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TV CALIBRATION RELATIONSHIPS ARE SIMILAR FOR HUMAN VERTEBRA AND FEMUR
S450
Presentation 1013 − Topic 32. Medical imaging
QCT-BMD TO MICROCT-BV/TV CALIBRATION RELATIONSHIPS ARE SIMILAR FOR HUMAN VERTEBRA AND FEMUR Enrico Dall’Ara (1), Dieter Pahr (1), Philippe Zysset (2)
1. Vienna University of Technology, Austria; 2. University of Bern, Switzerland
Introduction The quantitative computed tomography (QCT)based Finite Element (FE) method can be used to predict bone strength. The accuracy of FE models is strongly related to the accuracy of the prediction of bone volume fraction (BV/TV) from bone mineral density (BMD). A procedure to calibrate QCTBMD with μCTBV/TV has been already shown for vertebral body slices [Dall’Ara, 2011]. The goal of this study was to compare this calibration for human vertebral and femoral bone.
Methods Six human vertebral body slices [Dall’Ara, 2011] and three human proximal femora were scanned with clinical QCT (voxel size 0.39x0.39x0.45mm³ and 0.33x0.33x1mm³, respectively). BMD was evaluated in each voxel using a calibration phantom. Afterwards, six slices from the vertebrae and ten slices from the femora (from greater trochanter, head, lesser trochanter and neck) were cut to fit the largest sample holder of a μCT system and scanned with 0.018³ mm³ voxel size. Before scanning, the samples were immersed in 0.9% NaCl solution and left at least ten minutes under vacuum to remove air bubbles. The μCT and QCT images were registered by rigid transformation. Each μCT grey scale image was Gauss filtered and segmented with an iterative threshold [Ridler, 1977]. Two isotropic 3D grids, with cell dimension ~1.3mm³ (small) and ~4.5mm³ (large), were superimposed to the registered QCT and μCT images. The QCTBMD was compared to μCTBV/TV for each grid cell. The dimensions of the cells were chosen to simulate the voxel size in QCT-based FE models [Dall’Ara, 2010] (small) and to investigate the effect of image noise in a typical region of interest for trabecular bone (large). Standard error of the estimate (SEE), 95% confidence interval (CI) and coefficient of determination (R²) were computed for each linear regression. ANCOVA was used to compare the parameters of the regression equations.
Results The following linear regressions were found for vertebrae (eq. 1, 2) and femora (eq. 3, 4) with small (eq. 1, 3) and large (eq. 2, 4) grids (Figure1): BV/TV=0.095*BMD+0.127 (1) BV/TV=0.090*BMD+0.515 (2) BV/TV=0.096*BMD+0.638 (3) BV/TV=0.093*BMD+1.080 (4) Journal of Biomechanics 45(S1)
Calibration equations were found to be linear for the full range of BMD. Linear regressions showed intercepts close to 0 (CIvertebrae: 0.04-0.22%; CIfemura: 0.51-0.77%). Even though the anatomical position was found to affect the relationship between μCTBV/TV and QCTBMD (p<0.001 for large and for small regions), the differences in slope were small (1% for small and 3% for large regions). As a consequence, similar QCTBMD values for vertebra and femur were found to correspond to 100% μCTBV/TV (1057 and 1041 mgHA/cc, respectively). Grid size did not affect significantly the calibrations for both vertebra (p=0.131) and femur (p=0.169). However, higher R² and lower SEE were found in case of large regions for both vertebra (R²=0.82 and SEE=4.3 vs R²=0.95 and SEE=1.8) and femur (R²=0.92 and SEE=7.2 vs R²=0.97 and SEE=3.2).
Figure 1: μCTBV/TV –QCTBMD calibrations.
Discussion The results suggest that a similar QCTBMDμCTBV/TV calibration can be used for both anatomical sites while generating QCT-based FE models. Moreover, the higher SEE and lower R² in case of small regions suggest that the deleterious effect of QCT image noise on FE modelling increases by reducing element size. The higher SEE computed for the femora, is probably due to the lower original clinical CT image resolution.
