23Na TQF NMR imaging for the study of spinal disc tissue

Multidimensional compressed sensing to advance 23 Na multi-quantum coherences MRI

PURPOSE

Sodium (23 Na) multi-quantum coherences (MQC) MRI was accelerated using three-dimensional (3D) and a dedicated five-dimensional (5D) compressed sensing (CS) framework for simultaneous Cartesian single (SQ) and triple quantum (TQ) sodium imaging of in vivo human brain at 3.0 and 7.0 T.

THEORY AND METHODS

3D 23 Na MQC MRI requires multi-echo paired with phase-cycling and exhibits thus a multidimensional space. A joint reconstruction framework to exploit the sparsity in all imaging dimensions by extending the conventional 3D CS framework to 5D was developed. 3D MQC images of simulated brain, phantom and healthy brain volunteers obtained from 3.0 T and 7.0 T were retrospectively and prospectively undersampled. Performance of the CS models were analyzed by means of structural similarity index (SSIM), root mean squared error (RMSE), signal-to-noise ratio (SNR) and signal quantification of tissue sodium concentration and TQ/SQ ratio.

RESULTS

It was shown that an acceleration of three-fold, leading to less than2 × 10 $$ 2\times 10 $$ min of scan time with a resolution of8 × 8 × 20   mm 3 $$ 8\times 8\times 20\;{\mathrm{mm}}^3 $$ at 3.0 T, are possible. 5D CS improved SSIM by 3%, 5%, 1% and reduced RMSE by 50%, 30%, 8% for in vivo SQ, TQ, and TQ/SQ ratio maps, respectively. Furthermore, for the first time prospective undersampling enabled unprecedented high resolution from8 × 8 × 20   mm 3 $$ 8\times 8\times 20\;{\mathrm{mm}}^3 $$ to6 × 6 × 10   mm 3 $$ 6\times 6\times 10\;{\mathrm{mm}}^3 $$ MQC images of in vivo human brain at 7.0 T without extending acquisition time.

CONCLUSION

5D CS proved to allow up to three-fold acceleration retrospectively on 3.0 T data. 2-fold acceleration was demonstrated prospectively at 7.0 T to reach higher spatial resolution of 23 Na MQC MRI.

Frontiers of Sodium MRI Revisited: From Cartilage to Brain Imaging

Sodium magnetic resonance imaging (²³ Na-MRI) is a highly promising imaging modality that offers the possibility to noninvasively quantify sodium content in the tissue, one of the most relevant parameters for biochemical investigations. Despite its great potential, due to the intrinsically low signal-to-noise ratio (SNR) of sodium imaging generated by low in vivo sodium concentrations, low gyromagnetic ratio, and substantially shorter relaxation times than for proton (¹ H) imaging, ²³ Na-MRI is extremely challenging. In this article, we aim to provide a comprehensive overview of the literature that has been published in the last 10-15 years and which has demonstrated different technical designs for a range of ²³ Na-MRI methods applicable for disease diagnoses and treatment efficacy evaluations. Currently, a wider use of 3.0T and 7.0T systems provide imaging with the expected increase in SNR and, consequently, an increased image resolution and a reduced scanning time. A great interest in translational research has enlarged the field of sodium MRI applications to almost all parts of the body: articular cartilage tendons, spine, heart, breast, muscle, kidney, and brain, etc., and several pathological conditions, such as tumors, neurological and degenerative diseases, and others. The quantitative parameter, tissue sodium concentration, which reflects changes in intracellular sodium concentration, extracellular sodium concentration, and intra-/extracellular volume fractions is becoming acknowledged as a reliable biomarker. Although the great potential of this technique is evident, there must be steady technical development for ²³ Na-MRI to become a standard imaging tool. The future role of sodium imaging is not to be considered as an alternative to ¹ H MRI, but to provide early, diagnostically valuable information about altered metabolism or tissue function associated with disease genesis and progression. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1.

