The effect of voxel size on high-resolution peripheral computed tomography measurements of trabecular and cortical bone microstructure

Abstract

Purpose: Accurate quantification of bone microstructure plays a significant role in understanding bone mechanics and response to disease or treatment. High-resolution peripheral quantitative computed tomography (HR-pQCT) allows for the quantification of trabecular and cortical structure in vivo, with the capability of generating images at multiple voxel sizes (41, 82, and 123 μm). The aim of this study was to characterize the effect of voxel size on structural measures of trabecular and cortical bone and to determine accuracy in reference to micro-CT ([micro sign]CT), the gold standard for bone microstructure quantification.

Methods: Seventeen radii from human cadaver specimens were imaged at each HR-pQCT voxel size and subsequently imaged using [micro sign]CT. Bone density and microstructural assessment was performed in both the trabecular and cortical compartments, including cortical porosity quantification. Two distinct analysis techniques were applied to the 41 μm HR-pQCT data: the standard clinical indirect analysis and a direct analysis requiring no density or structural model assumptions. Analysis parameters were adjusted to enable segmentation and structure extraction at each voxel size.

Results: For trabecular microstructural measures, the 41 μm HR-pQCT data displayed the strongest correlations and smallest errors compared to [micro sign]CT data. The direct analysis technique applied to the 41 μm data yielded an additional improvement in accuracy, especially for measures of trabecular thickness. The 123 μm data performed poorly, with all microstructural measures either having moderate or nonsignificant correlations with [micro sign]CT data. Trabecular densitometric measures showed strong correlations to [micro sign]CT data across all voxel sizes. Cortical thickness was strongly correlated with [micro sign]CT values across all HR-pQCT voxel sizes. The accuracy of cortical porosity parameters was highly dependent on voxel size; again, the 41 μm data was most strongly correlated. Measures of cortical density and pore diameter at all HR-pQCT voxel sizes had either weak or nonsignificant correlations.

Conclusions: This study demonstrates the effect of voxel size on the accuracy of HR-pQCT measurements of trabecular and cortical microstructure and presents parameters for HR-pQCT analysis at nonstandard resolutions. For all parameters measured, correlations were strongest at 41 μm. Weak correlations for porosity measures indicate that a better understanding of pore structure and resolution dependence is needed.

Figure 1

MTF data for the HR-pQCT images at each voxel size.

Figure 2

Representative images of the trabecular segmentation for both µCT and HR-pQCT images. (A) Grayscale images acquired by the respective scanners. (B) Trabecular segmentation of a single slice. (C) 3D representation of the trabecular volume.

Figure 3

Representative images of the cortical segmentation. (A) Grayscale images acquired by the respective scanners. (B) Cortical segmentation of a single slice. (C) Cortical porosity segmentation of a single slice. (D) 3D representations of the cortical volume. (E) 3D representations of the cortical porosity volume.

Figure 4

Bland–Altman plots for trabecular parameters.

Figure 5

Bland–Altman plots for extended cortical parameters.