title: Theory of MRI contrast in the annulus fibrosus of the intervertebral disc.
abstract: OBJECTIVE
  Here we develop a three-dimensional analytic model for MR image contrast of collagen lamellae in the annulus fibrosus of the intervertebral disc of the spine, based on the dependence of the MRI signal on collagen fiber orientation.
  
  
  METHODS
  High-resolution MRI scans were performed at 1.5 and 7 T on intact whole disc specimens from ovine, bovine, and human spines. An analytic model that approximates the three-dimensional curvature of the disc lamellae was developed to explain inter-lamellar contrast and intensity variations in the annulus. The model is based on the known anisotropic dipolar relaxation of water in tissues with ordered collagen.
  
  
  RESULTS
  Simulated MRI data were generated that reproduced many features of the actual MRI data. The calculated inter-lamellar image contrast demonstrated a strong dependence on the collagen fiber angle and on the circumferential location within the annulus.
  
  
  CONCLUSIONS
  This analytic model may be useful for interpreting MR images of the disc and for predicting experimental conditions that will optimize MR image contrast in the annulus fibrosus.
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA, USA. alexander.wright@uphs.upenn.edu.
authors.forename: Alexander C
authors.initials: AC
authors.lastname: Wright
authors.affiliation: Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA, USA.
authors.forename: Jonathon H
authors.initials: JH
authors.lastname: Yoder
authors.affiliation: Department of Orthopaedics and Rehabilitation, Pennsylvania State University, Hershey, PA, USA.
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Vresilovic
authors.affiliation: Department of Biomedical Engineering, University of Delaware, Newark, DE, USA.
authors.forename: Dawn M
authors.initials: DM
authors.lastname: Elliott
mesh_terms: D000818:Animals; D016880:Anisotropy; D000070616:Annulus Fibrosus / Q000000981:diagnostic imaging*; D002417:Cattle; D003094:Collagen / Q000737:chemistry; D003198:Computer Simulation; D003287:Contrast Media / Q000737:chemistry*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional / Q000379:methods; D007403:Intervertebral Disc / Q000000981:diagnostic imaging*; D008279:Magnetic Resonance Imaging; D012756:Sheep
chemical_list: D003287:Contrast Media; D003094:Collagen
keywords: Annulus fibrosus; Collagen; Dipolar relaxation; Image contrast; Intervertebral disc