Quantitative analysis of trabecular microstructure by 400 MHz nuclear magnetic resonance imaging

Micron-resolution Imaging of Cortical Bone under 14 T Ultrahigh Magnetic Field

Compact, mineralized cortical bone tissues are often concealed on magnetic resonance (MR) images. Recent development of MR instruments and pulse techniques has yielded significant advances in acquiring anatomical and physiological information from cortical bone despite its poor 1 H signals. This work demonstrates the first MR research on cortical bones under an ultrahigh magnetic field of 14 T. The 1 H signals of different mammalian species exhibit multi-exponential decays of three characteristic T2 or T2 * values: 0.1-0.5 ms, 1-4 ms, and 4-8 ms. Systematic sample comparisons attribute these T2 /T2 * value ranges to collagen-bound water, pore water, and lipids, respectively. Ultrashort echo time (UTE) imaging under 14 T yielded spatial resolutions of 20-80 microns, which resolves the 3D anatomy of the Haversian canals. The T2 * relaxation characteristics further allow spatial classifications of collagen, pore water and lipids in human specimens. The study achieves a record of the spatial resolution for MR imaging in bone and shows that ultrahigh-field MR has the unique ability to differentiate the soft and organic compartments in bone tissues.

Analytical methodology to measure periodontal bone morphometry following orthodontic tooth movement in mice

Introduction

Basic research in orthodontics is commonly conducted in rodents. However, experimental studies on orthodontic tooth movement (OTM) lack a standard method to examine OTM and periodontal changes. This study describes a unifying protocol for the analysis of OTM and associated bone microarchitectural changes in mice using microcomputed tomography (µCT).

Methods

Mice (10 animals/group) were divided into control and OTM groups. OTM was generated by anchoring a nickel–titanium closed-coil spring to the upper incisors to pull the upper left first molar. A third group of TNFα −/− mice was added since these are known to have slower OTM. Using µCT, we implemented and tested a number of methods to measure OTM distance and examine 3D bone morphometric parameters associated with OTM in mice.

Results

In total, we tested five methods to measure the OTM distance in mice. The results indicated that measuring the intermolar diastema, and assessing tooth movement relative to the anterior root of the zygomatic arch, displayed the lowest standard deviation and enabled optimal detection of intergroup differences. We also developed two protocols for µCT analysis of the periradicular bone that yielded no false-positive results. Our results revealed that including the width of the periodontal ligament rather than excluding it from the region of interest in mice detected more statistically significant differences in the morphometric parameters between the OTM and control sides and between WT and TNFα −/− mice despite more subtle differences.

Conclusions

We, therefore, propose new guidelines for a standardized μCT-based method to analyse OTM and the extent of the periradicular bone structural changes in mice.

Update on bone density measurements and their interpretation in children and adolescents

Following the increased awareness about the central role of the pediatric age in building bone for life, clinicians face more than ever the necessity of assessing bone health in pediatric subjects at risk for early bone mass derangements or in healthy children, in order to optimize their bone mass accrual and prevent osteoporosis. Although the diagnosis of osteoporosis is not made solely upon bone mineral density measurements during growth, such determination can be very useful in the follow-up of pediatric patients with primary and secondary osteoporosis. The ideal instrument would give information on the mineral content and density of the bone, and on its architecture. It should be able to perform the measurements on the skeletal sites where fractures are more frequent, and it should be minimally invasive, accurate, precise and rapid. Unfortunately, none of the techniques currently utilized fulfills all requirements. In the present review, we focus on the pediatric use of dual-energy X-ray absorptiometry (DXA), quantitative computed tomography (QCT), peripheral QCT (pQCT), and magnetic resonance imaging (MRI), highlighting advantages and limits for their use and providing indications for bone densitometry interpretation and of vertebral fractures diagnosis in pediatric subjects.

Cylinders or walls? A new computational model to estimate the MR transverse relaxation rate dependence on trabecular bone architecture

OBJECTIVE

Bone density is distributed in a complex network of interconnecting trabecular plates and rods that are interspersed with bone marrow. A computational model to assess the dependence of the relaxation rate on the geometry of bone can consider the distribution of bone material in the form of two components: cylinders and open walls (walls with gaps). We investigate whether the experimentally known dependence of the transverse relaxation rate on the trabecular bone structure can be usefully interpreted in terms of these two components.

MATERIALS AND METHODS

We established a computer model based on an elementary computational cell. The model includes a variable number of open walls and infinitely long cylinders as well as multiple geometric parameters. The transverse relaxation rate is computed as a function of these parameters. Within the model, increasing the trabecular spacing with a fixed trabecular radius is equivalent to thinning the trabeculae while maintaining constant spacing.

RESULTS

Increasing the number of cylinder and wall gap elements beyond their nearest neighbors does not change the transverse relaxation rate. Although the absolute contribution to the relaxation due to open walls is on average more important than that due to cylinders, the latter drops off rapidly. The change on transverse relaxation rate is larger for changing cylinder geometry than for changing wall geometry, as it can be seen from the effect on the relaxation rate when trabecular spacing is varied, compared to varying the size of wall gaps.

CONCLUSION

Our results provide strong evidence that trabecular thinning, which is associated with increasing age, decreases the relaxation rates. The effect of thinning plates and rods on the transverse relaxation can be understood in terms of simple cylinders and open walls. A reduction in the relaxation rate can be seen as an indication of thinning cylinders, corresponding to reduced bone stability and ultimately, osteoporosis.

Imaging technologies for assessment of skeletal health in men

Conventional radiography can detect most fractures, evaluate their healing, and visualize characteristic skeletal abnormalities for some metabolic bone diseases. Dual-energy X-ray absorptiometry (DXA) is used to measure areal bone mineral density (BMD) in order to diagnose osteoporosis, estimate fracture risk, and monitor changes in BMD over time. Vertebral fracture assessment by DXA can diagnose vertebral fractures with less ionizing radiation, greater patient convenience, and lower cost than conventional radiography. Quantitative computed tomography (QCT) measures volumetric BMD separately in cortical and trabecular bone compartments. High resolution peripheral QCT and high resolution magnetic resonance imaging are noninvasive research tools that assess the microarchitecture of bone. The use of these technologies and others has been associated with special challenges in men compared with women, provided insights into differences in the pathogenesis of osteoporosis in men and women, and enhanced understanding of the mechanisms of action of osteoporosis treatments.

