Structural and mechanical parameters of trabecular bone estimated from in vivo high-resolution magnetic resonance images at 3 tesla field strength

Effect of Low-Intensity Vibration on Bone Strength, Microstructure, and Adiposity in Pre-Osteoporotic Postmenopausal Women: A Randomized Placebo-Controlled Trial

There has been evidence that cyclical mechanical stimulation may be osteogenic, thus providing opportunities for nonpharmacological treatment of degenerative bone disease. Here, we applied this technology to a cohort of postmenopausal women with varying bone mineral density (BMD) T-scores at the total hip (-0.524 ± 0.843) and spine (-0.795 ± 1.03) to examine the response to intervention after 1 year of daily treatment with 10 minutes of vibration therapy in a randomized double-blinded trial. The device operates either in an active mode (30 Hz and 0.3 g) or placebo. Primary endpoints were changes in bone stiffness at the distal tibia and marrow adiposity of the vertebrae, based on 3 Tesla high-resolution MRI and spectroscopic imaging, respectively. Secondary outcome variables included distal tibial trabecular microstructural parameters and vertebral deformity determined by MRI, volumetric and areal bone densities derived using peripheral quantitative computed tomography (pQCT) of the tibia, and dual-energy X-ray absorptiometry (DXA)-based BMD of the hip and spine. Device adherence was 83% in the active group (n = 42) and 86% in the placebo group (n = 38) and did not differ between groups (p = .7). The mean 12-month changes in tibial stiffness in the treatment group and placebo group were +1.31 ± 6.05% and -2.55 ± 3.90%, respectively (group difference 3.86%, p = .0096). In the active group, marrow fat fraction significantly decreased after 12 months of intervention (p = .0003), whereas no significant change was observed in the placebo group (p = .7; group difference -1.59%, p = .029). Mean differences of the changes in trabecular bone volume fraction (p = .048) and erosion index (p = .044) were also significant, as was pQCT-derived trabecular volumetric BMD (vBMD; p = .016) at the tibia. The data are commensurate with the hypothesis that vibration therapy is protective against loss in mechanical strength and, further, that the intervention minimizes the shift from the osteoblastic to the adipocytic lineage of mesenchymal stem cells. © 2020 American Society for Bone and Mineral Research (ASBMR).

Bone Structure Analysis of the Radius Using Ultrahigh Field (7T) MRI: Relevance of Technical Parameters and Comparison with 3T MRI and Radiography

Bone fractal signature analysis (FSA-also termed bone texture analysis) is a tool that assesses structural changes that may relate to clinical outcomes and functions. Our aim was to compare bone texture analysis of the distal radius in patients and volunteers using radiography and 3T and 7T magnetic resonance imaging (MRI)-a patient group (n = 25) and a volunteer group (n = 25) were included. Participants in the patient group had a history of chronic wrist pain with suspected or confirmed osteoarthritis and/or ligament instability. All participants had 3T and 7T MRI including T1-weighted turbo spin echo (TSE) sequences. The 7T MRI examination included an additional high-resolution (HR) T1 TSE sequence. Radiographs of the wrist were acquired for the patient group. When comparing patients and volunteers (unadjusted for gender and age), we found a statistically significant difference of horizontal and vertical fractal dimensions (FDs) using 7T T1 TSE-HR images in low-resolution mode (horizontal: p = 0.04, vertical: p = 0.01). When comparing radiography to the different MRI sequences, we found a statistically significant difference for low- and high-resolution horizontal FDs between radiography and 3T T1 TSE and 7T T1 TSE-HR. Vertical FDs were significantly different only between radiographs and 3T T1 TSE in the high-resolution mode; FSA measures obtained from 3T and 7T MRI are highly dependent on the sequence and reconstruction resolution used, and thus are not easily comparable between MRI systems and applied sequences.

Micro-Finite Element Analysis of the Proximal Femur on the Basis of High-Resolution Magnetic Resonance Images

PURPOSE OF REVIEW

Hip fractures have catastrophic consequences. The purpose of this article is to review recent developments in high-resolution magnetic resonance imaging (MRI)-guided finite element analysis (FEA) of the hip as a means to determine subject-specific bone strength.

RECENT FINDINGS

Despite the ability of DXA to predict hip fracture, the majority of fractures occur in patients who do not have BMD T scores less than - 2.5. Therefore, without other detection methods, these individuals go undetected and untreated. Of methods available to image the hip, MRI is currently the only one capable of depicting bone microstructure in vivo. Availability of microstructural MRI allows generation of patient-specific micro-finite element models that can be used to simulate real-life loading conditions and determine bone strength. MRI-based FEA enables radiation-free approach to assess hip fracture strength. With further validation, this technique could become a potential clinical tool in managing hip fracture risk.

The reproducibility of measuring trabecular bone parameters using a commercially available high-resolution magnetic resonance imaging approach: A pilot study

Bone imaging is currently the best non-invasive way to assess changes to bone associated with aging or chronic disease. However, common imaging techniques such as dual energy x-ray absorptiometry are associated with limitations. Magnetic resonance imaging (MRI) is a radiation-free technique that can measure bone microarchitecture. However, published MRI bone assessment protocols use specialized MRI coils and sequences and therefore have limited transferability across institutions. We developed a protocol on a Siemens 3 Tesla MRI machine, using a commercially available coil (Siemens 15 CH knee coil), and manufacturer supplied sequences to acquire images at the tibia. We tested the reproducibility of the FSE and the GE Axial sequences and hypothesized that both would generate reproducible trabecular bone parameters. Eight healthy adults (age 25.5 ± 5.4 years) completed three measurements of each MRI sequence at the tibia. Each of the images was processed for 8 different bone parameters (such as volumetric bone volume fraction). We computed the coefficient of variation (CV) and intraclass correlation coefficients (ICC) to assess reproducibility and reliability. Both sequences resulted in trabecular parameters that were reproducible (CV <5% for most) and reliable (ICC >80% for all). Our study is one of the first to report that a commercially available MRI protocol can result in reproducible data, and is significant as MRI may be an accessible method to measure bone microarchitecture in clinical or research environments. This technique requires further testing, including validation and evaluation in other populations.

