Inter-scanner differences in in vivo QCT measurements of the density and strength of the proximal femur remain after correction with anthropomorphic standardization phantoms

Abstract

In multicenter studies and longitudinal studies that use two or more different quantitative computed tomography (QCT) imaging systems, anthropomorphic standardization phantoms (ASPs) are used to correct inter-scanner differences and allow pooling of data. In this study, in vivo imaging of 20 women on two imaging systems was used to evaluate inter-scanner differences in hip integral BMD (iBMD), trabecular BMD (tBMD), cortical BMD (cBMD), femoral neck yield moment (My) and yield force (Fy), and finite-element derived strength of the femur under stance (FEstance) and fall (FEfall) loading. Six different ASPs were used to derive inter-scanner correction equations. Significant (p<0.05) inter-scanner differences were detected in all measurements except My and FEfall, and no ASP-based correction was able to reduce inter-scanner variability to corresponding levels of intra-scanner precision. Inter-scanner variability was considerably higher than intra-scanner precision, even in cases where the mean inter-scanner difference was statistically insignificant. A significant (p<0.01) effect of body size on inter-scanner differences in BMD was detected, demonstrating a need to address the effects of body size on QCT measurements. The results of this study show that significant inter-scanner differences in QCT-based measurements of BMD and bone strength can remain even when using an ASP.

Keywords: Biomechanics; Bone mineral density; Hip; Quantitative computed tomography.

Figure 1

Quantitative computed tomography images of the ASPs used in the study. All standardization phantoms were scanned atop the three-chamber solid calcium hydroxyapatite bone mineral reference phantom. Yellow squares and circles bound the ROIs used for BMD measurements in simulated soft tissue and vertebral trabecular bone. Yellow lines indicate the profiles used for BMD measurements in simulated femoral cortices, femoral trabecular bone, vertebral arches, and transverse processes.

Figure 2

Profiles used to obtain BMD measurements in ASPs. a.) For hip phantom cortices, the two peak values in each side of the cortex were averaged to obtain the profile’s cortical BMD. For QRM trabecular bone cores, the ten central values were averaged to obtain the profile’s trabecular BMD. b.) For transverse processes, the four central values of the peak were averaged to obtain the profile’s BMD. c.) for vertebral arches, the three central values of the peak were average to obtain the profile’s BMD.

Figure 3

Bland-Altman plots for inter-scanner differences in volumetric BMD measurements in 20 women. Solid lines indicate the mean inter-scanner difference, and dashed lines indicate 95% confidence intervals (mean ± 1.96 standard deviations).

Figure 4

Bland-Altman plots for inter-scanner differences in femoral neck sectional properties and finite element-based strength of the proximal femur in 20 women. Solid lines indicate the mean inter-scanner difference, and dashed lines indicate 95% confidence intervals (mean ± 1.96 standard deviations).