In vivo magnetic resonance detects rapid remodeling changes in the topology of the trabecular bone network after menopause and the protective effect of estradiol.

INTRODUCTION: Estrogen depletion after menopause is accompanied by bone loss and architectural deterioration of trabecular bone. The hypothesis underlying this work is that the microMRI-based virtual bone biopsy can capture the temporal changes of scale and topology of the trabecular network and that estrogen supplementation preserves the integrity of the trabecular network. MATERIALS AND METHODS: Subjects studied were early postmenopausal women, 45-55 yr of age (N = 65), of whom 32 were on estrogen (estradiol group), and the remainder were not (control group). Early menopause was defined by amenorrhea for 6-24 mo and elevated serum follicle-stimulating hormone (FSH) concentration. The subjects were evaluated with three imaging modalities at baseline and 12 and 24 mo to determine the temporal changes in trabecular and cortical architecture and density. microMRI of the distal radius and tibia was performed at 137 x 137 x 410-microm(3) voxel size. The resulting bone volume fraction maps were Fourier interpolated to a final voxel size of 45.7 x 45.7 x 136.7 microm(3), binarized, skeletonized, and subjected to 3D digital topological analysis (DTA). Skeletonization converts trabecular rods to curves and plates to surfaces. Parameters quantifying scale included BV/TV, whereas DTA parameters included the volume densities of curves (C) and surface (S)-type voxels, as well as composite parameters: the surface/curve ratio (S/C), and erosion index (EI, ratio of the sum of parameters expected to increase with osteoclastic resorption divided by the sum of those expected to decrease). For comparison, pQCT of the same peripheral locations was conducted, and trabecular density and cortical structural parameters were measured. Areal BMD of the lumbar vertebrae and hip was also measured. RESULTS: Substantial changes in trabecular architecture of the distal tibia, in particular as they relate to topology of the network, were detected after 12 mo in the control group. S/C decreased 5.6% (p < 0.0005), and EI increased 7.1% (p < 0.0005). Most curve- and profile-type voxels (representative of trabecular struts), increased significantly (p < 0.001). Curve and profile edges resulting from disconnection of rod-like trabeculae increased by 9.8% and 5.1% (p = 0.0001 and <0.001, respectively). Similarly, DXA BMD in the spine and hip decreased 2.6% and 1.3% (p < 0.0001 and <0.005, respectively), and pQCT cortical area decreased 3.6% (p = 0.0001). However, neither trabecular density nor BV/TV changed. Furthermore, none of the parameters measured in the estradiol group were significantly different after 12 mo. Substantial differences in the mean changes from baseline between the estradiol treatment and control groups, in particular after 24 mo, were observed, with relative group differences as large as 13% (S/C, p = 0.005), and the relative changes in the two groups had the opposite sign for most parameters. The observed temporal alterations in architecture are consistent with remodeling changes that involve gradual conversion of plate-like to rod-like trabecular bone along with disconnection of trabecular elements, even in the absence of a net loss of trabecular bone. The high-resolution 3D rendered images provide direct evidence of the above remodeling changes in individual subjects. The radius structural data indicated similar trends but offered no definitive conclusions. CONCLUSIONS: The short-term temporal changes in trabecular architecture after menopause, and the protective effects of estradiol ensuring maintenance of a more plate-like TB architecture, reported here, have not previously been observed in vivo. This work suggests that MRI-based in vivo micromorphometry of trabecular bone has promise as a tool for monitoring osteoporosis treatment.

Trabecular structure quantified with the MRI-based virtual bone biopsy in postmenopausal women contributes to vertebral deformity burden independent of areal vertebral BMD.

UNLABELLED: In postmenopausal women with a wide range of vertebral deformities, MRI-based structural measures of topology and scale at the distal radius are shown to account for as much as 30% of vertebral deformity, independent of integral vertebral BMD. INTRODUCTION: Trabecular bone architecture has been postulated to contribute to overall bone strength independent of vertebral BMD measured by DXA. However, there has thus far been only sparse in vivo evidence to support this hypothesis. MATERIALS AND METHODS: Postmenopausal women, 60-80 yr of age, were screened by DXA, and those with T-scores at either the hip or spine falling within the range of -2.5 +/- 1.0 were studied with the MRI-based virtual bone biopsy, along with heel broadband ultrasound absorption and pQCT of the tibia. The data from 98 subjects meeting the enrollment criteria were subjected to microMRI at the distal tibia and radius, and measures of topology and scale of the trabecular bone network were computed. A spinal deformity index (SDI) was obtained from morphometric measurements in midline sagittal MR images of the thoracic and lumbar spine to evaluate associations between structure and deformity burden. RESULTS: A number of structural indices obtained at the distal radius were correlated with the SDI. Among these were the topological surface density (a measure of trabecular plates) and trabecular bone volume fraction, which were inversely correlated with SDI (p < 0.0001). Combinations of two structural parameters accounted for up to 30% of the variation in SDI (p < 0.0001) independent of spinal BMD, which was not significantly correlated. pQCT trabecular BMD was also weakly associated, whereas broadband ultrasound absorption was not. No significant association between SDI and structural indices were found at the tibia. CONCLUSIONS: Structural measures at the distal radius obtained in vivo by microMRI explained a significant portion of the variation in total spinal deformity burden in postmenopausal women independent of areal BMD.

Mineral volume and morphology in carotid plaque specimens using high-resolution MRI and CT.

OBJECTIVE: High-resolution MRI methods have been used to evaluate carotid artery atherosclerotic plaque content. The purpose of this study was to assess the performance of high-resolution MRI in evaluation of the quantity and pattern of mineral deposition in carotid endarterectomy (CEA) specimens, with quantitative micro-CT as the gold standard. METHODS AND RESULTS: High-resolution MRI and CT were compared in 20 CEA specimens. Linear regression comparing mineral volumes generated from CT (VCT) and MRI (VMRI) data demonstrated good correlation using simple thresholding (VMRI=-0.01+0.98VCT; R2=0.90; threshold=4xnoise) and k-means clustering methods (VMRI=-0.005+1.38VCT; R2=0.93). Bone mineral density (BMD) and bone mineral content (BMC [mineral mass]) were calculated for CT data and BMC verified with ash weight. Patterns of mineralization like particles, granules, and sheets were more clearly depicted on CT. CONCLUSIONS: Mineral volumes generated from MRI or CT data were highly correlated. CT provided a more detailed depiction of mineralization patterns and provided BMD and BMC in addition to mineral volume. The extent of mineralization as well as the morphology may ultimately be useful in assessing plaque stability.