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Trabecular bone is difficult to measure using commercial MRI techniques

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Reproducibility of a MRI protocol measuring trabecular bone was assessed

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Tibia trabecular bone was reproducible using a knee coil and a FSE Axial sequence

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Tibia trabecular bone was reproducible using a knee coil and a GE Axial sequence

Accuracy of MRI-based finite element assessment of distal tibia compared to mechanical testing

High-resolution MRI-derived finite element analysis (FEA) has been used in translational research to estimate the mechanical competence of human bone. However, this method has yet to be validated adequately under in vivo imaging spatial resolution or signal-to-noise conditions. We therefore compared MRI-based metrics of bone strength to those obtained from direct, mechanical testing. The study was conducted on tibiae from 17 human donors (12 males and five females, aged 33 to 88years) with no medical history of conditions affecting bone mineral homeostasis. A 25mm segment from each distal tibia underwent MR imaging in a clinical 3-Tesla scanner using a fast large-angle spin-echo (FLASE) sequence at 0.137mm×0.137mm×0.410mm voxel size, in accordance with in vivo scanning protocol. The resulting high-resolution MR images were processed and used to generate bone volume fraction maps, which served as input for the micro-level FEA model. Simulated compression was applied to compute stiffness, yield strength, ultimate strength, modulus of resilience, and toughness, which were then compared to metrics obtained from mechanical testing. Moderate to strong positive correlations were found between computationally and experimentally derived values of stiffness (R²=0.77, p<0.0001), yield strength (R²=0.38, p=0.0082), ultimate strength (R²=0.40, p=0.0067), and resilience (R²=0.46, p=0.0026), but only a weak, albeit significant, correlation was found for toughness (R²=0.26, p=0.036). Furthermore, experimentally derived yield strength and ultimate strength were moderately correlated with MRI-derived stiffness (R²=0.48, p=0.0022 and R²=0.58, p=0.0004, respectively). These results suggest that high-resolution MRI-based finite element (FE) models are effective in assessing mechanical parameters of distal skeletal extremities.

The Efficacy of Low-intensity Vibration to Improve Bone Health in Patients with End-stage Renal Disease Is Highly Dependent on Compliance and Muscle Response

RATIONAL AND OBJECTIVES

Low intensity vibration (LIV) may represent a nondrug strategy to mitigate bone deficits in patients with end-stage renal disease.

MATERIALS AND METHODS

Thirty end-stage renal patients on maintenance hemodialysis were randomized to stand for 20 minutes each day on either an active or placebo LIV device. Analysis at baseline and completion of 6-month intervention included magnetic resonance imaging (tibia and fibula stiffness; trabecular thickness, number, separation, bone volume fraction, plate-to-rod ratio; and cortical bone porosity), dual-energy X-ray absorptiometry (hip and spine bone mineral density [BMD]), and peripheral quantitative computed tomography (tibia trabecular and cortical BMD; calf muscle cross-sectional area).

RESULTS

Intention-to-treat analysis did not show any significant changes in outcomes associated with LIV. Subjects using the active device and with greater than the median adherence (70%) demonstrated an increase in distal tibia stiffness (5.3%), trabecular number (1.7%), BMD (2.3%), and plate-to-rod ratio (6.5%), and a decrease in trabecular separation (-1.8%). Changes in calf muscle cross-sectional area were associated with changes in distal tibia stiffness (R = 0.85), trabecular bone volume/total volume (R = 0.91), number (R = 0.92), and separation (R = -0.94) in the active group but not in the placebo group. Baseline parathyroid hormone levels were positively associated with increased cortical bone porosity over the 6-month study period in the placebo group (R = 0.55) but not in the active group (R = 0.01). No changes were observed in the nondistal tibia locations for either group except a decrease in hip BMD in the placebo group (-1.7%).

CONCLUSION

Outcomes and adherence thresholds identified from this pilot study could guide future longitudinal studies involving vibration therapy.

MRI assessment of bone structure and microarchitecture

Osteoporosis is a disease of weak bone and increased fracture risk caused by low bone mass and microarchitectural deterioration of bone tissue. The standard-of-care test used to diagnose osteoporosis, dual-energy x-ray absorptiometry (DXA) estimation of areal bone mineral density (BMD), has limitations as a tool to identify patients at risk for fracture and as a tool to monitor therapy response. Magnetic resonance imaging (MRI) assessment of bone structure and microarchitecture has been proposed as another method to assess bone quality and fracture risk in vivo. MRI is advantageous because it is noninvasive, does not require ionizing radiation, and can evaluate both cortical and trabecular bone. In this review article, we summarize and discuss research progress on MRI of bone structure and microarchitecture over the last decade, focusing on in vivo translational studies. Single-center, in vivo studies have provided some evidence for the added value of MRI as a biomarker of fracture risk or treatment response. Larger, prospective, multicenter studies are needed in the future to validate the results of these initial translational studies.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 5 J. MAGN. RESON. IMAGING 2017;46:323-337.