Microarchitectural changes in the aging skeleton

The age-related reduction in bone mass is disproportionally related to skeletal weakening, suggesting that microarchitectural changes are also important determinants of bone quality. The study of cortical and trabecular microstructure, which for many years was mainly based on two-dimensional histologic and scanning electron microscopy imaging, gained a tremendous momentum in the last decade and a half, due to the introduction of microcomputed tomography (μCT). This technology provides highly accurate qualitative and quantitative analyses based on three-dimensional images at micrometer resolution, which combined with finite elemental analysis predicts the biomechanical implications of microstructural changes. Global μCT analyses of trabecular bone have repeatedly suggested that the main age-related change in this compartment is a decrease in trabecular number with unaltered, or even increased, trabecular thickness. However, we show here that this may result from a bias whereby thick trabeculae near the cortex and the early clearance of thin struts mask authentic trabecular thinning. The main cortical age-related change is increased porosity due to negatively balanced osteonal remodeling and expansion of Haversian canals, which occasionally merge with endosteal and periosteal resorption bays, thus leading to rapid cortical thinning and cortical weakening. The recent emergence of CT systems with submicrometer resolution provides novel information on the age-related decrease in osteocyte lacunar density and related micropetrosis, the result of lacunar hypermineralization. Last but not least, the use of the submicrometer CT systems confirmed the occurrence of microcracks in the skeletal mineralized matrix and vastly advanced their morphologic characterization and mode of initiation and propagation.

ASSESSMENT AND CLASSIFICATION OF MECHANICAL STRENGTH COMPONENTS OF HUMAN FEMUR TRABECULAR BONE USING DIGITAL IMAGE PROCESSING AND NEURAL NETWORKS

In this work, the assessment of the mechanical strength of human femur trabecular bone and its classification into normal or abnormal are carried out using digital image processing and neural networks. The mechanical strength components of femur trabeculae, such as primary compressive (PC), primary tensile (PT), secondary tensile (ST), and Ward's triangle (WT), are delineated by the semiautomatic image processing procedure from the planar radiographic images (N = 90) of subjects that are acquired under controlled clinical settings. Parameters such as apparent mineralization and total area of the individual mechanical strength components are calculated for normal and abnormal samples. The data are trained with neural networks and validated. The classifications are carried out using feed-forward neural networks trained with the standard backpropagation algorithm. The abnormal and normal outputs are validated by sensitivity and specificity measurements. The observation shows that the investigation of bone mechanical strength at the various strength components is useful in classifying normal and abnormal human femur trabeculae from conventional radiographs. Furthermore, the results confirm the effectiveness of the neural network–based classification of femur trabeculae into normal and abnormal conditions. The sensitivity and specificity are found to be 100% and 80%, respectively. In this paper, the methodology, data collection procedures, and neural network–based analysis and results are discussed in detail.

QUALITATIVE ASSESSMENT OF TENSILE STRENGTH COMPONENTS OF HUMAN FEMUR TRABECULAR BONE USING RADIOGRAPHIC IMAGING AND SPECTRAL ANALYSIS

In this work, the primary and secondary tensile strength components of human femur trabecular bone are qualitatively assessed using planar radiographic images and spectral analysis. Normal and abnormal femur trabecular images (N = 40) were recorded using planar radiography following standard image acquisition protocol. From the images, the tensile strength components of the trabeculae are delineated using image processing procedures and are then subjected to Fourier transform. The zero (DC), First (FMOI), and Second Moments of Inertia (SMOI) are the parameters considered and are correlated with presence and absence of mineralization in the image. Results show that the values of moments correlate well with percentage mineralization in normal images when compared to abnormal images for both primary and secondary tensile strength components. Further, no or poor correlations were found for abnormals in all cases. Among all, the values of second moment showed highest correlation in the secondary tensile region. In this paper the objectives, methodology, significance results and the conclusions are presented.

Methods for assessing bone quality: a review

BACKGROUND

Bone mass, geometry, and tissue material properties contribute to bone structural integrity. Thus, bone strength arises from both bone quantity and quality. Bone quality encompasses the geometric and material factors that contribute to fracture resistance.

QUESTIONS/PURPOSES

This review presents an overview of the methods for assessing bone quality across multiple length scales, their outcomes, and their relative advantages and disadvantages.

METHODS

A PubMed search was conducted to identify methods related to bone mechanical testing, imaging, and compositional analysis. Using various exclusion criteria, articles were selected for inclusion.

RESULTS

Methods for assessing mechanical properties include whole-bone, bulk tissue, microbeam, and micro- and nanoindentation testing techniques. Outcomes include structural strength and material modulus. Advantages include direct assessment of bone strength; disadvantages include specimen destruction during testing. Methods for characterizing bone geometry and microarchitecture include quantitative CT, high-resolution peripheral quantitative CT, high-resolution MRI, and micro-CT. Outcomes include three-dimensional whole-bone geometry, trabecular morphology, and tissue mineral density. The primary advantage is the ability to image noninvasively; disadvantages include the lack of a direct measure of bone strength. Methods for measuring tissue composition include scanning electron microscopy, vibrational spectroscopy, nuclear magnetic resonance imaging, and chemical and physical analytical techniques. Outcomes include mineral density and crystallinity, elemental composition, and collagen crosslink composition. Advantages include the detailed material characterization; disadvantages include the need for a biopsy.

CONCLUSIONS

Although no single method can completely characterize bone quality, current noninvasive imaging techniques can be combined with ex vivo mechanical and compositional techniques to provide a comprehensive understanding of bone quality.

Advanced imaging assessment of bone fragility in glucocorticoid-induced osteoporosis

Advanced bone imaging techniques provide structural information, beyond bone mineral density (BMD), and growing evidence indicates that BMD only partially explains bone strength and fracture resistance. Assessing glucocorticoid-induced osteoporosis (GIO) is important, especially the documentation of glucocorticoid (GC) impact on trabecular and cortical bone and on macro and microstructural features. Advanced methods for assessing macrostructure of bone include volumetric quantitative computed tomography (vQCT), high-resolution computed tomography (hrCT), and high-resolution magnetic resonance imaging (hrMRI). The methods for assessing bone microstructure include micro computed tomography (μCT) and micro magnetic resonance imaging (μMRI). Many advanced imaging techniques have been used in vitro and in vivo to examine structural effects of GIO in animals and in humans, and these applications are explored in this review. In human in vitro studies, investigators have used standard bone histomorphometry and μCT to compare trabecular microarchitecture and bone remodeling in postmenopausal women and in males with GIO, and have found that high-dose GC produces dramatic bone loss, accompanied by major reduction in trabecular connectivity and increases in trabecular perforations. In animal studies, investigators have used standard histomorphometry along with pQCT, vQCT, hrMRI or μCT to examine GIO in a variety of animal models including rats, minipigs and sheep. They generally have found excellent relationships between treatment-induced structural changes assessed by these advanced imaging techniques and changes in BMD and biomechanical properties. They also have examined various therapeutic interventions in animals and monitored their efficacy using quantitative imaging methods. In human in vivo studies, investigators have serially examined postmenopausal women and males with GIO in order to assess the extent of skeletal deterioration and to determine the best advanced measures of BMD and structure, with which to monitor disease activity and therapeutic response, and to predict fracture risk. They generally have found that bone density and structural measures obtained by pQCT, vQCT and hrMRI contributed substantially to understanding the skeletal effects of glucocorticoids and to predicting the risk of fracture in human GIO. These animal and human applications, illustrating advanced imaging in GIO, are still in early stages of development. However, as discussed in this review, the novelty and power of the imaging approaches are compelling, and their utility is promising.