References Dall’Ara et al, JBiomech, 43: 2374-2380, 2010 Dall’Ara et al, Medic Phys, 38: 2602-2608, 2011 Ridler and Calvard, IEEE Trans Syst Man Cybern, SMC-8: 630-632, 1978
ESB2012: 18th Congress of the European Society of Biomechanics
Presentation 1013 − Topic 32. Medical imaging
QCT-BMD TO MICROCT-BV/TV CALIBRATION RELATIONSHIPS ARE SIMILAR FOR HUMAN VERTEBRA AND FEMUR Enrico Dall’Ara (1), Dieter Pahr (1), Philippe Zysset (2)
1. Vienna University of Technology, Austria; 2. University of Bern, Switzerland
Introduction The quantitative computed tomography (QCT)based Finite Element (FE) method can be used to predict bone strength. The accuracy of FE models is strongly related to the accuracy of the prediction of bone volume fraction (BV/TV) from bone mineral density (BMD). A procedure to calibrate QCTBMD with μCTBV/TV has been already shown for vertebral body slices [Dall’Ara, 2011]. The goal of this study was to compare this calibration for human vertebral and femoral bone.
Methods Six human vertebral body slices [Dall’Ara, 2011] and three human proximal femora were scanned with clinical QCT (voxel size 0.39x0.39x0.45mm³ and 0.33x0.33x1mm³, respectively). BMD was evaluated in each voxel using a calibration phantom. Afterwards, six slices from the vertebrae and ten slices from the femora (from greater trochanter, head, lesser trochanter and neck) were cut to fit the largest sample holder of a μCT system and scanned with 0.018³ mm³ voxel size. Before scanning, the samples were immersed in 0.9% NaCl solution and left at least ten minutes under vacuum to remove air bubbles. The μCT and QCT images were registered by rigid transformation. Each μCT grey scale image was Gauss filtered and segmented with an iterative threshold [Ridler, 1977]. Two isotropic 3D grids, with cell dimension ~1.3mm³ (small) and ~4.5mm³ (large), were superimposed to the registered QCT and μCT images. The QCTBMD was compared to μCTBV/TV for each grid cell. The dimensions of the cells were chosen to simulate the voxel size in QCT-based FE models [Dall’Ara, 2010] (small) and to investigate the effect of image noise in a typical region of interest for trabecular bone (large). Standard error of the estimate (SEE), 95% confidence interval (CI) and coefficient of determination (R²) were computed for each linear regression. ANCOVA was used to compare the parameters of the regression equations.
Results The following linear regressions were found for vertebrae (eq. 1, 2) and femora (eq. 3, 4) with small (eq. 1, 3) and large (eq. 2, 4) grids (Figure1): BV/TV=0.095*BMD+0.127 (1) BV/TV=0.090*BMD+0.515 (2) BV/TV=0.096*BMD+0.638 (3) BV/TV=0.093*BMD+1.080 (4) Journal of Biomechanics 45(S1)
Calibration equations were found to be linear for the full range of BMD. Linear regressions showed intercepts close to 0 (CIvertebrae: 0.04-0.22%; CIfemura: 0.51-0.77%). Even though the anatomical position was found to affect the relationship between μCTBV/TV and QCTBMD (p<0.001 for large and for small regions), the differences in slope were small (1% for small and 3% for large regions). As a consequence, similar QCTBMD values for vertebra and femur were found to correspond to 100% μCTBV/TV (1057 and 1041 mgHA/cc, respectively). Grid size did not affect significantly the calibrations for both vertebra (p=0.131) and femur (p=0.169). However, higher R² and lower SEE were found in case of large regions for both vertebra (R²=0.82 and SEE=4.3 vs R²=0.95 and SEE=1.8) and femur (R²=0.92 and SEE=7.2 vs R²=0.97 and SEE=3.2).
Figure 1: μCTBV/TV –QCTBMD calibrations.
Discussion The results suggest that a similar QCTBMDμCTBV/TV calibration can be used for both anatomical sites while generating QCT-based FE models. Moreover, the higher SEE and lower R² in case of small regions suggest that the deleterious effect of QCT image noise on FE modelling increases by reducing element size. The higher SEE computed for the femora, is probably due to the lower original clinical CT image resolution.
References Dall’Ara et al, JBiomech, 43: 2374-2380, 2010 Dall’Ara et al, Medic Phys, 38: 2602-2608, 2011 Ridler and Calvard, IEEE Trans Syst Man Cybern, SMC-8: 630-632, 1978
ESB2012: 18th Congress of the European Society of Biomechanics