A new magnetic resonance-based technique for high-resolution quantification of amorphous and quasi-amorphous structures

We present a novel, high-resolution magnetic resonance technique, fine structure analysis (FSA) for the quantification and analysis of amorphous and quasi-amorphous biological structures. The one-dimensional technique is introduced mathematically and then applied to one simulated phantom, two physical phantoms and a set of ex vivo biological samples, scanned with interpoint spacings of 0.0038-0.195 mm and cross-sectional sizes of 3 × 3 or 5 × 5 mm. The simulated phantom and one of the physical phantoms consists of randomly arranged beads of known size in two and three dimensions, respectively. The second physical phantom was constructed by etching lines on Perspex. The ex vivo samples are human bone specimens. We show that for all three phantoms, the FSA technique is able to elucidate the average spacing of the structures present within each sample using structural spectroscopy, the smallest of which was 180 µm in size. We further show that in samples of trabecular bone, FSA is able to produce comparable results to micro-computed tomography, the current gold standard for measuring bone microstructure, but without the need for ionizing radiation. Many biological structures are too small to be captured by conventional, clinically deployed medical imaging techniques. FSA has the potential for use in the analysis of pathologies where such small-scale repeating structures are disrupted or their size, and spacing is otherwise altered.

3 Tesla MRI detects deterioration in proximal femur microarchitecture and strength in long-term glucocorticoid users compared with controls

BACKGROUND

Glucocorticoid-induced osteoporosis (GIO) is the most common secondary form of osteoporosis, and glucocorticoid users are at increased risk for fracture compared with nonusers. There is no established relationship between bone mineral density (BMD) and fracture risk in GIO. We used 3 Tesla (T) MRI to investigate how proximal femur microarchitecture is altered in subjects with GIO.

METHODS

This study had institutional review board approval. We recruited 6 subjects with long-term (> 1 year) glucocorticoid use (median age = 52.5 (39.2-58.7) years) and 6 controls (median age = 65.5 [62-75.5] years). For the nondominant hip, all subjects underwent dual-energy x-ray absorptiometry (DXA) to assess BMD and 3T magnetic resonance imaging (MRI, 3D FLASH) to assess metrics of bone microarchitecture and strength.

RESULTS

Compared with controls, glucocorticoid users demonstrated lower femoral neck trabecular number (-50.3%, 1.12 [0.84-1.54] mm(-1) versus 2.27 [1.88-2.73] mm(-1) , P = 0.02), plate-to-rod ratio (-20.1%, 1.48 [1.39-1.71] versus 1.86 [1.76-2.20], P = 0.03), and elastic modulus (-64.8% to -74.8%, 1.54 [1.22-3.19] GPa to 2.31 [1.87-4.44] GPa versus 6.15 [5.00-7.09] GPa to 6.59 [5.58-7.31] GPa, P < 0.05), and higher femoral neck trabecular separation (+192%, 0.705 [0.462-1.00] mm versus 0.241 [0.194-0.327] mm, P = 0.02). There were no differences in femoral neck trabecular thickness (-2.7%, 0.193 [0.184-0.217] mm versus 0.199 [0.179-0.210] mm, P = 0.94) or femoral neck BMD T-scores (+20.7%, -2.1 [-2.8 to -1.4] versus -2.6 [-3.3 to -2.5], P = 0.24) between groups.

CONCLUSION

The 3T MRI can potentially detect detrimental changes in proximal femur microarchitecture and strength in long-term glucocorticoid users.

In vivo measurement reproducibility of femoral neck microarchitectural parameters derived from 3T MR images

PURPOSE

To evaluate the within-day and between-day measurement reproducibility of in vivo 3D MRI assessment of trabecular bone microarchitecture of the proximal femur.

MATERIALS AND METHODS

This Health Insurance Portability and Accountability Act (HIPPA)-compliant, Institutional Review Board (IRB)-approved study was conducted on 11 healthy subjects (mean age = 57.4 ± 14.1 years) with written informed consent. All subjects underwent a 3T MRI hip scan in vivo (0.234 × 0.234 × 1.5 mm) at three timepoints: baseline, second scan same day (intrascan), and third scan 1 week later (interscan). We applied digital topological analysis and volumetric topological analysis to compute the following microarchitectural parameters within the femoral neck: total bone volume, bone volume fraction, markers of trabecular number (skeleton density), connectivity (junctions), plate-like structure (surfaces), plate width, and trabecular thickness. Reproducibility was assessed using root-mean-square coefficient of variation (RMS-CV) and intraclass correlation coefficient (ICC).

RESULTS

The within-day RMS-CVs ranged from 2.3% to 7.8%, and the between-day RMS-CVs ranged from 4.0% to 7.3% across all parameters. The within-day ICCs ranged from 0.931 to 0.989, and the between-day ICCs ranged from 0.934 to 0.971 across all parameters.

CONCLUSION

These results demonstrate high reproducibility for trabecular bone microarchitecture measures derived from 3T MR images of the proximal femur. The measurement reproducibility is within a range suitable for clinical cross-sectional and longitudinal studies in osteoporosis.