Implications of noise and resolution on mechanical properties of trabecular bone estimated by image-based finite-element analysis

Recent advances in micro-magnetic resonance imaging (microMRI) now allow noninvasive assessment of mechanical properties of trabecular bone (TB) in vivo by micro finite-element analysis. The first aim of this work was to address the implications of limited resolution and signal-to-noise ratio on elastic properties of TB derived under conditions of in vivo imaging via simulation at various resolutions and noise levels on the basis of models derived from microCT images at 21 microm isotropic voxel size from cores of cadaveric human TB (n = 13) from three anatomic sites. The second aim was to compare how elastic constants derived from actual MR images at 9.4 Tesla at 50 microm isotropic voxel size compare with those from high-resolution microCT. Elastic moduli computed from simulated in vivo microMR images were highly correlated with those obtained from microCT (R(2) = 0.99) and the data were relatively immune to noise. Correlations of similar strength were obtained between estimated moduli from microCT and acquired high-field MR images. Systematic errors manifesting in significant deviations of the slopes from unity are caused by higher apparent bone-volume fraction of the MR images but can potentially be corrected with appropriate histogram-standardization techniques.

Spatial autocorrelation and mean intercept length analysis of trabecular bone anisotropy applied to in vivo magnetic resonance imaging

Osteoporosis is characterized by bone loss and deterioration of the trabecular bone (TB) architecture that leads to impaired overall mechanical strength of the bone. Bone mineral density (BMD) measured by dual-energy x-ray absorptiometry is currently the standard clinical metric assessing bone integrity but it fails to capture the structural changes in the TB. Recent research suggests that structure contributes to bone strength in a manner complementary to BMD. Besides parameters of scale such as the mean TB thickness and mean bone volume fraction, parameters describing the anisotropy of the trabecular architecture play an important role in the characterization of TB since trabeculae are preferentially oriented along the direction of local loading. Therefore, the degree of structural anisotropy is of pivotal importance to the bone's mechanical competence. The most common method for measuring structural anisotropy of TB is the mean-intercept length (MIL). In this work we present a method, based on the three-dimensional spatial autocorrelation function (ACF), for mapping of the full structural anisotropy ellipsoid of both TB thickness and spacing and we examine its performance as compared to that of MIL. Not only is the ACF method faster by several orders of magnitude, it is also considerably more robust to noise. Further, it is applicable at lower spatial resolution and is relatively insensitive to image shading. The chief reason for ACF's superior performance is that it does not require binarization, which is difficult to achieve in the limited spatial regime of in vivo magnetic resonance imaging. MIL and ACF have been applied to high-resolution magnetic resonances images of the tibia in a group of ten healthy postmenopausal women by comparing the structural anisotropy and principal direction of the computed fabric tensor for each method. While there is fair agreement between the two methods, ACF analysis yielded greater anisotropy than MIL for both TB thickness and spacing. There was good agreement between the two techniques as far as the eigenvectors of the fabric ellipsoids were concerned, which parallel the bone's macroscopic axis.

Development of a compact MRI system for measuring the trabecular bone microstructure of the finger

A compact MRI system for measuring the trabecular bone (TB) microstructure of the finger using a high-field-strength (1.0T) permanent magnet was developed. The entire system was installed in a 0.6 mx1.2 m space. One male and 36 female subjects participated in the imaging experiments. The TB of the distal phalanx of the middle finger was imaged at a voxel resolution of (160 microm)3 using a three-dimensional (3D) driven equilibrium spin-echo (SE) imaging sequence (imaging time=approximately 14 min). The image data sets obtained yielded two distinct peaks for the bone and marrow in image intensity histograms when no motion was present. The structural parameters obtained through 3D image analysis show that this compact system is potentially useful for evaluating bone quality.

Structural and functional assessment of trabecular and cortical bone by micro magnetic resonance imaging

Osteoporosis is a multifactorial disorder of bone mineral homeostasis affecting the elderly. It is a major public health issue with significant socioeconomic consequences. Recent findings suggest that bone loss-the key manifestation of the disease-is accompanied by architectural deterioration, both affecting the bone's mechanical competence and susceptibility to fracture. This article reviews the potential of quantitative micro MRI (mu-MRI), including a discussion of the technical requirements for image acquisition, processing, and analysis for assessing the architectural implications of osteoporosis and as a means to monitor the response to treatment. With current technology, the resolution achievable in clinically acceptable scan times and necessary signal-to-noise ratio (SNR) is comparable to trabecular thickness. This limited spatial resolution regime demands processing and analysis algorithms designed to operate under such limiting conditions. It is shown that three different classes of structural parameters can be distinguished, characterizing scale, topology, and orientation. There is considerable evidence that osteoporotic bone loss affects all three classes but that topological changes, resulting from conversion of trabecular plates to rods, with the latter's eventual disconnection, are particularly prominent. Clinical applications discussed can be divided into those dealing with assessment of osteoporotic fracture risk as opposed to the study of the effect of disease progression and regression in response to treatment. Current data suggest that noninvasive assessment of cortical and trabecular bone (TB) architecture by mu-MRI may provide new surrogate endpoints to assess the efficacy of intervention in osteoporosis treatment and prevention.

Quantitative MRI for the assessment of bone structure and function

Osteoporosis is the most common degenerative disease in the elderly. It is characterized by low bone mass and structural deterioration of bone tissue, leading to morbidity and increased fracture risk in the hip, spine and wrist-all sites of predominantly trabecular bone. Bone densitometry, currently the standard methodology for diagnosis and treatment monitoring, has significant limitations in that it cannot provide information on the structural manifestations of the disease. Recent advances in imaging, in particular MRI, can now provide detailed insight into the architectural consequences of disease progression and regression in response to treatment. The focus of this review is on the emerging methodology of quantitative MRI for the assessment of structure and function of trabecular bone. During the past 10 years, various approaches have been explored for obtaining image-based quantitative information on trabecular architecture. Indirect methods that do not require resolution on the scale of individual trabeculae and therefore can be practiced at any skeletal location, make use of the induced magnetic fields in the intertrabecular space. These fields, which have their origin in the greater diamagnetism of bone relative to surrounding marrow, can be measured in various ways, most typically in the form of R2', the recoverable component of the total transverse relaxation rate. Alternatively, the trabecular network can be quantified by high-resolution MRI (micro-MRI), which requires resolution adequate to at least partially resolve individual trabeculae. Micro-MRI-based structure analysis is therefore technically demanding in terms of image acquisition and algorithms needed to extract the structural information under conditions of limited signal-to-noise ratio and resolution. Other requirements that must be met include motion correction and image registration, both critical for achieving the reproducibility needed in repeat studies. Key clinical applications targeted involve fracture risk prediction and evaluation of the effect of therapeutic intervention.

