title: Technical Note: Cortical thickness and density estimation from clinical CT using a prior thickness-density relationship.
abstract: OBJECTIVE
  Cortical thickness and density are critical components in determining the strength of bony structures. Computed tomography (CT) is one possible modality for analyzing the cortex in 3D. In this paper, a model-based approach for measuring the cortical bone thickness and density from clinical CT images is proposed.
  
  
  METHODS
  Density variations across the cortex were modeled as a function of the cortical thickness and density, location of the cortex, density of surrounding tissues, and imaging blur. High resolution micro-CT data of cadaver proximal femurs were analyzed to determine a relationship between cortical thickness and density. This thickness-density relationship was used as prior information to be incorporated in the model to obtain accurate measurements of cortical thickness and density from clinical CT volumes. The method was validated using micro-CT scans of 23 cadaver proximal femurs. Simulated clinical CT images with different voxel sizes were generated from the micro-CT data. Cortical thickness and density were estimated from the simulated images using the proposed method and compared with measurements obtained using the micro-CT images to evaluate the effect of voxel size on the accuracy of the method. Then, 19 of the 23 specimens were imaged using a clinical CT scanner. Cortical thickness and density were estimated from the clinical CT images using the proposed method and compared with the micro-CT measurements. Finally, a case-control study including 20 patients with osteoporosis and 20 age-matched controls with normal bone density was performed to evaluate the proposed method in a clinical context.
  
  
  RESULTS
  Cortical thickness (density) estimation errors were 0.07 ± 0.19 mm (-18 ± 92 mg/cm(3)) using the simulated clinical CT volumes with the smallest voxel size (0.33 × 0.33 × 0.5 mm(3)), and 0.10 ± 0.24 mm (-10 ± 115 mg/cm(3)) using the volumes with the largest voxel size (1.0 × 1.0 × 3.0 mm(3)). A trend for the cortical thickness and density estimation errors to increase with voxel size was observed and was more pronounced for thin cortices. Using clinical CT data for 19 of the 23 samples, mean errors of 0.18 ± 0.24 mm for the cortical thickness and 15 ± 106 mg/cm(3) for the density were found. The case-control study showed that osteoporotic patients had a thinner cortex and a lower cortical density, with average differences of -0.8 mm and -58.6 mg/cm(3) at the proximal femur in comparison with age-matched controls (p-value < 0.001).
  
  
  CONCLUSIONS
  This method might be a promising approach for the quantification of cortical bone thickness and density using clinical routine imaging techniques. Future work will concentrate on investigating how this approach can improve the estimation of mechanical strength of bony structures, the prevention of fracture, and the management of osteoporosis.
authors.affiliation: Galgo Medical, Barcelona 08036, Spain.
authors.forename: Ludovic
authors.initials: L
authors.lastname: Humbert
authors.affiliation: Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven 5600 MB, The Netherlands.
authors.forename: Javad
authors.initials: J
authors.lastname: Hazrati Marangalou
authors.affiliation: CETIR Centre Medic, Barcelona 08029, Spain.
authors.forename: Luis Miguel
authors.initials: LM
authors.lastname: Del Río Barquero
authors.affiliation: Biomechanics Section, KU Leuven-University of Leuven, Leuven 3001, Belgium.
authors.forename: G Harry
authors.initials: GH
authors.lastname: van Lenthe
authors.affiliation: Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven 5600 MB, The Netherlands.
authors.forename: Bert
authors.initials: B
authors.lastname: van Rietbergen
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D015519:Bone Density; D000071538:Cortical Bone / Q000000981:diagnostic imaging* / Q000502:physiology*; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging / Q000502:physiology; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D014057:Tomography, X-Ray Computed