Potential of in vivo MRI-based nonlinear finite-element analysis for the assessment of trabecular bone post-yield properties

PURPOSE

Bone strength is the key factor impacting fracture risk. Assessment of bone strength from high-resolution (HR) images have largely relied on linear micro-finite element analysis (μFEA) even though failure always occurs beyond the yield point, which is outside the linear regime. Nonlinear μFEA may therefore be more informative in predicting failure behavior. However, existing nonlinear models applied to trabecular bone (TB) have largely been confined to micro-computed tomography (μCT) and, more recently, HR peripheral quantitative computed tomography (HR-pQCT) images, and typically have ignored evaluation of the post-yield behavior. The primary purpose of this work was threefold: (1) to provide an improved algorithm and program to assess TB yield as well as post-yield properties; (2) to explore the potential benefits of nonlinear μFEA beyond its linear counterpart; and (3) to assess the feasibility and practicality of performing nonlinear analysis on desktop computers on the basis of micro-magnetic resonance (μMR) images obtained in vivo in patients.

METHODS

A method for nonlinear μFE modeling of TB yield as well as post-yield behavior has been designed where material nonlinearity is captured by adjusting the tissue modulus iteratively according to the tissue-level effective strain obtained from linear analysis using a computationally optimized algorithm. The software allows for images at in vivo μMRI resolution as input with retention of grayscale information. Associations between axial stiffness estimated from linear analysis and yield as well as post-yield parameters from nonlinear analysis were investigated from in vivo μMR images of the distal tibia (N = 20; ages: 58-84) and radius (N = 20; ages: 50-75).

RESULTS

All simulations were completed in 1 h or less for 61 strain levels using a desktop computer (dual quad-core Xeon 3.16 GHz CPUs equipped with 40 GB of RAM). Although yield stress and ultimate stress correlated strongly (R(2) > 0.95, p < 0.001) with axial stiffness, toughness correlated moderately at the distal tibia (R(2) = 0.81, p < 0.001) and only weakly at the distal radius (R(2) = 0.34, p = 0.007). Further, toughness was found to vary by up to 16% for bone of very similar axial stiffness (<2%).

CONCLUSIONS

The work demonstrates the practicality of nonlinear μFE simulations at in vivo μMRI resolution, as well as its potential for providing additional information beyond that obtainable from linear analysis. The data suggest that a direct assessment of toughness may provide information not captured by stiffness.

Assessment of trabecular bone yield and post-yield behavior from high-resolution MRI-based nonlinear finite element analysis at the distal radius of premenopausal and postmenopausal women susceptible to osteoporosis

RATIONALE AND OBJECTIVES

To assess the performance of a nonlinear microfinite element model on predicting trabecular bone yield and post-yield behavior based on high-resolution in vivo magnetic resonance images via the serial reproducibility.

MATERIALS AND METHODS

The nonlinear model captures material nonlinearity by iteratively adjusting tissue-level modulus based on tissue-level effective strain. It enables simulations of trabecular bone yield and post-yield behavior from micro magnetic resonance images at in vivo resolution by solving a series of nonlinear systems via an iterative algorithm on a desktop computer. Measures of mechanical competence (yield strain/strength, ultimate strain/strength, modulus of resilience, and toughness) were estimated at the distal radius of premenopausal and postmenopausal women (N = 20, age range 50-75) in whom osteoporotic fractures typically occur. Each subject underwent three scans (20.2 ± 14.5 days). Serial reproducibility was evaluated via coefficient of variation (CV) and intraclass correlation coefficient (ICC).

RESULTS

Nonlinear simulations were completed in an average of 14 minutes per three-dimensional image data set involving analysis of 61 strain levels. The predicted yield strain/strength, ultimate strain/strength, modulus of resilience, and toughness had a mean value of 0.78%, 3.09 MPa, 1.35%, 3.48 MPa, 14.30 kPa, and 32.66 kPa, respectively, covering a substantial range by a factor of up to 4. Intraclass correlation coefficient ranged from 0.986 to 0.994 (average 0.991); CV ranged from 1.01% to 5.62% (average 3.6%), with yield strain and toughness having the lowest and highest CV values, respectively.

CONCLUSIONS

The data suggest that the yield and post-yield parameters have adequate reproducibility to evaluate treatment effects in interventional studies within short follow-up periods.

Micro-finite element analysis applied to high-resolution MRI reveals improved bone mechanical competence in the distal femur of female pre-professional dancers

Micro-finite element analysis applied to high-resolution (0.234-mm length scale) MRI reveals greater whole and cancellous bone stiffness, but not greater cortical bone stiffness, in the distal femur of female dancers compared to controls. Greater whole bone stiffness appears to be mediated by cancellous, rather than cortical bone adaptation.

INTRODUCTION

The purpose of this study was to compare bone mechanical competence (stiffness) in the distal femur of female dancers compared to healthy, relatively inactive female controls.

METHODS

This study had institutional review board approval. We recruited nine female modern dancers (25.7±5.8 years, 1.63±0.06 m, 57.1±4.6 kg) and ten relatively inactive, healthy female controls matched for age, height, and weight (32.1±4.8 years, 1.6±0.04 m, 55.8±5.9 kg). We scanned the distal femur using a 7-T MRI scanner and a three-dimensional fast low-angle shot sequence (TR/TE=31 ms/5.1 ms, 0.234 mm×0.234 mm×1 mm, 80 slices). We applied micro-finite element analysis to 10-mm-thick volumes of interest at the distal femoral diaphysis, metaphysis, and epiphysis to compute stiffness and cross-sectional area of whole, cortical, and cancellous bone, as well as cortical thickness. We applied two-tailed t-tests and ANCOVA to compare groups.