Image segmentation of trabecular spongiosa by visual inspection of the gradient magnitude

Recent advances in physical models of skeletal dosimetry utilize high-resolution 3-dimensional microscopic computed tomography images of trabecular spongiosa. These images are coupled to radiation transport codes to assess energy deposition within active bone marrow and trabecular endosteum. These transport codes rely primarily on the segmentation of the spongiosa images into bone and marrow voxels. Image thresholding has been the segmentation of choice for bone sample images because of its extreme simplicity. However, the ability of the segmentation to reproduce the physical boundary between bone and marrow depends on the selection of the threshold value. Statistical models, as well as visual inspection of the image, have been employed extensively to determine the correct threshold. Both techniques are affected by partial volume effect and can provide unexpected results if performed without care. In this study, we propose a new technique to threshold trabecular spongiosa images based on visual inspection of the image gradient magnitude. We first show that the gradient magnitude of the image reaches a maximum along a surface that remains almost independent of partial volume effect and that is a good representation of the physical boundary between bone and marrow. A computer program was then developed to allow a user to compare the position of the iso-surface produced by a threshold with the gradient magnitude. The threshold that produces the iso-surface that best coincides with the maximum gradient is chosen. The technique was finally tested with a set of images of a true bone sample with different resolutions, as well as with three images of a cube of Duocell aluminium foam of known mass and density. Both tests demonstrate the ability of the gradient magnitude technique to retrieve sample volumes or media volume fractions with 1% accuracy at 30 microm voxel size.

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.

Non-destructive studies of tissue-engineered phalanges by magnetic resonance microscopy and X-ray microtomography

One of the intents of tissue engineering is to fabricate biological materials for the augmentation or replacement of impaired, damaged, or diseased human tissue. In this context, novel models of the human phalanges have been developed recently through suturing of polymer scaffolds supporting osteoblasts, chondrocytes, and tenocytes to mimic bone, cartilage, and tendon, respectively. Characterization of the model constructs has been accomplished previously through histological and biochemical means, both of which are necessarily destructive to the constructs. This report describes the application of two complementary, non-destructive, non-invasive techniques, magnetic resonance microscopy (MRM) and X-ray microtomography (XMT or quantitative computed tomography), to evaluate the spatial and temporal growth and developmental status of tissue elements within tissue-engineered constructs obtained after 10 and 38 weeks of implantation in athymic (nude) mice. These two times represent respective points at which model middle phalanges are comprised principally of organic components while being largely unmineralized and later become increasingly more mineralized. The spatial distribution of mineralized deposits within intact constructs was readily detected by XMT (qCT) and was comparable to low intensity zones observed on MRM hydration maps. Moreover, the MRM-derived hydration values for mineralized zones were inversely correlated with mineral densities measured by XMT. In addition, the MRM method successfully mapped fat deposits, collagenous tissues, and the hydration state of the soft tissue elements comprising the specimens. These results support the application of non-destructive, non-invasive, quantitative MRM and XMT for the evaluation of constituent tissue elements within complex constructs of engineered implants.

Implications of pulse sequence in structural imaging of trabecular bone

PURPOSE

To investigate the SNR and image properties of 3D steady-state free precession (SSFP), fast large-angle spin echo (FLASE), gradient-recalled acquisition in steady state (GRASS), and spoiled GRASS (SPGR) for structural imaging of trabecular bone (TB).

MATERIALS AND METHODS

SNR was examined theoretically and experimentally on phantoms, bone specimens, and in vivo. The bone volume fraction, TB thickness, and echo time (TE) dependence of the thickness were compared. The trabecula was modeled as a cylinder in simulations to examine the intra-voxel spin-dephasing in SSFP and GRASS. Images were acquired on a 1.5 T Siemens Sonata system (40 mT/m maximum gradient, 200 T/m/s peak slew rate).

RESULTS

Within the hardware and safety limit constraints, SNR of FLASE was superior, followed by SSFP, GRASS, and SPGR. The trabecular thickness derived from gradient-echo images was 10-45% greater than that obtained with FLASE. Conversely, SSFP images delineated partial volume trabeculae better than FLASE. Simulations indicated that the artifactual thickening was more severe in SSFP than in GRASS, which was attributed to off-resonance effects from the induced gradients at the bone/marrow interface.

CONCLUSION

FLASE had the highest SNR and was insensitive to susceptibility dephasing. Although SSFP has superior SNR compared to GRASS, off-resonance effects and duty cycle limitations may compromise its practicality in this application. Inc.

Stereological measures of trabecular bone structure: comparison of 3D micro computed tomography with 2D histological sections in human proximal tibial bone biopsies

Stereology applied on histological sections is the 'gold standard' for obtaining quantitative information on cancellous bone structure. Recent advances in micro computed tomography (microCT) have made it possible to acquire three-dimensional (3D) data non-destructively. However, before the 3D methods can be used as a substitute for the current 'gold standard' they have to be verified against the existing standard. The aim of this study was to compare bone structural measures obtained from 3D microCT data sets with those obtained by stereology performed on conventional histological sections using human tibial bone biopsies. Furthermore, this study forms the first step in introducing the proximal tibia as a potential bone examination location by peripheral quantitative CT and CT. Twenty-nine trabecular bone biopsies were obtained from autopsy material at the medial side of the proximal tibial metaphysis. The biopsies were embedded in methylmetacrylate before microCT scanning in a Scanco microCT 40 scanner at a resolution of 20 x 20 x 20 microm3, and the 3D data sets were analysed with a computer program. After microCT scanning, 16 sections were cut from the central 2 mm of each biopsy and analysed with a computerized method. Trabecular bone volume (BV/TV) and connectivity density (CD) were estimated in both modalities, whereas trabecular bone pattern factor (TBPf) was estimated on the histological sections only. Trabecular thickness (Tb.Th), number (Tb.N) and separation (Tb.Sp), and structure model index (SMI) were estimated with the microCT method only. Excellent correlations were found between the two techniques for BV/TV (r = 0.95) and CD (r = 0.95). Additionally, an excellent relationship (r = 0.95) was ascertained between TBPf and SMI. The study revealed high correlations between measures of bone structure obtained from conventional 2D sections and 3D microCT data. This indicates that 3D microCT data sets can be used as a substitute for conventional histological sections for bone structural evaluations.

Comparison of high-resolution MRI, optical microscopy and SEM for quantitation of trabecular architecture in the rat femur

Magnetic resonance imaging (MRI) has been used to analyze trabecular bone architecture in femur heads taken from adult Wistar rats. The aim of this study was to validate the use of MRI in assessing trabecular structure and morphology by comparing standard measures of bone morphology in the rat femur obtained from high resolution MRI with those obtained by conventional optical microscopy and by scanning electron microscopy (SEM). MR images were obtained on a Bruker 4.7 T micro-imaging system using a three-dimensional spin echo sequence with spatial resolution of 23 microm in-plane and a slice thickness of 39 microm. Optical images were obtained by de-calcifying the bone in EDTA and then sectioning 5-microm-thick slices. SEM images were obtained from bone embedded in epoxy resin with surface preparation by diamond polishing. Values of standard bone morphological parameters were compared and correlation coefficients between the MRI and the optical- and SEM-derived measures of morphology were calculated. Partial volume effects in MRI were minimized in this study by the use of very thin slices, yielding better agreement with optical- and SEM-derived measures of trabecular bone morphology than have been obtained in previous studies. Correlations between the MRI and optical data were significantly lower than those between the MRI and SEM data. Effects of de-calcification were also investigated. The results indicate that comparison of MRI with thin (de-calcified) optical images may be inherently flawed due to the destructive de-calcification and sectioning process used to prepare samples for the optical imaging.

