title: A calibration methodology of QCT BMD for human vertebral body with registered micro-CT images.
abstract: OBJECTIVE
  The accuracy of QCT-based homogenized finite element (FE) models is strongly related to the accuracy of the prediction of bone volume fraction (BV/TV) from bone mineral density (BMD). The goal of this study was to establish a calibration methodology to relate the BMD computed with QCT with the BV/TV computed with micro-CT (microCT) over a wide range of bone mineral densities and to investigate the effect of region size in which BMD and BV/TV are computed.
  
  
  METHODS
  Six human vertebral bodies were dissected from the spine of six donors and scanned submerged in water with QCT (voxel size: 0.391 x 0.391 x 0.450 mm3) and microCT (isotropic voxel size: 0.018(3) mm3). The microCT images were segmented with a single level threshold. Afterward, QCT-grayscale, microCT-grayscale, and microCT-segmented images were registered. Two isotropic grids of 1.230 mm (small) and 4.920 mm (large) were superimposed on every image, and QCT(BMD) was compared both with microCT(BMD) and microCT(BV/TV) for each grid cell.
  
  
  RESULTS
  The ranges of QCT(BMD) for large and small regions were 9-559 mg/cm3 and -90 to 1006 mg/cm3, respectively. QCT(BMD) was found to overestimate microCT(BMD). No significant differences were found between the QCT(BMD)-microCT(BV/TV) regression parameters of the two grid sizes. However, the R2 was higher, and the standard error of the estimate (SEE) was lower for large regions when compared to small regions. For the pooled data, an extrapolated QCTBMD value equal to 1062 mg/ cm3 was found to correspond to 100% microCT(BV/TV).
  
  
  CONCLUSIONS
  A calibration method was defined to evaluate BV/TV from QCTBMD values for cortical and trabecular bone in vitro. The QCT(BMD-microCT(BV/TV) calibration was found to be dependent on the scanned vertebral section but not on the size of the regions. However, the higher SEE computed for small regions suggests that the deleterious effect of QCT image noise on FE modelling increases with decreasing voxel size.
authors.affiliation: Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Gusshausstrasse 27-19, A-1040, Vienna, Austria. edallara@ilsb.tuwien.ac.at
authors.forename: E
authors.initials: E
authors.lastname: Dall'Ara
authors.forename: P
authors.initials: P
authors.lastname: Varga
authors.forename: D
authors.initials: D
authors.lastname: Pahr
authors.forename: P
authors.initials: P
authors.lastname: Zysset
mesh_terms: D000465:Algorithms; D001317:Austria; D015519:Bone Density / Q000502:physiology*; D002138:Calibration; D003720:Densitometry / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013131:Spine / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D014057:Tomography, X-Ray Computed / Q000379:methods* / Q000592:standards