RESULTS

Dancers demonstrated greater whole and cancellous bone stiffness and cross-sectional area at all locations (p<0.05). Cortical bone stiffness, cross-sectional area, and thickness did not differ between groups (>0.08). At all locations, the percent of intact whole bone stiffness for cortical bone alone was lower in dancers (p<0.05). Adjustment for cancellous bone cross-sectional area eliminated significant differences in whole bone stiffness between groups (p>0.07), but adjustment for cortical bone cross-sectional area did not (p<0.03).

CONCLUSIONS

Modern dancers have greater whole and cancellous bone stiffness in the distal femur compared to controls. Elevated whole bone stiffness in dancers may be mediated via cancellous, rather than cortical bone adaptation.

Magnetic resonance of calcified tissues

MRI of the human body is largely made possible by the favorable relaxation properties of protons of water and triacyl glycerides prevalent in soft tissues. Hard tissues--key among them bone--are generally less amenable to measurement with in vivo MR imaging techniques, not so much as a result of the lower proton density but rather due to the extremely short life-times of the proton signal in water bound to solid-like entities, typically collagen, or being trapped in micro-pores. Either mechanism can enhance T2 relaxation by up to three orders of magnitude relative to their soft-tissue counterparts. Detection of these protons requires solid-state techniques that have emerged in recent years and that promise to add a new dimension to the study of hard tissues. Alternative approaches to probe calcified tissues exploit their characteristic magnetic properties. Bone, teeth and extra-osseous calcium-containing biomaterials are unique in that they are more diamagnetic than all other tissues and thus yield information indirectly by virtue of the induced magnetic fields present in their vicinity. Progress has also been made in methods allowing very high-resolution structural imaging of trabecular and cortical bone relying on detection of the surrounding soft-tissues. This brief review, much of it drawn from work conducted in the author's laboratory, seeks to highlight opportunities with focus on early-stage developments for image-based assessment of structure, function, physiology and mechanics of calcified tissues in humans via liquid and solid-state approaches, including proton, deuteron and phosphorus NMR and MRI.

Predicting trabecular bone elastic properties from measures of bone volume fraction and fabric on the basis of micromagnetic resonance images

The relationship between fabric (a measure of structural anisotropy) and elastic properties of trabecular bone was examined by invoking morphology and homogenization theory on the basis of micromagnetic resonance images from the distal tibia in specimens (N = 30) and human subjects (N = 16) acquired at a 160 × 160 × 160 μm(3) voxel size. The fabric tensor was mapped in 7.5 × 7.5 × 7.5 mm(3) cubic subvolumes by a three-dimensional mean-intercept-length method. Elastic constants (three Young's and three shear moduli) were derived from linear microfinite element simulations of three-dimensional grayscale bone volume fraction-mapped images. In the specimen data, moduli fit power laws of bone volume fraction (bone volume/total volume) for all three test directions and subvolumes (R(2) = 0.92-0.98) with exponents ranging from 1.3 to 1.8. Weaker linear relationships were found for the in vivo data because of a narrower range in bone volume/total volume. When pooling the data for all test directions and subvolumes, bone volume/total volume predicted elastic moduli less well in the specimens (mean R(2) = 0.74) and not at all in vivo. A model of bone volume/total volume and fabric was highly predictive of microfinite element-derived Young's moduli: mean R(2) s of 0.98 and 0.82 (in vivo). The results show that fabric, an important predictor of bone mechanical properties, can be assessed in the limited resolution and signal-to-noise ratio regime of micromagnetic resonance images.

Micro-MR imaging-based computational biomechanics demonstrates reduction in cortical and trabecular bone strength after renal transplantation

PURPOSE

To examine the ability of three-dimensional micro-magnetic resonance (MR) imaging-based computational biomechanics to detect mechanical alterations in trabecular bone and cortical bone in the distal tibia of incident renal transplant recipients 6 months after renal transplantation and compare them with bone mineral density (BMD) outcomes.

MATERIALS AND METHODS

The study was approved by the institutional review board and complied with HIPAA guidelines. Written informed consent was obtained from all subjects. Micro-MR imaging of distal tibial metaphysis was performed within 2 weeks after renal transplantation (baseline) and 6 months later in 49 participants (24 female; median age, 44 years; range, 19-61 years) with a clinical 1.5-T whole-body imager using a modified three-dimensional fast large-angle spin-echo pulse sequence. Micro-finite-element models for cortical bone, trabecular bone, and whole-bone section were generated from each image by delineating the endosteal and periosteal boundaries. Mechanical parameters (stiffness and failure load) were estimated with simulated uniaxial compression tests on the micro-finite-element models. Structural parameters (trabecular bone volume fraction [BV/TV, bone volume to total volume ratio], trabecular thickness [TbTh], and cortical thickness [CtTh]) were computed from micro-MR images. Total hip and spine areal BMD were determined with dual-energy x-ray absorptiometry (DXA). Parameters obtained at the follow-up were compared with the baseline values by using parametric or nonparametric tests depending on the normality of data.

RESULTS

All mechanical parameters were significantly lower at 6 months compared with baseline. Decreases in cortical bone, trabecular bone, and whole-bone stiffness were 3.7% (P = .03), 4.9% (P = .03), and 4.3% (P = .003), respectively. Decreases in cortical bone, trabecular bone, and whole-bone failure strength were 7.6% (P = .0003), 6.0% (P = .004), and 5.6% (P = .0004), respectively. Conventional structural measures, BV/TV, TbTh, and CtTh, did not change significantly. Spine BMD decreased by 2.9% (P < .0001), while hip BMD did not change significantly at DXA.

