A new method of segmentation of compact-appearing, transitional and trabecular compartments and quantification of cortical porosity from high resolution peripheral quantitative computed tomographic images

Abstract

A transitional or cortico-trabecular junctional zone exists at any location composed of both cortical and trabecular bones such as the metaphyses of tubular bones and short bones like the femoral neck. The transitional zone comprises the inner cortex adjacent to the medullary canal and trabeculae abutting against the cortex contiguous with the endocortical surface. This is a site of vigorous remodeling. Intracortical remodeling cavitates the inner cortex expanding this transitional zone at the price of compact-appearing cortex so that it contains porosity, cortical fragments that resemble trabeculae, and trabeculae abutting the eroding cortex. The porosity of the transitional zone is an important source of bone loss. It reduces bone strength exponentially and is a quantifiable `fingerprint' of structural deterioration. A new automated method of segmentation of bone from background and bone into its compact-appearing cortex, transitional zone, and trabecular compartment is described, with a new approach to quantification of cortical porosity. Segmentation is achieved by automatically selecting attenuation profile curves perpendicular to the periosteal surface. Local bone edges are identified as the beginning and the end of the rising and falling S-shaped portions of the curve enabling the delineation of the compartments. Analyzing ~3600 consecutive overlapping profiles around the perimeter of each cross-sectional slice segments the compartments. Porosity is quantified as the average void volume fraction of all voxels within each compartment. To assess accuracy at the distal radius and tibia, μCT images of cadaveric specimens imaged at 19 μm voxel size served as the gold standard. To assess accuracy at the proximal femur, scanning electron microscopy (SEM) images of specimens collected at 2.5 μm resolution served as the gold standard. Agreement between HRpQCT and the gold standards for segmentation and quantification of porosity at the distal radius and tibia ranged from R(2)=0.87 to 0.99, and for the proximal femur ranged from 0.93 to 0.99. The precision error in vivo for segmentation and quantification of porosity in HRpQCT images at the distal radius, given by the root mean square error of the coefficient of variation, ranged from 0.54% for porosity of the transitional zone to 3.98% for area of the compact-appearing cortex. Segmentation of the transitional zone minimizes errors in apportioning cortical fragments and cortical porosity to the medullary compartment and so is likely to allow accurate assessment of fracture risk and the morphological effects of growth, aging, diseases and therapies.

Irreversible Deterioration of Cortical and Trabecular Microstructure Associated With Breastfeeding

“…Porosity within the total cortex and its compartments (compact cortex, outer and inner transitional zones), matrix mineralization density, trabecular number, thickness, separation, and bone volume/tissue volume (BV/TV) were quantified using StrAx1.0, a non-threshold-based method for quantifying microarchitecture in vivo. (34) The method automatically segments bone into its compartments (Straxcorp, Melbourne, Australia). The in vivo precision was 0.5 to 3.0%.…”

“…The in vivo precision was 0.5 to 3.0%. (34) Daily quality control was carried out by scanning a phantom containing rods of hydroxyapatite (QRM, Moehrendorf, Germany).…”

The clinical contribution of cortical porosity to fragility fractures

“…8,11 Studies using high-resolution peripheral quantitative computed tomography (HR-pQCT) to quantify porosity (XtremeCT; Scanco Medical AG, Bruttisellen, Switzerland) present low values of porosity (between 1% and 15%) because of quantifying only porosity of the compactappearing cortex [12][13][14][15][16][17] and only pores over 100 mm, although 60% of cortical pores are under 100 mm in diameter. [18][19][20] This threshold-based image analysis underestimates porosity by including only completely empty voxels and excluding the voxels containing both void and bone matrix.…”

“…[18][19][20] This threshold-based image analysis underestimates porosity by including only completely empty voxels and excluding the voxels containing both void and bone matrix. 11 Furthermore, direct measurements of cortical bone water content using deuterium oxide or dehydration experiments report a void volume between 15 and 40%, suggesting an under-reporting of porosity. [21][22][23] To avoid exclusion of voxels that contain both void volume and bone matrix, a new approach has been developed, taking into account this process of partial volume effect.…”

“…StrAx1.0 software is a non-threshold-based approach (StraxCorp Pty Ltd, Melbourne, VIC, Australia) that automatically selects attenuation profile curves, and segments the bone into the compact-appearing cortex, transitional zones, and trabecular compartment in high-resolution CT images 11,24,25 as well as in low-resolution CT images. 26,27 Bone is segmented by analyzing B3600 consecutive overlapping profiles around the perimeter of each cross-sectional slice.…”