Relationship between susceptibility induced field inhomogeneities, restricted diffusion, and relaxation in sedimentary rocks

Assessment of Calcaneal Spongy Bone Magnetic Resonance Characteristics in Women: A Comparison between Measures Obtained at 0.3 T, 1.5 T, and 3.0 T

BACKGROUND

There is a growing interest in bone tissue MRI and an even greater interest in using low-cost MR scanners. However, the characteristics of bone MRI remain to be fully defined, especially at low field strength. This study aimed to characterize the signal-to-noise ratio (SNR), T2, and T2* in spongy bone at 0.3 T, 1.5 T, and 3.0 T. Furthermore, relaxation times were characterized as a function of bone-marrow lipid/water ratio content and trabecular bone density.

METHODS

Thirty-two women in total underwent an MR-imaging investigation of the calcaneus at 0.3 T, 1.5 T, and 3.0 T. MR-spectroscopy was performed at 3.0 T to assess the fat/water ratio. SNR, T2, and T2* were quantified in distinct calcaneal regions (ST, TC, and CC). ANOVA and Pearson correlation statistics were used.

RESULTS

SNR increase depends on the magnetic field strength, acquisition sequence, and calcaneal location. T2* was different at 3.0 T and 1.5 T in ST, TC, and CC. Relaxation times decrease as much as the magnetic field strength increases. The significant linear correlation between relaxation times and fat/water found in healthy young is lost in osteoporotic subjects.

CONCLUSION

The results have implications for the possible use of relaxation vs. lipid/water marrow content for bone quality assessment and the development of quantitative MRI diagnostics at low field strength.

Low-field permanent magnets for industrial process and quality control

In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.

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.

A fast random walk algorithm for computing the pulsed-gradient spin-echo signal in multiscale porous media

A new method for computing the signal attenuation due to restricted diffusion in a linear magnetic field gradient is proposed. A fast random walk (FRW) algorithm for simulating random trajectories of diffusing spin-bearing particles is combined with gradient encoding. As random moves of a FRW are continuously adapted to local geometrical length scales, the method is efficient for simulating pulsed-gradient spin-echo experiments in hierarchical or multiscale porous media such as concrete, sandstones, sedimentary rocks and, potentially, brain or lungs.

The dependence on magnetic field strength of correlated internal gradient relaxation time distributions in heterogeneous materials

Magnetic susceptibility differences in porous media produce local gradients within the pore space. At high magnetic fields, these inhomogeneities have the potential to greatly affect nuclear magnetic resonance measurements. We undertake a study using a new NMR technique to measure the internal gradients present in highly heterogeneous samples over a wide range of magnetic field strengths. Our results show that even at ultra-high fields there can exist signal at internal gradient strengths sufficiently small that techniques for suppressing unwanted side effects have the possibility to be used. Our findings encourage the use of these high and ultra-high field strengths for a broader range of samples. Our results also give experimental evidence to support the theory of internal gradient scaling as a function of field strength within pores.

Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique

A magic asymmetric gradient stimulated echo (MAGSTE) sequence was recently proposed to improve molecular diffusion measurements in the presence of spatially varying background gradients. Its effectiveness has been demonstrated previously with simulated background gradients and in phantoms that contain bulk susceptibility differences. In this study, we investigated the MAGSTE technique in microscopically heterogeneous systems, and compared it with the conventional bipolar pulsed gradient stimulated echo (bPGSTE) sequence. We demonstrated that the MASGTE measurements, compared to the bPGSTE method, varied significantly less when the diffusion encoding/decoding interval (delta) was changed. In addition, the MAGSTE technique provided good characterization of the surface area-to-volume ratio for heterogeneous systems investigated in this study. In sum, this study showed that the MAGSTE technique provided diffusion measurements superior to those of the bPGSTE sequence, especially in the presence of severe heterogeneous background gradients.