CONCLUSION

MR imaging-based micro-finite-element analysis suggests that stiffness and failure strength of the distal tibia decrease over a 6-month interval after renal transplantation.

Imaging-Based Methods for Non-invasive Assessment of Bone Properties Influenced by Mechanical Loading

PURPOSE

To describe the most common in vivo imaging-based research tools used to assess bone properties that are influenced by mechanical loading associated with exercise, habitual physical activity, or disease states. Bone is a complex metabolically active tissue that adapts to changes in mechanical loading by altering the amount and spatial organization of mineral.

METHOD

Using a narrative review design, the authors provide an overview of bone biology and biomechanics to emphasize the importance of bone size scale, porosity, and degree of mineralization when interpreting measures acquired using quantitative ultrasound (QUS), dual-energy X-ray absorptiometry (DXA), computed tomography (CT), magnetic resonance imaging (MRI), and finite element analysis (FEA). For each imaging modality, basic imaging principles, typical outcome measures associated with changes in mechanical loading, and salient features for physiotherapists are described.

MAIN RESULTS

While each imaging modality has strengths and limitations, currently CT-based methods are best suited for determining the effects of mechanical loading on bone properties-particularly in the peripheral skeleton.

CONCLUSIONS

Regardless of the imaging technology used, the physiotherapist must carefully consider the assumptions of the imaging-based method, the clinical context, the nature of the change in mechanical loading, and the expected time course for change in bone properties.

Purpose: To describe the most common in vivo imaging-based research tools used to assess bone properties that are influenced by mechanical loading associated with exercise, habitual physical activity, or disease states. Bone is a complex metabolically active tissue that adapts to changes in mechanical loading by altering the amount and spatial organization of mineral. Method: Using a narrative review design, the authors provide an overview of bone biology and biomechanics to emphasize the importance of bone size scale, porosity, and degree of mineralization when interpreting measures acquired using quantitative ultrasound (QUS), dual-energy X-ray absorptiometry (DXA), computed tomography (CT), magnetic resonance imaging (MRI), and finite element analysis (FEA). For each imaging modality, basic imaging principles, typical outcome measures associated with changes in mechanical loading, and salient features for physiotherapists are described. Main Results: While each imaging modality has strengths and limitations, currently CT-based methods are best suited for determining the effects of mechanical loading on bone properties—particularly in the peripheral skeleton. Conclusions: Regardless of the imaging technology used, the physiotherapist must carefully consider the assumptions of the imaging-based method, the clinical context, the nature of the change in mechanical loading, and the expected time course for change in bone properties.

Determination of vertebral and femoral trabecular morphology and stiffness using a flat-panel C-arm-based CT approach

The importance of assessing trabecular architecture together with bone mineral density to determine bone stiffness and fracture risk in osteoporosis has been well established. However, no imaging modalities are available to assess trabecular architecture at clinically relevant sites in the axial skeleton. Recently developed flat-panel CT devices, however, offer resolutions that are potentially good enough to resolve bone architecture at these sites. The goal of the present study was to investigate how accurate trabecular architecture and stiffness can be determined based on images from such a device (XperCT, Philips Healthcare). Ten cadaver human C3 vertebrae, twelve T12 vertebrae and 12 proximal femora were scanned with XperCT while mimicking in-vivo scanning conditions and compared to scans of the same bones with microCT. Standard segmentation and morphology quantification algorithms were applied as well as finite element (FE) simulation based on segmented and gray value images. Results showed that mean trabecular separation (Tb.Sp) and number (Tb.N) can be accurately determined at all sites. The accuracy of other parameters, however, depended on the site. For T12 no other structural parameters could be accurately quantified and no FE-results could be obtained from segmented images. When using gray-level images, however, accurate determination of cancellous bone stiffness was possible. For the C3 vertebrae and proximal femora, mean bone volume fraction (BV/TV), Tb.Sp, Tb.N, and anisotropy (C3 only) could be determined accurately. For Tb.Th, structure model index (SMI, femur only), and anisotropy good correlations were obtained but the values were not determined accurately. FE simulations based on segmented images were accurate for the C3 vertebrae, but severely underestimated bone stiffness for the femur. Here also, this was improved by using the gray value models. In conclusion, XperCT does provide a resolution that is good enough to determine trabecular architecture, but the signal to noise ratio is key to the accuracy of the morphology measurement. When the trabeculae are thick e.g. in the femur or the noise is low, e.g. cervical spine, architecture and stiffness could be determined accurately, but when the trabeculae are thin and the noise is high, e.g. thoracic spine, architecture could not be determined accurately and the connectivity was lost and hence no mechanical properties could be calculated directly.

On the significance of motion degradation in high-resolution 3D μMRI of trabecular bone

RATIONALE AND OBJECTIVES

Subtle subject movement during high-resolution three-dimensional micro-magnetic resonance imaging of trabecular bone (TB) causes blurring, thereby rendering the data unreliable for quantitative analysis. In this work, the effects of translational and rotational motion displacements were evaluated qualitatively and quantitatively.

MATERIALS AND METHODS

In experiment 1, motion was induced by applying various simulated and previously observed in vivo trajectories as phase shifts to k-space or rotation angles to k-space segments of a virtually motion-free data set. In experiment 2, images that were visually free of motion artifacts from two groups of 10 healthy individuals, differing in age, were selected to probe the effects of motion on TB parameters. In both experiments, images were rated for motion severity, and the scores were compared to a focus criterion, the normalized gradient squared.