Advances in imaging: impact on studying craniofacial bone structure

Methods for measuring the structure of craniofacial bones are discussed in this paper. In addition to the three-dimensional macro-structure of the craniofacial skeleton, there is considerable interest in imaging the bone at a microscopic resolution in order to depict the micro-architecture of the trabecular bone itself. In addition to the density of the bone, the microarchitecture reflects bone quality. An understanding of bone quality and density changes has implications for a number of craniofacial pathologies, as well as for implant design and understanding the biomechanical function and loading of the jaw. Trabecular bone micro-architecture has been recently imaged using imaging methods such as micro-computed tomography, magnetic resonance imaging, and the images have been used in finite element models to assess bone mechanical properties. In this paper, some of the recent advances in micro-computed tomography and magnetic resonance imaging are reviewed, and their potential for imaging the trabecular bone in mandibular bones is presented. Examples of in vitro and in vivo images are presented.

Noninvasive assessment of bone structure

Bone fragility is determined by bone mass and trabecular structure. While bone mass can be readily measured as bone density, bone trabecular structure cannot be easily assessed by currently available methods. The realization of the importance of bone structure in determining fracture risk has led to the development of several imaging modalities aimed at evaluating the contribution of bone quality to its biomechanical strength and fragility. High-resolution magnetic resonance imaging and computed tomography have limited spatial resolution and high cost but have a potential to generate true three-dimensional images of trabecular structure in vivo. Bone radiographs subjected to various forms of texture analysis have higher resolution and lower cost but provide only a two-dimensional representation of bone structure. Both two- and three-dimensional methods have been shown to predict biomechanical strength in vitro and to differentiate between subjects with and without fractures in vivo. Therefore, all of these methods deserve closer evaluation and also need further technical improvements before they can be considered for use in clinical practice.

Local 3D scaling properties for the analysis of trabecular bone extracted from high-resolution magnetic resonance imaging of human trabecular bone: comparison with bone mineral density in the prediction of biomechanical strength in vitro

RATIONALE AND OBJECTIVES

A novel, nonlinear morphologic measure [DeltaP(alpha)] based on local 3D scaling properties was applied to high-resolution magnetic resonance images (HR-MRI) of human trabecular bone to predict biomechanical strength in vitro.

METHODS

We extracted DeltaP(alpha) and traditional morphologic parameters (apparent trabecular volume fraction, apparent trabecular separation) from HR-MR images of 32 femoral and 13 spinal bone specimens. Furthermore, bone mineral density (BMD) and maximum compressive strength (MCS) were determined. The morphologic measures were compared with BMD in predicting the biomechanical strength.

RESULTS

In the vertebral (femoral) specimens, R2 for MCS versus DeltaP(alpha) was 0.87 (0.61) (P < 0.001). Correlation between BMD and MCS was 0.53 (P = 0.05) (0.79 [P < 0.001]) for the vertebral (femoral) specimens. For the femoral specimens, prediction of MCS could be improved further by combining BMD and morphologic parameters by multiple regression (R2 = 0.88).

CONCLUSIONS

Morphologic measures extracted from HR-MRI considering local 3D-scaling properties can be used to predict biomechanical properties of bone in vitro. They are superior to 2-dimensional standard linear morphometric measures and, depending on the anatomic location, more reliably predict bone strength as measured by MCS than does BMD.

Local differences in the trabecular bone structure of the proximal femur depicted with high-spatial-resolution MR imaging and multisection CT

RATIONALE AND OBJECTIVES

The authors performed this study to investigate structural variations in the trabecular bone of the proximal femur at high-resolution magnetic resonance (MR) imaging and high-resolution multisection computed tomography (CT).

MATERIALS AND METHODS

Bone mineral density (BMD) was measured in 36 proximal human femur specimens by using dual x-ray absorptiometry. High-resolution MR imaging was performed at 1.5 T with an in-plane spatial resolution of 0.195 x 0.195 mm and a section thickness of 0.3 and 0.9 mm. Multisection CT was performed with an ultra-high-resolution protocol; images were obtained with an in-plane spatial resolution of 0.25 mm and a section thickness of 1 mm. In a subset of these specimens, micro CT was performed with an isotropic spatial resolution of 30 microm. Identical regions of interest (ROIs) were used to analyze images obtained with MR imaging, multisection CT, and micro CT. Trabecular bone structural parameters were obtained, and the parameters from the individual imaging modalities and BMD were correlated.

RESULTS

Significant differences concerning the trabecular microarchitecture between the individual ROIs were demonstrated with multisection CT and MR imaging. A number of the correlations between structural parameters derived with multisection CT, MR imaging, micro CT, and BMD measurements were significant. For MR imaging, threshold technique and section thickness had an effect on structural parameters.

CONCLUSION

Structural parameters obtained in the proximal femur with multisection CT and high-resolution MR imaging show regional differences. These techniques may be useful for depicting the trabecular architecture in the diagnosis of osteoporosis.

Role of magnetic resonance for assessing structure and function of trabecular bone

The strength of trabecular bone and its resistance to fracture traditionally have been associated with apparent density. This paradigm assumes that neither the ultrastructural nor microstructural make-up of the bone is altered during aging and osteoporosis. During the past decade there has been growing evidence from both laboratory and clinical studies against this view. Recent advances in noninvasive imaging technology, notably micro-magnetic resonance imaging (micro MRI) and computed tomography, offer an opportunity to test the hypothesis that architecture is an independent contributor to bone strength. MRI appears to be ideally suited for this task because bone marrow has uniform high signal intensity while bone appears with background intensity, thus yielding a binary system tomographic system. However, in vivo trabecular bone imaging is hampered by the limited signal-to-noise ratio that precludes voxel sizes much smaller than trabecular thickness, which would be required to yield a bimodal intensity histogram for segmentation of the image into bone and marrow. The resulting partial volume blurring leads to fuzzy boundaries. Successful structure analysis thus demands more elaborate processing strategies. This article reviews new approaches conceived in the authors' laboratory toward acquisition, processing, and structural analysis of trabecular bone images in the limited spatial resolution regimen of in vivo micro MRI. These methods are shown to provide detailed insight into the three-dimensional trabecular network topology and scale at the distal radius or distal tibia that typically serve as surrogate sites. The micro MRI-derived structural parameters are shown to be associated with the bone's biomechanical properties and fracture resistance. Further, the technology has advanced to a stage permitting serial studies in laboratory animals and humans as a means to evaluate the effects of treatment. The method currently is confined to peripheral skeletal sites, and its extension to typical fracture sites such as the proximal femur hinges on further advances in detection sensitivity.

