Structural analysis of high resolution in vitro MR images compared to stained grindings

Advanced Imaging Assessment of Bone Quality

Noninvasive and/or nondestructive techniques can provide structural information about bone, beyond simple bone densitometry. While the latter provides important information about osteoporotic fracture risk, many studies indicate that bone mineral density (BMD) only partly explains bone strength. Quantitative assessment of macrostructural characteristics, such as geometry, and microstructural features, such as relative trabecular volume, trabecular spacing, and connectivity, may improve our ability to estimate bone strength. Methods for quantitatively assessing macrostructure include (besides conventional radiographs) dual X ray absorptiometry (DXA) and computed tomography (CT), particularly volumetric quantitative computed tomography (vQCT). Methods for assessing microstructure of trabecular bone noninvasively and/or nondestructively include high-resolution computed tomography (hrCT), microcomputed tomography (micro-CT), high-resolution magnetic resonance (hrMR), and micromagnetic resonance (micro-MR). vQCT, hrCT, and hrMR are generally applicable in vivo; micro-CT and micro-MR are principally applicable in vitro. Despite progress, problems remain. The important balances between spatial resolution and sampling size, or between signal-to-noise and radiation dose or acquisition time, need further consideration, as do the complexity and expense of the methods versus their availability and accessibility. Clinically, the challenges for bone imaging include balancing the advantages of simple bone densitometry versus the more complex architectural features of bone, or the deeper research requirements versus the broader clinical needs. The biological differences between the peripheral appendicular skeleton and the central axial skeleton must be further addressed. Finally, the relative merits of these sophisticated imaging techniques must be weighed with respect to their applications as diagnostic procedures, requiring high accuracy or reliability, versus their monitoring applications, requiring high precision or reproducibility.

Bone structure of the calcaneus: analysis with magnetic resonance imaging and correlation with histomorphometric study

The purpose of this study was to compare structural measurements obtained from MR images of the calcaneus with those obtained from conventional histomorphometry. Sagittal magnetic resonance (MR) images of the calcaneus of 24 fresh human cadaveric feet were obtained at a spatial resolution achievable in vivo. A three-dimensional gradient echo-sequence was used with a slice thickness of 700 microm and in plane resolution of 172 x 172 microm. Structural analysis (four histomorphometric parameters; seven connectivity parameters) was performed in the superior region of the calcaneus. Bone biopsy specimens were obtained in the same area and were sectioned for histomorphometric study. Most of the MR histomorphometric parameters were overestimated (by a factor ranging from 0.8 to 3), as compared with histomorphometry. However, significant ( P<0.05) correlations were found between MR imaging and histomorphometric measurements for bone volume/tissue volume, trabecular separation, trabecular number, star volume of the marrow space, node count and terminus count. MR histomorphometric parameters correlated much better with histomorphometry than connectivity parameters. This study suggests that structural parameters characterizing cancellous bone in the calcaneus can be derived from MR images in the limited spatial resolution regime applicable in vivo.

Evaluation of 9.4-T MR microimaging in assessing normal and defective fetal bone development: comparison of MR imaging and histological findings

We evaluated 9.4-T magnetic resonance (MR) microimaging in assessing normal and defective bone development in mouse embryos. For this purpose, we performed 9.4-T MR microimaging on developing bones in normal embryos, and also in Runx2/Cbfa1-/- embryos with severely defective bone development. MR images were compared with the histological and histochemical features of these fetal bones. MR microimaging delineate successfully the normal long bone development in embryos. The T1- and T2-weighted MR microimaging demonstrated chondrocyte maturation in different regions of growing cartilage, such as epiphysis, physis, hypertrophic cartilage, and zone of provisional calcification. These developmental changes were detectable in as early as E14.5 embryos. The MR microimaging clearly demonstrated defective bone development in Runx2/Cbfa1-/- embryos. The femur from E18.5 homozygous Runx2/Cbfa1-/- embryos lacked MR signal intensity patterns including the hypertrophic cartilage, which are characteristic of the bone from the age-matched Runx2/Cbfa1+/+ embryos. Interestingly, however, the tibia from the same mutants was associated with MR signal patterns indicative of hypertrophic cartilage but not of the primary spongiosa and ossifying perichondrium, suggesting that bone development is differently regulated in these two long bones. On the other hand, the bones from heterozygous Runx2/Cbfa1+/- embryos exhibited an MR phenotype intermediate between the Runx2/Cbfa1+/+ and Runx2/Cbfa1-/- embryos; the primary spongiosa and ossifying perichondrium formation occurred normally even in the absence of preceding organized maturation of chondrocytes, a phenotype that was not detected by histological examinations. We concluded that MR microimaging is useful in assessing the bone development.

Patient-specific microarchitecture of vertebral cancellous bone: a peripheral quantitative computed tomographic and histological study

This study directly compares peripheral quantitative computed tomography (pQCT) and histology for the assessment of 11 morphological parameters. Sixty-eight cylindrical cancellous bone samples were cored from the thoracic (T-9) thoracolumbar (T-12 or L-1), and lumbar (L-4) vertebral bodies of nine autopsy subjects (aged 44-88 years). Four transverse slices were acquired by pQCT from the bottom to the top of each cylinder. Slice thickness was 300 microm and pixel size was 70 x 70 microm. Thin sections (5 microm) were obtained at the same location in the samples, stained with Von Kossa, and photographed. Classical morphological parameters and strut analysis parameters were measured on all images (272 pQCT and 272 matched histological sections). Because of the partial volume effect and specific thresholding procedure, pQCT overestimated the absolute value of the bone volume fraction (BV/TV) and trabecular thickness (Tb.Th) by a factor 2. The trabecular number (Tb.N), trabecular spacing (Tb.Sp), and total strut length (TSL) were correctly estimated. However, the direct correlation between pQCT and histology was excellent (r2 > 0.85, p < 0.001) for BV/TV, Tb.N, Tb.Sp, TSL, and star surface. For Tb.Th, number of nodes, and number of free ends, the correlation was also good (r(2) > 0.6, p < 0.001). Using a random regression model, we also explored the ability of these parameters to add structural information to the readily available BV/TV or apparent density. The model identified significant (p < 0.001) differences between subjects. For a given BV/TV, some patients had more trabeculae (Tb.N) that were thinner (Tb.Th) and more disconnected (higher free ends and star). This was observed for both histology and pQCT morphometrical data. Our analysis demonstrates the capacity of both histology and pQCT to detect subjects with specific structural patterns in vertebral cancellous bone.