RESULTS

Strong correlations were observed between the motion quality scores and the corresponding normalized gradient squared values (R(2) = 0.52-0.64, P < .01). The results from experiment 1 demonstrated consistently lower image quality and alterations in structural parameters of 9% to 45% with increased amplitude of displacements. In experiment 2, the significant differences in structural parameter group means of the motion-free images were lost upon motion degradation. Autofocusing, a postprocessing correction method, partially recovered the sharpness of the original motion-free images in 13 of 20 subjects.

CONCLUSIONS

Quantitative TB structural measures are highly sensitive to subtle motion-induced degradation, which adversely affects precision and statistical power. The results underscore the influence of subject movement in high-resolution three-dimensional micro-magnetic resonance imaging and its correction for TB structure analysis.

Performance of the MRI-based virtual bone biopsy in the distal radius: serial reproducibility and reliability of structural and mechanical parameters in women representative of osteoporosis study populations

Serial reproducibility and reliability critically determine sensitivity to detect changes in response to intervention and provide a basis for sample size estimates. Here, we evaluated the performance of the MRI-based virtual bone biopsy in terms of 26 structural and mechanical parameters in the distal radius of 20 women in the age range of 50 to 75 years (mean=62.0 years, S.D.=8.1 years), representative of typical study populations in drug intervention trials and fracture studies. Subjects were examined three times at average intervals of 20.2 days (S.D.=14.5 days) by MRI at 1.5 T field strength at a voxel size of 137×137×410 μm(3). Methods involved prospective and retrospective 3D image registration and auto-focus motion correction. Analyses were performed from a central 5×5×5 mm(3) cuboid subvolume and trabecular volume consisting of a 13 mm axial slab encompassing the entire medullary cavity. Whole-volume axial stiffness and sub-regional Young's and shear moduli were computed by finite-element analysis. Whole-volume-derived aggregate mean coefficient of variation of all structural parameters was 4.4% (range 1.8% to 7.7%) and 4.0% for axial stiffness; corresponding data in the subvolume were 6.5% (range 1.6% to 13.0%) for structural, and 5.5% (range 4.6% to 6.5%) for mechanical parameters. Aggregate ICC was 0.976 (range 0.947 to 0.986) and 0.992 for whole-volume-derived structural parameters and axial stiffness, and 0.946 (range 0.752 to 0.991) and 0.974 (range 0.965 to 0.978) for subvolume-derived structural and mechanical parameters, respectively. The strongest predictors of whole-volume axial stiffness were BV/TV, junction density, skeleton density and Tb.N (R(2) 0.79-0.87). The same parameters were also highly predictive of sub-regional axial modulus (R(2) 0.88-0.91). The data suggest that the method is suited for longitudinal assessment of the response to therapy. The underlying technology is portable and should be compatible with all general-purpose MRI scanners, which is appealing considering the very large installed base of this modality.

Quantitative assessment of trabecular bone micro-architecture of the wrist via 7 Tesla MRI: preliminary results

OBJECT

The goal of this study was to determine the feasibility of performing quantitative 7 T magnetic resonance imaging (MRI) assessment of trabecular bone micro-architecture of the wrist, a common fracture site.

MATERIALS AND METHODS

The wrists of 4 healthy subjects (1 woman, 3 men, 28 ± 8.9 years) were scanned on a 7 T whole body MR scanner using a 3D fast low-angle shot (FLASH) sequence (TR/TE = 20/4.5 m s, 0.169 × 0.169 × 0.5 mm). Trabecular bone was segmented and divided into 4 or 8 angular subregions. Total bone volume (TBV), bone volume fraction (BVF), surface-curve ratio (SC), and erosion index (EI) were computed. Subjects were scanned twice to assess measurement reproducibility.

RESULTS

Group mean subregional values for TBV, BVF, SC, and EI (8 subregion analysis) were as follows: 8489 ± 3686, 0.27 ± 0.045, 9.61 ± 6.52; and 1.43 ± 1.25. Within each individual, there was subregional variation in TBV, SC, and EI (>5%), but not BVF (<5%). Intersubject variation (≥12%) existed for all parameters. Within-subject coefficients of variation were ≤10%.

CONCLUSION

This is the first study to perform quantitative 7T MRI assessment of trabecular bone micro-architecture of the wrist. This method could be utilized to study perturbations in bone structure in subjects with osteoporosis or other bone disorders.

Performance of μMRI-Based virtual bone biopsy for structural and mechanical analysis at the distal tibia at 7T field strength

PURPOSE

To assess the performance of a 3D fast spin echo (FSE) pulse sequence utilizing out-of-slab cancellation through phase alternation and micro-magnetic resonance imaging (μMRI)-based virtual bone biopsy processing methods to probe the serial reproducibility and sensitivity of structural and mechanical parameters of the distal tibia at 7.0T.

MATERIALS AND METHODS

The distal tibia of five healthy subjects was imaged at three timepoints with a 3D FSE sequence at 137 × 137 × 410 μm(3) voxel size. Follow-up images were retrospectively 3D registered to baseline images. Coefficients of variation (CV) and intraclass correlation coefficients (ICCs) for measures of scale and topology of the whole tibial trabecular bone (TB) cross-section as well as finite-element-derived Young's and shear moduli of central cuboidal TB subvolumes (8 × 8 × 5 mm(3) ) were evaluated as measures of reproducibility and reliability. Four additional cubic TB subregions (anterior, medial, lateral, and posterior) of similar dimensions were extracted and analyzed to determine associations between whole cross-section and subregional structural parameters.

