Diffusion-based MR methods for bone structure and evolution

Diffusion MRI for Assessment of Bone Quality; A Review of Findings in Healthy Aging and Osteoporosis

Diffusion MRI (dMRI) is a growing imaging technique with the potential to provide biomarkers of tissue variation, such as cellular density, tissue anisotropy, and microvascular perfusion. However, the role of dMRI in characterizing different aspects of bone quality, especially in aging and osteoporosis, has not yet been fully established, particularly in clinical applications. The reason lies in the complications accompanied with implementation of dMRI in assessment of human bone structure, in terms of acquisition and quantification. Bone is a composite tissue comprising different elements, each contributing to the overall quality and functional competence of bone. As diffusion is a critical biophysical process in biological tissues, early changes of tissue microstructure and function can affect diffusive properties of the tissue. While there are multiple MRI methods to detect variations of individual properties of bone quality due to aging and osteoporosis, dMRI has potential to serve as a superior method for characterizing different aspects of bone quality within the same framework but with higher sensitivity to early alterations. This is mainly because several properties of the tissue including directionality and anisotropy of trabecular bone and cell density can be collected using only dMRI. In this review article, we first describe components of human bone that can be potentially detected by their diffusivity properties and contribute to variations in bone quality during aging and osteoporosis. Then we discuss considerations and challenges of dMRI in bone imaging, current status, and suggestions for development of dMRI in research studies and clinics to segregate different contributing components of bone quality in an integrated acquisition. Level of Evidence: 5 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:975-992.

Direct measurement of internal magnetic fields in natural sands using scanning SQUID microscopy

NMR experiments are ideally carried out in well-controlled magnetic fields. When samples of natural porous materials are studied, the situation can be complicated if the sample itself contains magnetic components, giving rise to internal magnetic fields in the pore space that modulate the externally applied fields. If not properly accounted for, the internal fields can lead to misinterpretation of relaxation, diffusion, or imaging data. To predict the potential effect of internal fields, and develop effective mitigation strategies, it is important to develop a quantitative understanding of the magnitude and distribution of internal fields occurring in natural porous media. To develop such understanding, we employ scanning SQUID microscopy, a technique that can detect magnetic field variations very accurately at high spatial resolution (∼3μm). We prepared samples from natural unconsolidated aquifer material, and scanned areas of about 200×200μm in a very low background magnetic field of ∼2μT. We found large amplitude variations with a magnitude of about 2mT, across a relatively long spatial scale of about 200μm, that are associated with a large magnetic grain (>50μm radius) with a strong magnetic remanence. We also detected substantial variations exceeding 60μT on small spatial scales of about ∼10μm. We attribute these small-scale variations to very fine-grained magnetic material. Because we made our measurements at very low background field, the observed variations are not induced by the background field but due to magnetic remanence. Consequently, the observed internal fields will affect even low-field NMR experiments.

Time-dependent diffusion in skeletal muscle with the random permeable barrier model (RPBM): application to normal controls and chronic exertional compartment syndrome patients

The purpose of this work was to carry out diffusion tensor imaging (DTI) at multiple diffusion times Td in skeletal muscle in normal subjects and chronic exertional compartment syndrome (CECS) patients and analyze the data with the random permeable barrier model (RPBM) for biophysical specificity. Using an institutional review board approved HIPAA-compliant protocol, seven patients with clinical suspicion of CECS and eight healthy volunteers underwent DTI of the calf muscle in a Siemens MAGNETOM Verio 3 T scanner at rest and after treadmill exertion at four different T(d) values. Radial diffusion values λ(rad) were computed for each of seven different muscle compartments and analyzed with RPBM to produce estimates of free diffusivity D(0), fiber diameter a, and permeability κ. Fiber diameter estimates were compared with measurements from literature autopsy reference for several compartments. Response factors (post/pre-exercise ratios) were computed and compared between normal controls and CECS patients using a mixed-model two-way analysis of variance. All subjects and muscle compartments showed nearly time-independent diffusion along and strongly time-dependent diffusion transverse to the muscle fibers. RPBM estimates of fiber diameter correlated well with corresponding autopsy reference. D(0) showed significant (p < 0.05) increases with exercise for volunteers, and a increased significantly (p < 0.05) in volunteers. At the group level, response factors of all three parameters showed trends differentiating controls from CECS patients, with patients showing smaller diameter changes (p = 0.07), and larger permeability increases (p = 0.07) than controls. Time-dependent diffusion measurements combined with appropriate tissue modeling can provide enhanced microstructural specificity for in vivo tissue characterization. In CECS patients, our results suggest that high-pressure interfiber edema elevates free diffusion and restricts exercise-induced fiber dilation. Such specificity may be useful in differentiating CECS from other disorders or in predicting its response to either physical therapy or fasciotomy.