Analysis of trabecular bone texture by modified Hurst orientation transform method

There is a growing need for noninvasive and inexpensive methods that can effectively be used on a large scale, to detect an onset of early osteoarthritis in human knee joints. Of many possible options, fractal analysis of two-dimensional projection x-ray images of trabecular bone (TB) texture, appears as one of the best approaches. However, there are some problems associated with the characterization of the roughness and anisotropy of the bone texture. To resolve these problems, a modified Hurst orientation transform (HOT) method, previously developed by the authors, has been used in this study. The advantages of the HOT method over other techniques used to analyze bone texture, are that it calculates a two-dimensional fractal dimension in all possible directions and also provides a measure of anisotropy for both surfaces exhibiting strong anisotropy and surfaces exhibiting weak anisotropy. In this study, the accuracy of the HOT method in measuring the bone texture roughness and anisotropy; together with the effects of image noise, blur, exposure, magnification, and projection angle on its performance were investigated. Computer-generated images of fractal surfaces and x-ray images obtained for a human tibia head were used. Results obtained show that the HOT method can effectively be used to characterize the roughness and anisotropy (isotropy) of TB texture.

Skeletal dosimetry via NMR microscopy: investigations of sample reproducibility and signal source

Nuclear magnetic resonance microscopy has been used for several years as a means of quantifying the 3D microarchitecture of the cancellous regions of the skeleton. These studies were originally undertaken for the purpose of developing non-invasive techniques for the early detection of osteoporosis and other bone structural changes. Recently, nuclear magnetic resonance microscopy has also been used to acquire this same 3D data for the purpose of both (1) generating chord length data across bone trabeculae and marrow cavities and (2) generating 3D images for direct coupling to Monte Carlo radiation transport codes. In both cases, one is interested in the reproducibility of the dosimetric data obtained from nuclear magnetic resonance microscopy. In the first of two studies, a trabecular bone sample from the femoral head of a 51-y-old male cadaver was subjected to repeated image acquisition, image processing, image coupling, and radiation transport simulations. The resulting absorbed fractions at high electron energies (4 MeV) were shown to vary less than 4% among four different imaging sessions of the same sample. In a separate study, two femoral head samples were imaged under differing conditions of the NMR signal source. In the first case, the samples were imaged with intact marrow. These samples were then subjected to marrow digestion and immersed in Gd-doped water, which then filled the marrow cavities. Energy-dependent absorbed fraction profiles for both the marrow-intact and marrow-free samples showed essentially equivalent results. These studies thus provide encouragement that skeletal dosimetry models of improved patient specificity can be achieved via NMR microscopy in vivo.

Characterization of trabecular bone by dipolar demagnetizing field MRI

A multiple spin-echo (MSE) sequence has been applied for the first time to study trabecular bone ex vivo. The second echo generated by the demagnetizing field presents discrete drops in signal intensity for certain values of the pitch of the magnetization helix created by the correlation gradient. These dips may reflect characteristic pore sizes in the trabecular bone specimens. This hypothesis is supported by similar experiments performed on a phantom with uniform pore size distribution. Trabecular bone images weighted in the MSE contrast mechanism are reported.

Trabecular structure assessment in lumbar vertebrae specimens using quantitative magnetic resonance imaging and relationship with mechanical competence

The purpose of this study was to use quantitative magnetic resonance imaging (MRI; high-resolution [HR] and relaxometry) to assess trabecular bone structure in lumbar vertebrae specimens and to compare these techniques with bone mineral density (BMD) in predicting stress values obtained from mechanical tests. Fourteen vertebral midsagittal sections from lumbar vertebrae L3 were obtained from cadavers (aged 22-76 years). HR images with a slice thickness of 300 microm and an in-plane spatial resolution of 117 microm2 x 117 microm2 were obtained. Transverse relaxation time T2' distribution was measured by using an asymmetric spin-echo (ASE) sequence. Traditional morphometric measures of bone structure such as apparent trabecular bone fraction (app. BV/TV), apparent trabecular bone number (app. Tb.N), apparent trabecular bone separation (app. Tb.Sp), and apparent trabecular bone thickness (app. Tb.Th) as well as the directional mean intercept length (MIL) were calculated. Additionally, BMD measurements of these sections were obtained by dual-energy X-ray absorptiometry (DXA) and biomechanical properties such as directional stress values (to fracture) were determined on adjacent specimens. With the exception of T2', all morphological parameters correlated very well with age, BMD, and stress values (R between 0.79 and 0.92). However, in the direction perpendicular to the magnetic field, T2' values enhanced the adjusted R2 correlation value with horizontal (M/L) stress values in addition to BMD from 0.70 to 0.91 (p < 0.05).

Future methods in the assessment of bone mass and structure

There have been major advances in the diagnosis of osteoporosis over the last few decades not only in the definitions that are now used but also in the technology that is available. The future will see further development of the techniques currently in common clinical use, such us dual energy X-ray absorptiometry and quantitative ultrasound. In addition new techniques for assessing bone structure, including MRI and fractal analysis of X-rays, may add significantly to our understanding of the pathophysiology of osteoporosis and to the prediction of fracture risk.

Chord distributions across 3D digital images of a human thoracic vertebra

Radiation dose estimates to the trabecular region of the skeleton are of primary importance due to recent advancements in nuclear medicine. Establishing methods for accurately calculating dose in these regions is difficult due to the complex microstructure of this anatomic site and the typical ranges of beta-particles in both bone and marrow tissues. At the present time, models of skeletal dosimetry used in clinical medicine rely upon measured distributions of straight-line path lengths (chord lengths) through bone and marrow regions. This work develops a new three-dimensional, digital method for acquiring these distributions within voxelized images. In addition, the study details the characteristics of measuring chord distributions within digital images and provides a methodology for avoiding undesirable pixel or voxel effects. The improved methodology has been applied to a digital image (acquired via NMR microscopy) of the trabecular region of a human thoracic vertebra. The resulting chord-length distributions across both bone trabeculae and bone marrow cavities were found to be in general agreement with those measured in other studies utilizing different methods. In addition, this study identified that bone and marrow space chord-length distributions are not statistically independent, a condition implicitly assumed within all current skeletal dosimetry models of electron transport. The study concludes that the use of NMR microscopy combined with the digital measurement techniques should be used to further expand the existing Reference Man database of trabecular chord distributions to permit the development of skeletal dosimetry models which are more age and gender specific.

Magnetic resonance microscopy of morphological alterations in mouse trabecular bone structure under conditions of simulated microgravity

This work describes the use of magnetic resonance (MR) microscopy to examine changes in tibial trabecular bone structure in mice following 28 days of hindlimb suspension, a model simulating the effects of microgravity in rodents. In this first MR study involving mice, analysis of 3D images showed that apparent bone volume fraction, trabecular number, and trabecular thickness were decreased, and apparent trabecular spacing increased, significantly (P < 0.05) in hindlimb-suspended mice compared to controls. These changes agreed well with light microscopy measurements from an independent study and also with actual spaceflight experiments with rats.

