted visualization of AC features using micro-CT in non-calcified cartilage. After labeling the samples for 36 h in PTA, the spatial distribution of X-ray attenuation correlated highly with the collagen distribution determined by FTIRI in both equine (mean ± S.D. of the Pearson correlation coefficients, r = 0.96 ± 0.03, n = 12) and human AC (r = 0.82 ± 0.15, n = 4).
  
  
  CONCLUSIONS
  PTA-induced X-ray attenuation is a potential marker for non-destructive detection of AC collagen distributions in 3D. This approach opens new possibilities in development of non-destructive 3D histopathological techniques for characterization of OA.
authors.affiliation: Department of Physics, University of Helsinki, Helsinki, Finland; Research Center Group for Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Oulu, Finland. Electronic address: heikki.nieminen@helsinki.fi.
authors.forename: H J
authors.initials: HJ
authors.lastname: Nieminen
authors.affiliation: Department of Physics, University of Helsinki, Helsinki, Finland; Research Center Group for Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Oulu, Finland. Electronic address: tuomo.ylitalo@helsinki.fi.
authors.forename: T
authors.initials: T
authors.lastname: Ylitalo
authors.affiliation: Research Center Group for Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Oulu, Finland. Electronic address: sakari.karhula@oulu.fi.
authors.forename: S
authors.initials: S
authors.lastname: Karhula
authors.affiliation: Department of Physics, University of Helsinki, Helsinki, Finland. Electronic address: jussi-petteri.suuronen@helsinki.fi.
authors.forename: J-P
authors.initials: JP
authors.lastname: Suuronen
authors.affiliation: Research Center Group for Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Oulu, Finland; Medical Research Center Oulu, Oulu University Hospital and University of Oulu, Finland. Electronic address: sami.kauppinen@oulu.fi.
authors.forename: S
authors.initials: S
authors.lastname: Kauppinen
authors.affiliation: Department of Physics, University of Helsinki, Helsinki, Finland. Electronic address: ritva.serimaa@helsinki.fi.
authors.forename: R
authors.initials: R
authors.lastname: Serimaa
authors.affiliation: Department of Physics, University of Helsinki, Helsinki, Finland. Electronic address: edward.haeggstrom@helsinki.fi.
authors.forename: E
authors.initials: E
authors.lastname: Hæggström
authors.affiliation: Department of Laboratory Medicine and Pathobiology, University of Toronto and Mount Sinai Hospital, Toronto, Canada. Electronic address: kenpritzker@gmail.com.
authors.forename: K P H
authors.initials: KP
authors.lastname: Pritzker
authors.affiliation: Department of Surgery and Intensive Care, University of Oulu and Oulu University Hospital, Finland. Electronic address: maarit.valkealahti@ppshp.fi.
authors.forename: M
authors.initials: M
authors.lastname: Valkealahti
authors.affiliation: Medical Research Center Oulu, Oulu University Hospital and University of Oulu, Finland; Department of Anatomy and Cell Biology, University of Oulu, Finland; Department of Surgery and Intensive Care, University of Oulu and Oulu University Hospital, Finland. Electronic address: petri.lehenkari@oulu.fi.
authors.forename: P
authors.initials: P
authors.lastname: Lehenkari
authors.affiliation: Research Center Group for Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Oulu, Finland; Medical Research Center Oulu, Oulu University Hospital and University of Oulu, Finland. Electronic address: mikko.finnila@oulu.fi.
authors.forename: M
authors.initials: M
authors.lastname: Finnilä
authors.affiliation: Research Center Group for Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Oulu, Finland; Medical Research Center Oulu, Oulu University Hospital and University of Oulu, Finland; Department of Diagnostic Radiology, Oulu University Hospital, Oulu, Finland. Electronic address: simo.saarakkala@oulu.fi.
authors.forename: S
authors.initials: S
authors.lastname: Saarakkala
mesh_terms: D000368:Aged; D000818:Animals; D002358:Cartilage, Articular / Q000737:chemistry*; D003094:Collagen / Q000032:analysis*; D003287:Contrast Media; D006736:Horses; D006801:Humans; D008297:Male; D008875:Middle Aged; D008982:Molybdenum; D010003:Osteoarthritis / Q000378:metabolism; D010756:Phosphoric Acids; D010772:Phosphotungstic Acid; D014018:Tissue Distribution; D055114:X-Ray Microtomography / Q000379:methods*
chemical_list: D003287:Contrast Media; D010756:Phosphoric Acids; D010772:Phosphotungstic Acid; D008982:Molybdenum; D003094:Collagen; C003125:phosphomolybdic acid
keywords: Articular cartilage; Collagen; Imaging; Osteoarthritis; Tomography; X-ray