Selecting ordered environments in NMR of spin 3/2 nuclei via frequency-sweep pulses

Sodium NMR/MRI for anisotropic systems

Sodium ((23)Na) plays a central role in many physiological processes, and its high NMR sensitivity makes it an attractive nucleus for biomedical NMR and MRI research. Many biological tissues contain structures such as fibers and membranes that impose anisotropic translational and rotational motions on the sodium ions. Translational motion can be studied by diffusion measurements. Anisotropic rotational motion results in non-vanishing quadrupolar interaction that it is best studied by exploiting multiple quantum coherences for (23)Na NMR spectroscopy and MRI. The current review covers the application of the various NMR techniques to the study of (23)Na in anisotropic compartments in cartilage, tendon, intervertebral discs, red blood cells, nervous system and muscles.

MR imaging of articular cartilage physiology

The newer magnetic resonance (MR) imaging methods can give insights into the initiation, progression, and eventual treatment of osteoarthritis. Sodium imaging is specific for changes in proteoglycan (PG) content without the need for an exogenous contrast agent. T1ρ imaging is sensitive to early PG depletion. Delayed gadolinium-enhanced MR imaging has high resolution and sensitivity. T2 mapping is straightforward and is sensitive to changes in collagen and water content. Ultrashort echo time MR imaging examines the osteochondral junction. Magnetization transfer provides improved contrast between cartilage and fluid. Diffusion-weighted imaging may be a valuable tool in postoperative imaging.

Optimal nuclear magnetic resonance excitation schemes for the central transition of a spin 3/2 in the presence of residual quadrupolar coupling

Optimal control theory is applied for enhancing the intensity of the central peak of a spin 3/2 signal in the presence of a residual quadrupolar coupling. While a maximum enhancement is always possible in the regime omega(rf) << omega(Q) via the use of modulated and shaped pulses, the intermediate rf-power regime omega(rf)-omega(Q) does not admit simple solutions based on intuition. In this work we present optimized shaped pulses that have been derived using an optimization algorithm based on optimal control and test these with (23)Na NMR in this regime. In addition to enhancing the intensity of the central transition signal, the satellite peaks can be effectively suppressed, which is a useful feature for the implementation in (23)Na imaging sequences.

Clean demarcation of cartilage tissue 23Na by inversion recovery

Monitoring the sodium concentration in vivo using 23Na MRI can be an important tool for assessing the onset of tissue disorders. Practical clinical 23Na MRI methods furthermore often do not allow one to use sufficiently small voxel sizes such that only the tissue of interest is seen, but a large signal contamination can arise from sodium in synovial fluid. Here we demonstrate that applying an inversion recovery (IR) technique allows one to distinctly select either the cartilage-bound or the free sodium for visualization in an image. The results are validated both ex vivo and in vivo.

Anisotropy of collagen fibre alignment in bovine cartilage: comparison of polarised light microscopy and spatially resolved diffusion-tensor measurements

OBJECTIVE

To compare collagen fibre alignment angles obtained from polarised light microscopy (PLM) and diffusion-tensor imaging (DTI) in bovine articular cartilage.

METHODS

Five samples of bovine articular cartilage from five different animals were studied using magnetic resonance imaging and PLM techniques. T(2)-weighted, diffusion-tensor (DT), and PLM images were acquired for each sample and average depth profiles of the PLM and DTI angles, as well as the banding patterns observed in T(2)-weighted magnetic resonance (MR) images, were compared. Statistical properties of the distributions of the DTI and PLM angles were examined.

RESULTS

The samples exhibited a range of alignment morphologies. In the samples with the "conventional" three-zone alignment pattern, a correlation between the PLM and DTI alignment zones and the banding in T(2)-weighted MR images was observed. The shapes of the depth profiles of the PLM and DTI alignment angles were qualitatively similar for each sample. Three samples showed good quantitative correlation between the DT and PLM alignment angles. The correlation between the diffusion and PLM alignment angles was best in the regions of low degree of disorder of fibre alignment.

CONCLUSIONS

This study provides the first quantitative comparison of DTI of cartilage with the more established PLM techniques. The correlation between alignment angles derived from PLM and DTI data was evident across a wide range of alignment morphologies. The results support the use of DTI for the quantitative measurement of collagen fibre alignment. The microscopic-scale (~10 microm) dispersion of fibre alignment angles appears to be an important factor for understanding the extent of quantitative correlation between PLM and DTI results.