Three-dimensional microimaging (MRmicroI and microCT), finite element modeling, and rapid prototyping provide unique insights into bone architecture in osteoporosis

With the proportion of elderly people increasing in many countries, osteoporosis has become a growing public health problem, with rising medical, social, and economic consequences. It is well recognized that a combination of low bone mass and the deterioration of the trabecular architecture underlies osteoporotic fractures. A comprehensive understanding of the relationships between bone mass, the three-dimensional (3D) architecture of bone and bone function is fundamental to the study of new and existing therapies for osteoporosis. Detailed analysis of 3D trabecular architecture, using high-resolution digital imaging techniques such as magnetic resonance microimaging (MRmicroI), micro-computed tomography (microCT), and direct image analysis, has become feasible only recently. Rapid prototyping technology is used to replicate the complex trabecular architecture on a macroscopic scale for visual or biomechanical analysis. Further, a complete set of 3D image data provides a basis for finite element modeling (FEM) to predict mechanical properties. The goal of this paper is to describe how we can integrate three-dimensional microimaging and image analysis techniques for quantitation of trabecular bone architecture, FEM for virtual biomechanics, and rapid prototyping for enhanced visualization. The integration of these techniques provide us with an unique ability to investigate the role of bone architecture in osteoporotic fractures and to support the development of new therapies.

Does the trabecular bone structure depicted by high-resolution MRI of the calcaneus reflect the true bone structure?

RATIONALE AND OBJECTIVES

The purpose of this study was to compare trabecular bone structure parameters assessed with high-resolution magnetic resonance imaging (HR-MRI) with those determined in specimen sections.

METHODS

High-resolution MR images were obtained for 30 calcaneus specimens with a three-dimensional, T1-weighted spin-echo sequence (spatial in-plane resolution 0.195 mm, slice thicknesses of 0.3 and 0.9 mm). Thirty-eight sections were obtained from the specimens, and contact radiography was performed. In the corresponding sections, structural parameters analogous to bone histomorphometry were determined.

RESULTS

Significant correlations between MRI-derived structural parameters and those derived from macro pathological sections were found: r values of up to 0.75 were obtained (P < 0.01). The highest correlations were found for apparent bone volume/total volume and trabecular thickness. Image thresholding techniques showed a significant impact on these correlations (P < 0.01). The thinner MR sections were less susceptible to the different thresholding algorithms.

CONCLUSIONS

Trabecular bone structure depicted by HR-MR images is significantly correlated with that shown in macro sections (P < 0.01); however, a number of limitations have to be considered, including the substantial impact of thresholding techniques and slice thickness.

Evaluation of changes in trabecular bone architecture and mechanical properties of minipig vertebrae by three-dimensional magnetic resonance microimaging and finite element modeling

The study objective was to analyze the three-dimensional (3D) trabecular architecture and mechanical properties in vertebral specimens of young and mature Sinclair minipigs to assess the relative contribution of architecture to bone strength. We used 3D magnetic resonance microimaging (MRmicroI) and direct image analysis to evaluate a set of standard structural measurements and new architectural descriptors of trabecular bone in biopsy specimens from L2, L3, and L4 vertebrae (n = 16 in each group) from young (mean age, 1.2 years) and mature (mean age, 4.8 years) minipigs. The measurements included bone volume/tissue volume (BV/TV), marrow star volume (Ma.St.V), connectivity density (ConnD), and two new parameters, percent platelike trabeculae (% plate) and percent bone in the load direction (% boneLD). The % plate, calculated from surface curvature, allowed the delineation of plates from rods. The % boneLD quantified the percentage of bone oriented along the long axis of the vertebral body. We showed that 3D MRmicroI can detect the subtle changes in trabecular architecture between the two age groups. ConnD, star volume, % plate, % boneLD, and BV/TV were found to be more effective than the model-based, derived indices (trabecular thickness [Tb.Th], trabecular separation [Tb.Sp], and trabecular number [Tb.N]) in differentiating the structural changes. BV/TV, % plate, and % boneLD significantly increased (p < 0.05) in all three vertebral sites of the mature minipigs. The significant decrease in ConnD and star volume in the mature vertebra was consistent with the concurrent increase of platelike trabecular bone (p < 0.05). Overall, ConnD, star volume, % plate, and % boneLD provided a coherent picture of the architectural changes between the two age groups. Apparent modulus and maximum stress were determined experimentally on biopsy specimens from L2 vertebrae (n = 16). When apparent modulus was predicted using 3D MRmicroI data sets as input for finite element modeling (FEM), the results were similar to the experimentally determined apparent modulus (p = 0.12). Both methods were then used to compare the young and the mature animals; the experimental and predicted apparent modulus were significantly higher for the mature group (p = 0.003 and 0.012, respectively). The experimental maximum stress in the vertebra of the mature animals was twice as high as that for the young animals (p = 0.006). Bone quantity (BV/TV or bone mineral content [BMC]) alone could explain approximately 74-85% of the total variability in stress and modulus. The inclusion of either ConnD or % boneLD with BV/TV in a multiple regression analysis significantly improved the predictability of maximum stress, indicating that architecture makes additional contributions to compressive strength in normal minipig vertebra.

Fractal dimension of trabecular bone projection texture is related to three-dimensional microarchitecture

The purpose of this work was to understand how fractal dimension of two-dimensional (2D) trabecular bone projection images could be related to three-dimensional (3D) trabecular bone properties such as porosity or connectivity. Two alteration processes were applied to trabecular bone images obtained by magnetic resonance imaging: a trabeculae dilation process and a trabeculae removal process. The trabeculae dilation process was applied from the 3D skeleton graph to the 3D initial structure with constant connectivity. The trabeculae removal process was applied from the initial structure to an altered structure having 99% of porosity, in which both porosity and connectivity were modified during this second process. Gray-level projection images of each of the altered structures were simply obtained by summation of voxels, and fractal dimension (Df) was calculated. Porosity (phi) and connectivity per unit volume (Cv) were calculated from the 3D structure. Significant relationships were found between Df, phi, and Cv. Df values increased when porosity increased (dilation and removal processes) and when connectivity decreased (only removal process). These variations were in accordance with all previous clinical studies, suggesting that fractal evaluation of trabecular bone projection has real meaning in terms of porosity and connectivity of the 3D architecture. Furthermore, there was a statistically significant linear dependence between Df and Cv when phi remained constant. Porosity is directly related to bone mineral density and fractal dimension can be easily evaluated in clinical routine. These two parameters could be associated to evaluate the connectivity of the structure.