RESULTS

The mean signal-to-noise ratio (SNR) over the 15 image acquisitions was 27.5 ± 2.1. Retrospective registration yielded an average common analysis volume of 67% across the three exams per subject. Reproducibility (mean CV = 3.6%; range, 1.5%-5%) and reliability (ICCs, 0.95-0.99) of all parameters permitted parameter-based discrimination of the five subjects in spite of the narrow age range (26-36 years) covered. Parameters characterizing topology were better able to distinguish two individuals who demonstrated similar values for scalar measurements (≈ 34% difference, P < 0.001). Whole-section axial stiffness encompassing the cortex was superior at distinguishing two individuals relative to its central subregional TB counterpart (≈ 8% difference; P < 0.05). Interregion comparisons showed that although all parameters were correlated (mean R(2) = 0.78; range 0.57-0.99), the strongest associations observed were those for the erosion index (mean R(2) = 0.95, P ≤ 0.01).

CONCLUSION

The reproducibility and structural and mechanical parameter-based discriminative ability achieved in five healthy subjects suggests that 7T-derived μMRI of TB can be applied towards serial patient studies of osteoporosis and may enable earlier detection of disease or treatment-based effects.

Helmholtz-pair transmit coil with integrated receive array for high-resolution MRI of trabecular bone in the distal tibia at 7T

A Helmholtz-pair local transmit RF coil with an integrated four-element receive array RF coil and foot immobilization platform was designed and constructed for imaging the distal tibia in a whole-body 7T MRI scanner. Simulations and measurements of the B(1) field distribution of the transmit coil are described, along with SAR considerations for operation at 7T. Results of imaging the trabecular bone of three volunteers at 1.5T, 3T and 7T are presented, using identical 1.5T and 3T versions of the 7T four-element receive array. The spatially registered images reveal improved visibility for individual trabeculae and show average gains in SNR of 2.8× and 4.9× for imaging at 7T compared to 3T and 1.5T, respectively. The results thus display an approximately linear dependence of SNR with field strength and enable the practical utility of 7T scanners for micro-MRI of trabecular bone.

Reproducibility of subregional trabecular bone micro-architectural measures derived from 7-Tesla magnetic resonance images

High-resolution magnetic resonance imaging (MRI) of trabecular bone combined with quantitative image analysis represents a powerful technique to gain insight into trabecular bone micro-architectural derangements in osteoporosis and osteoarthritis. The increased signal-to-noise ratio of ultra high-field MR (≥7 Tesla) permits images to be obtained with higher resolution and/or decreased scan time compared to scanning at 1.5/3T. In this small feasibility study, we show high measurement precision for subregional trabecular bone micro-architectural analysis performed on 7T knee MR images. The results provide further support for the use of trabecular bone measures as biomarkers in clinical studies of bone disorders.

Non-destructive characterization of microdamage in cortical bone using low field pulsed NMR

The microcracking and damage accumulation process in human cortical bone was characterized by performing cyclic loading under four-point bending at ambient temperature. A non-destructive nuclear magnetic resonance (NMR) spin-spin (T(2)) relaxation technique was applied to quantify the apparent changes in bone porosity as a function of cyclic loading and prior damage accumulation, first to unloaded cortical bone to quantify the initial porosity and then to fatigued cortical bone that was subjected to cyclic loading to various levels of modulus degradation and microdamage in the form of microcracks. The NMR T(2) relaxation time and amplitude data of the fatigued bone were compared against the undamaged state. The difference in the T(2) relaxation time data was taken as a measure of the increase in pore size, bone porosity or microcrack density due to microdamage induced by cyclic loading. A procedure was developed to deduce the number and size distributions of microcracks formed in cortical bone. Serial sectioning of the fatigued bone showed the formation of microcracks along the cement lines or within the interstitial tissue. The results on the evolution of microdamage derived from NMR measurements were verified by independent experimental measurements of microcrack density using histological characterization techniques. The size distribution and population of the microcracks were then utilized in conjunction with an analytical model to predict the degradation of the elastic modulus of cortical bone as a function of damage accumulation.

Computational biomechanics of the distal tibia from high-resolution MR and micro-CT images

The mechanical properties of bone estimated by micro-finite element (microFE) analysis on the basis of in vivo micro-MR images (microMRIs) of the distal extremities provide a new tool for direct assessment of the mechanical consequences of intervention. However, the accuracy of the method has not previously been investigated. Here, we compared microFE-derived mechanical parameters obtained from microMRIs at 160 microm isotropic voxel size now achievable in vivo with those derived from 25 microm isotropic (reference) microCT images of 30 cadaveric tibiae from 15 donors (4 females and 11 males, aged 55-84 years). Elastic and shear moduli estimated from 5mm(3) subvolumes of trabecular bone (TB) derived from microMRIs were significantly correlated with those derived from volume-matched reference microCT images (R(2)=0.60-0.67). Axial stiffness of whole-bone sections (including both cortical and trabecular compartments) derived from microMR-based models were highly correlated (R(2)=0.85) with those from high-resolution reference images. Further, microFE models generated from microCT images after downsampling to lower resolutions relevant to in vivo microMRI (100-160 microm) showed mechanical parameters to be strongly correlated (R(2)>0.93) with those derived at reference resolution (25 microm). Incorporation of grayscale image information into the microMR-based microFE model yielded slopes closer to unity than binarized models (1.07+/-0.15 vs. 0.71+/-0.11) when correlated with reference subregional elastic and shear moduli. This work suggests that elastic properties of distal tibia can be reliably estimated by microFE analysis from microMRIs obtainable at in vivo resolution.