Assessment of glycosaminoglycan concentration in vivo by chemical exchange-dependent saturation transfer (gagCEST)

Glycosaminogycans (GAGs) are involved in numerous vital functions in the human body. Mapping the GAG concentration in vivo is desirable for the diagnosis and monitoring of a number of diseases such as osteoarthritis, which affects millions of individuals. GAG loss in cartilage is typically an initiating event in osteoarthritis. Another widespread pathology related to GAG is intervertebral disk degeneration. Currently existing techniques for GAG monitoring, such as delayed gadolinium-enhanced MRI contrast (dGEMRIC), T(1)(rho), and (23)Na MRI, have some practical limitations. We show that by exploiting the exchangeable protons of GAG one may directly measure the localized GAG concentration in vivo with high sensitivity and therefore obtain a powerful diagnostic MRI method.

Behavior of ordered sodium in enzymatically depleted cartilage tissue

The onset of cartilage tissue disorders can be characterized by a loss of proteoglycans (PGs) and diagnosed by contrast-enhanced proton ((1)H) MRI techniques, as well as sodium MRI. The behavior of sodium located in anisotropic environments, is examined as a function of cartilage degeneration. PGs are proteolytically depleted from the cartilage samples, which gives rise to a decrease of the ordered sodium content. More surprisingly, however, the residual quadrupolar couplings are shown to increase with increasing depletion levels. Since the residual quadrupolar couplings are intimately related to local order and anisotropic motion, measuring their distribution in cartilage may provide insight into the structural changes that occur within the tissue upon degradation. In this study relatively mild orientational dependence of the couplings was found. Little or no free sodium was observed in the cartilage specimens under study.

Sodium and T1rho MRI for molecular and diagnostic imaging of articular cartilage

In this article, both sodium magnetic resonance (MR) and T1rho relaxation mapping aimed at measuring molecular changes in cartilage for the diagnostic imaging of osteoarthritis are reviewed. First, an introduction to structure of cartilage, its degeneration in osteoarthritis (OA) and an outline of diagnostic imaging methods in quantifying molecular changes and early diagnostic aspects of cartilage degeneration are described. The sodium MRI section begins with a brief overview of the theory of sodium NMR of biological tissues and is followed by a section on multiple quantum filters that can be used to quantify both bi-exponential relaxation and residual quadrupolar interaction. Specifically, (i) the rationale behind the use of sodium MRI in quantifying proteoglycan (PG) changes, (ii) validation studies using biochemical assays, (iii) studies on human OA specimens, (iv) results on animal models and (v) clinical imaging protocols are reviewed. Results demonstrating the feasibility of quantifying PG in OA patients and comparison with that in healthy subjects are also presented. The section concludes with the discussion of advantages and potential issues with sodium MRI and the impact of new technological advancements (e.g. ultra-high field scanners and parallel imaging methods). In the theory section on T1rho, a brief description of (i) principles of measuring T1rho relaxation, (ii) pulse sequences for computing T1rho relaxation maps, (iii) issues regarding radio frequency power deposition, (iv) mechanisms that contribute to T1rho in biological tissues and (v) effects of exchange and dipolar interaction on T1rho dispersion are discussed. Correlation of T1rho relaxation rate with macromolecular content and biomechanical properties in cartilage specimens subjected to trypsin and cytokine-induced glycosaminoglycan depletion and validation against biochemical assay and histopathology are presented. Experimental T1rho data from osteoarthritic specimens, animal models, healthy human subjects and as well from osteoarthritic patients are provided. The current status of T1rho relaxation mapping of cartilage and future directions is also discussed.

Selective detection of ordered sodium signals via the central transition

Given the correlation between the concentrations of ordered (23)Na and the onset of tissue disorders, the ability to select the signal from ordered (23)Na over that of free (23)Na is of particular importance and can greatly enhance the potential of (23)Na-MRI as a diagnostic tool. Here, we describe a simple method that selectively detects the central transition of ordered sodium while minimizing the signal from free sodium. Our method relies upon the influence of the quadrupolar interaction on nutation frequencies and may also benefit solid-state imaging experiments. Both a liquid crystalline environment and a cartilage sample are used to demonstrate a clean separation between anisotropic and isotropic regions in the experiments.

Frequency-selective quadrupolar MRI contrast

A method for the selective detection of quadrupolar nuclei located in anisotropic environments is presented. The image contrast can be tuned to the degree of anisotropy in the sample by using frequency-swept pulsed. These methods are particularly useful in the field of sodium-MRI, where sodium signals from locally-ordered environments provide diagnostic information. In solid-state MRI, these methods could be useful for probing structural defects within the sample. We demonstrate here one-dimensional images, in which the pixel contrast indicates the presence or absence of quadrupolar coupling within a certain frequency range.