In vivo MR micro imaging with conventional radiofrequency coils cooled to 77 degrees K

Cryogenically cooled conventional surface coils are shown to provide significant signal-to-noise ratio (SNR) gains for MR micro imaging of tissue structure in vivo. Measurements are described which employ a simple, all-polyvinyl chloride (PVC) vacuum dewar capable of maintaining a bath of liquid nitrogen around the coil, within 5 mm of the tissue to be imaged. Images acquired in vivo at 64 MHz with a 2-cm diameter copper coil cooled to 77 K demonstrated a gain in SNR of approximately 2.7 +/- 0.3 relative to those obtained with the same coil at room temperature under otherwise identical conditions. This increase is consistent with the reduction in coil resistance and the minor contribution to overall resistance from the imaging object. The performance of the coil is illustrated with images from the human finger and rabbit eye and potential applications are discussed.

Bone labelling on micro-magnetic resonance images

The study of trabecular bone is receiving increasing interest within the medical community working in the field of skeletal diseases, such as osteoporosis. Quantification of trabecular bone structure usually requires as a starting point a correct segmentation of the trabecular network. We have developed a probabilistic relaxation labelling technique, which uses local features of the trabecular bone images to improve segmentation. Tests on synthetic images show that bone labelling performs a more accurate segmentation than conventional techniques such as thresholding, especially by preserving the connectivity of the trabecular network. Tests on acquired data show that porosity values obtained after segmentation are in good agreement with experimental values obtained by weighing the bone samples.

In vivo micro-imaging using alternating navigator echoes with applications to cancellous bone structural analysis

In micro-magnetic resonance imaging of cancellous bone architecture, involuntary subject motion even on a sub-millimeter scale is detrimental and generally precludes accurate quantification of architectural parameters. In this work a navigator-assisted three-dimensional spin-echo technique is described and evaluated for imaging at 137 microm resolution in humans. The method is based on gradient navigator echoes following the spin-echo readout for sensing translational displacements alternately in x- and y-directions with a spatial resolution of 273 microm and a temporal resolution of 0.2 sec. The technique is shown to improve micro-images of the distal forearm significantly and to enhance accuracy and reproducibility of bone volume fraction, transverse contiguity, and tubularity, parameters introduced in prior work to characterize the trabecular network. It is further shown that a fourfold reduction in navigator sampling time, along with zero-filling, improves the accuracy of the navigator correction while reducing the minimum pulse repetition time or gradient heating. The data indicate that navigator-assisted micro-imaging is capable of effectively correcting sub-millimeter displacements in micro-imaging.

Effect of prostaglandin and bisphosphonate on cancellous bone volume and structure in the ovariectomized rat studied by quantitative three-dimensional nuclear magnetic resonance microscopy

The purpose of this work was to evaluate the potential of nuclear magnetic resonance microscopy (NMRM) in conjunction with a processing technique to monitor the effect of preventive agents in an ovariectomized (OVX) rat. Twenty-five female Sprague-Dawley rats were OVX at 6 months of age (except for the intact control group), allowed to lose bone for 60 days, and then treated for 60 days. During treatment, animals were administered vehicle, prostaglandin E2 (PGE2; 6 mg/kg), or alendronate (3 microg/kg) subcutaneously once a day. Subsequently, tibiae were harvested and the marrow removed. NMRM was carried out at 9.4 T, with the specimens immersed in 1.2 mM diethylenetriaminepentaacetic acid-gadolinium salt (Gd-DTPA) aqueous solution. A three-dimensional (3D) partial flip-angle pulse sequence was used, providing a 1283 array of (46 microm)3 isotropic voxels. Fifty of the 128 axial images in the 3D data set comprising approximately 2.4 mm volume distal to the growth plate were processed from each specimen using a probability-based method for determining bone volume fraction (BVF), tubularity, contiguity, as well as the mean trabecular plate thickness and separation. PGE2 and alendronate altered BVF consistently at all tibial regions. The effect of alendronate was to keep BVF about midway between intact and OVX, whereas PGE2 returned BVF to intact levels. The other parameters showed similar responses to treatment. The strongest discriminator was trabecular BVF, which could obviously differentiate the groups. The study establishes NMRM as a nondestructive histomorphometric method for the quantitative evaluation of drug response in a rat ovariectomy model.

In vivo high resolution MRI of the calcaneus: differences in trabecular structure in osteoporosis patients

The purpose of this study was to use high resolution (HR) magnetic resonance (MR) images of the calcaneus to investigate the trabecular structure of patients with and without osteoporotic hip fractures and to compare these techniques with bone mineral density (BMD) in differentiating fracture and nonfracture patients. Axial and sagittal HR MR images of the calcaneus were obtained in 50 female (23 postmenopausal patients with osteoporotic hip fractures and 27 postmenopausal controls). A three-dimensional gradient-echo sequence was used with a slice thickness of 500 micron and in plane resolution of 195 x 195 micron. Texture analysis was performed using morphological features, analogous to standard histomorphometry and fractal dimension. Additionally, BMd measurements of the hip (dual-energy X-ray absorptiometry) were obtained in all patients. Significant differences between both patient groups were obtained using morphological parameters and fractal dimension as well as hip BMD (p < 0.05). Odds ratios for the texture parameters apparent (app.) bone volume/total volume and app. trabecular separation were higher than for hip BMD. Receiver operator characteristic values of texture measures and hip BMD were comparable. In conclusion, trabecular structure measures derived from HR MR images of the calcaneus can differentiate between postmenopausal women with and without osteoporotic hip fractures.

Assessment of the pore geometry of stereolithographic models by high resolution MRI

It is becoming increasingly evident that material strength and other mechanical properties depend not only on the density but also on the internal architecture of the structure in question. The internal structures of five different stereolithographic samples were examined noninvasively using high resolution magnetic resonance imaging (MRI). The image analysis techniques of segmentation applied to the acquired images allowed the quantification of the structural parameters mean pore size and pore separation. These parameters were quantified along many angular orientations with a spatial resolution of 0.12 mm, and their distributions clearly revealed the structural anisotropy of the samples.

Noninvasive assessment of bone density and structure using computed tomography and magnetic resonance

For several reasons, including low cost and radiation dose, simplicity, and the ability to image several skeletal sites, dual X-ray absoptiometry (DXA) is the most widely employed technique for diagnostic and serial assessment of integral bone mass in osteoporosis and other metabolic bone diseases. However, three-dimensional imaging modalities such as quantitative computed tomography (QCT) and magnetic resonance (MR) imaging offer the ability to separately examine different factors that may play independent and important roles in osteoporosis. These factors include the density of the trabecular and cortical compartments as well as the pattern of trabecular microarchitecture. New developments in QCT include volumetric approaches for precise compartmental assessment of the spine and proximal femur as well as thin-slice tomography of the vertebral body for assessment of trabecular texture. In addition, ultrahigh resolution CT scanners (spatial resolution ë50-150(i)i) have been developed for imaging of trabecular structure in specimens and in some cases for the peripheral skeleton (distal radius and phalanges). High resolution MR measurements may be employed for assessment of the trabecular texture at a range of peripheral sites, including the calcaneus, distal radius, and phalanges.