Influence of bone marrow composition on measurements of trabecular microstructure using decay due to diffusion in the internal field MRI: simulations and clinical studies

PURPOSE

Decay due to diffusion in the internal field (DDIF) MRI allows for measurements of microstructures of porous materials at low spatial resolution and thus has potential for trabecular bone quality measurements. In trabecular bone, solid bone changes (osteoporosis) as well as changes in bone marrow composition occur. The influence of such changes on DDIF MRI was studied by simulations and in vivo measurements.

METHODS

Monte Carlo simulations of DDIF in various trabecular bone models were conducted. Changes in solid bone and marrow composition were simulated with numerical bone erosion and marrow susceptibility variations. Additionally, in vivo measurements were performed in the lumbar spine of healthy volunteers aged 23-62 years.

RESULTS

Simulations and in vivo results showed that 1) DDIF decay times decrease with increasing marrow fat and 2) the marrow fat percentage needs to be incorporated in the DDIF analysis to discriminate between healthy and osteoporotic solid bone structures.

CONCLUSIONS

Bone marrow composition plays an important role in DDIF MRI: incorporation of marrow fat percentage into DDIF MRI allowed for differentiation of young and old age groups (in vivo experiments). DDIF MRI may develop into a means of assessing osteoporosis and disorders that affect marrow composition.

Potential diagnostic role of the MRI-derived internal magnetic field gradient in calcaneus cancellous bone for evaluating postmenopausal osteoporosis at 3T

INTRODUCTION

Bone mineral density (BMD) result has a low predictive value on patients' risk for future fractures. Thus, new approaches for examining patients at risk for developing osteoporosis would be desirable. Magnetic resonance (MR) investigations in cancellous bone have been shown to yield useful quantitative information on both trabecular-bone microstructure and bone marrow composition. This work was undertaken to address the hypothesis that the effective internal magnetic field gradient (IMFG), a new MR parameter, discriminates between healthy, osteopenic and osteoporotic postmenopausal women, classified on the basis of bone mineral density (BMD) criteria. The work builds on preliminary results indicating that IMFG, measured in trabecular-bone pores and quantified by spin-echo decay and water diffusion MR near the bone-bone marrow interface depends on both the bone marrow water rate of diffusion and the magnetic susceptibility difference (ΔX) between water and bone.

MATERIALS AND METHODS

MR relaxometry, MR spectroscopy and diffusion-weighted MR imaging of the heel was performed in fifty-five women (mean age, 62.9±6.6years) at 3T. Moreover, in order to study the reproducibility of IMFG measurement, five young women (mean age 31.0±3.2years; age range, 28-36years) were scanned and rescanned. The study protocol was approved by the local Ethics Committee. Quantitative Computer Tomography (QCT) of the L1-L3 vertebral segments was performed to classify the postmenopausal women into three groups according to QCT BMD: healthy (n=8); osteopenic (n=25); and osteoporotic (n=22). In all subjects, BMD T-scores, marrow fat content (Mfc), T2*, apparent diffusion coefficient (ADC) and IMFG (estimated from the additional spin-echo decay due to diffusion of water in local magnetic field gradients), were assessed in the whole calcaneus as well as in three calcaneal subregions: subtalar, tuber calcaneus, and cavum calcaneus. Between-group comparisons to assess group differences and Pearson correlation analysis were performed. Short and long-term coefficients of variation (CVS and CVL, respectively) were evaluated in young subjects.

RESULTS

Reproducibility of the IMFG measurement was satisfactory. No significant difference was found in the IMFG measurement performed in both calcaneus and subtalar calcaneal region between the two separate sessions comprised of five young women. Mfc did not significantly differ between groups. The IMFG in the subtalar region was significantly different between all three groups (P<0.01), being greatest in healthy women, intermediate in those with osteopenia, and lowest in osteoporotic subjects. Conversely neither T2* nor ADC is able to discriminate healthy subjects from those with osteopenia and osteoporosis. Increased inter-trabecular space, as it typically occurs in patients with osteoporosis, modifies water diffusion, conferring higher ADC values, thereby lowering the IMFG.

CONCLUSION

The IMFG measured in the calcaneal subtalar region shows a high ability in identifying healthy subjects. The new quantitative MR method based on measurement of the IMFG may provide a new means for assessing patients with osteoporosis.

Magnetic resonance of porous media (MRPM): a perspective

Porous media are ubiquitous in our environment and their application is extremely broad. The common connection between these diverse materials is the importance of the microstructure (μm to mm scale) in determining the physical, chemical and biological functions and properties. Magnetic resonance and its imaging modality have been essential for noninvasive characterization of these materials, in the development of catalysts, understanding cement hydration, fluid transport in rocks and soil, geological prospecting, and characterization of tissue properties for medical diagnosis. The past two decades have witnessed significant development of MRPM that couples advances in physics, chemistry and engineering with a broad range of applications. This article will summarize key advances in basic physics and methodology, examine their limitations and envision future R&D directions.

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.

Magnetic Resonance Characterization of Porous Media Using Diffusion through Internal Magnetic Fields

When a porous material is inserted into a uniform magnetic field, spatially varying fields typically arise inside the pore space due to susceptibility contrast between the solid matrix and the surrounding fluid. As a result, direct measurement of the field variation may provide a unique opportunity to characterize the pore geometry. The sensitivity of nuclear magnetic resonance (NMR) to inhomogeneous field variations through their dephasing effects on diffusing spins is unique and powerful. Recent theoretical and experimental research sheds new light on how to utilize susceptibility-induced internal field gradients to quantitatively probe the microstructure of porous materials. This article reviews ongoing developments based on the stimulated echo-pulse sequence to extend the characterization of porous media using both spatially resolved and unresolved susceptibility-induced internal gradients that operate on a diffusing-spin ensemble.

MRI of trabecular bone using a decay due to diffusion in the internal field contrast imaging sequence

PURPOSE

To characterize the DDIF (Decay due to Diffusion in the Internal Field) method using intact animal trabecular bone specimens of varying trabecular structure and porosity, under ex vivo conditions closely resembling in vivo physiological conditions. The DDIF method provides a diffusion contrast which is related to the surface-to-volume ratio of the porous structure of bones. DDIF has previously been used successfully to study marrow-free trabecular bone, but the DDIF contrast hitherto had not been tested in intact specimens containing marrow and surrounded by soft tissue.

MATERIALS AND METHODS

DDIF imaging was implemented on a 4.7 Tesla (T) small-bore, horizontal, animal scanner. Ex vivo results on fresh bone specimens containing marrow were obtained at body temperature. Control measurements were carried out in surrounding tissue and saline.

RESULTS

Significant DDIF effect was observed for trabecular bone samples, while it was considerably smaller for soft tissue outside the bone and for lipids. Additionally, significant differences were observed between specimens of different trabecular structure.

CONCLUSION

The DDIF contrast is feasible despite the reduction of the diffusion constant and of T(1) in such conditions, increasing our confidence that DDIF imaging in vivo may be clinically viable for bone characterization.

GPU accelerated Monte Carlo simulation of pulsed-field gradient NMR experiments

The simulation of diffusion by Monte Carlo methods is often essential to describing NMR measurements of diffusion in porous media. However, simulation timescales must often span hundreds of milliseconds, with large numbers of trajectories required to ensure statistical convergence. Here we demonstrate that by parallelising code to run on graphics processing units (GPUs), these calculations may be accelerated by over three orders of magnitude, opening new frontiers in experimental design and analysis. As such cards are commonly installed on most desktop computers, we expect that this will prove useful in many cases where simple analytical descriptions are not available or appropriate, e.g. in complex geometries or where short gradient pulse approximations do not hold, or for the analysis of diffusion-weighted MRI in complex tissues such as the lungs and brain.

Non-invasive predictors of human cortical bone mechanical properties: T(2)-discriminated H NMR compared with high resolution X-ray

Recent advancements in magnetic resonance imaging (MRI) have enabled clinical imaging of human cortical bone, providing a potentially powerful new means for assessing bone health with molecular-scale sensitivities unavailable to conventional X-ray-based diagnostics. To this end, ¹H nuclear magnetic resonance (NMR) and high-resolution X-ray signals from human cortical bone samples were correlated with mechanical properties of bone. Results showed that ¹H NMR signals were better predictors of yield stress, peak stress, and pre-yield toughness than were the X-ray derived signals. These ¹H NMR signals can, in principle, be extracted from clinical MRI, thus offering the potential for improved clinical assessment of fracture risk.

Visualization of inhomogeneous local magnetic field gradient due to susceptibility contrast

We visualized inhomogeneous local magnetic field (internal magnetic field) gradients arising from susceptibility contrast between an array of cylindrical glass tubes (solid matrix) and surrounding water (pore fluids) in a uniform applied magnetic field. MRI was performed to determine the spatially resolved decay rates due to diffusion in the internal magnetic field which were proportional to the strength of local gradient. We also spatially resolved the interference pattern of the cross-terms between the internal and the applied field gradient in order to extract the orientation of the internal field gradient. These experimental results were found to be consistent with the theoretical calculations. This work demonstrates a simple yet representative case for visualizing the strength and orientation of the local susceptibility induced magnetic field gradients in porous media.

High-resolution MRI of internal field diffusion-weighting in trabecular bone

Trabecular bone structure is known to play a crucial role in the overall strength, and thus fracture risk, of such areas of the skeleton as the vertebrae, spine, femur, tibiae, or radius. Several MR methods devoted to probing this structure depend upon the susceptibility difference between the solid bone matrix and the intervening fluid/marrow/fat, usually in the context of a linewidth (1/T(2)') measurement or mapping technique. A recently demonstrated new approach to this system involves using internal gradients to encode diffusion weighting, and extracting structural information (e.g., surface-to-volume ratio) from the resulting signal decay. This contrast method has been demonstrated in bulk measurements on cleaned, water-saturated bovine trabecular bone samples. In the present work, microscopic imaging (0.156 mm in-plane resolution) is performed in order to spatially resolve this contrast on the trabecular level, and confirm its interpretation for the bulk measurements. It is found that the local rate of decay due to diffusion in the internal field (DDIF) is maximal close to the trabecular surfaces. The overall decay rate in a lower resolution scan probes the abundance of these surfaces, and provides contrast beyond that found in conventional proton density weighted or T(1)-weighted imaging. Furthermore, a microscopic calculation of internal field distributions shows a qualitative distinction between the structural sensitivities of DDIF and T(2)'. DDIF contrast is highly localized around trabecular walls than is the internal field itself, making it a less sensitive but more specific measure of such important properties as trabecular number.

Magnetic resonance in porous media: recent progress

Recent years have seen significant progress in the NMR study of porous media from natural and industrial sources and of cultural significance such as paintings. This paper provides a brief outline of the recent technical development of NMR in this area. These advances are relevant for broad NMR applications in material characterization.