Aggrecan-like biomimetic proteoglycans (BPGs) composed of natural chondroitin sulfate bristles grafted onto a poly(acrylic acid) core for molecular engineering of the extracellular matrix

Biomimetic proteoglycans (BPGs) were designed to mimic the three-dimensional (3D) bottlebrush architecture of natural extracellular matrix (ECM) proteoglycans, such as aggrecan. BPGs were synthesized by grafting native chondroitin sulfate bristles onto a synthetic poly(acrylic acid) core to form BPGs at a molecular weight of approximately ∼1.6 MDa. The aggrecan mimics were characterized chemically, physically, and structurally, confirming the 3D bottlebrush architecture as well as a level of water uptake, which is greater than that of the natural proteoglycan, aggrecan. Aggrecan mimics were cytocompatible at physiological concentrations. Fluorescently labeled BPGs were injected into the nucleus pulposus of the intervertebral disc ex vivo and were retained in tissue before and after static loading and equilibrium conditioning. BPGs infiltrated the tissue, distributed and integrated with the ECM on a molecular scale, in the absence of a bolus, thus demonstrating a new molecular approach to tissue repair: molecular matrix engineering. Molecular matrix engineering may compliment or offer an acellular alternative to current regenerative medicine strategies.

STATEMENT OF SIGNIFICANCE

Aggrecan is a natural biomolecule that is essential for connective tissue hydration and mechanics. Aggrecan is composed of negatively charged chondroitin sulfate bristles attached to a protein core in a bottlebrush configuration. With age and degeneration, enzymatic degradation of aggrecan outpaces cellular synthesis resulting in a loss of this important molecule. We demonstrate a novel biomimetic molecule composed of natural chondroitin sulfate bristles grafted onto an enzymatically-resistant synthetic core. Our molecule mimics a 3D architecture and charge density of the natural aggrecan, can be delivered via a simple injection and is retained in tissue after equilibrium conditioning and loading. This novel material can serve as a platform for molecular repair, drug delivery and tissue engineering in regenerative medicine approaches.

Quantitative techniques for musculoskeletal MRI at 7 Tesla

Whole-body 7 Tesla MRI scanners have been approved solely for research since they appeared on the market over 10 years ago, but may soon be approved for selected clinical neurological and musculoskeletal applications in both the EU and the United States. There has been considerable research work on musculoskeletal applications at 7 Tesla over the past decade, including techniques for ultra-high resolution morphological imaging, 3D T2 and T2* mapping, ultra-short TE applications, diffusion tensor imaging of cartilage, and several techniques for assessing proteoglycan content in cartilage. Most of this work has been done in the knee or other extremities, due to technical difficulties associated with scanning areas such as the hip and torso at 7 Tesla. In this manuscript, we first provide some technical context for 7 Tesla imaging, including challenges and potential advantages. We then review the major quantitative MRI techniques being applied to musculoskeletal applications on 7 Tesla whole-body systems.

Quantitative sodium magnetic resonance imaging of cartilage, muscle, and tendon

Sodium magnetic resonance imaging (MRI), or imaging of the 23Na nucleus, has been under exploration for several decades, and holds promise for potentially revealing additional biochemical information about the health of tissues that cannot currently be obtained from conventional hydrogen (or proton) MRI. This additional information could serve as an important complement to conventional MRI for many applications. However, despite these exciting possibilities, sodium MRI is not yet used routinely in clinical practice, and will likely remain strictly in the domain of exploratory research for the coming decade. This paper begins with a technical overview of sodium MRI, including the nuclear magnetic resonance (NMR) signal characteristics of the sodium nucleus, the challenges associated with sodium MRI, and the specialized pulse sequences, hardware, and reconstruction techniques required. Various applications of sodium MRI for quantitative analysis of the musculoskeletal system are then reviewed, including the non-invasive assessment of cartilage degeneration in vivo, imaging of tendinopathy, applications in the assessment of various muscular pathologies, and assessment of muscle response to exercise.

Effects of instrumental artifacts on triple quantum filtered NMR spectra for spin I=3/2

In this work, the effects of various instrumental artifacts on the triple quantum filtered NMR spectra for spin I=3/2 nuclei are investigated. The studied artifacts include finite pulse widths, phase errors, radio frequency field inhomogeneity and pulse transients, which are commonly encountered in practice. The triple quantum filtered spectra are numerically simulated, based on the evolution of the spin density operator under the Hamiltonian for the artifacts. The results show that the presence of the artifacts introduces a shape distortion in the spectrum as well as a variation in the peak intensity, compared with the spectrum without any artifacts. This work indicates that the existence of the instrumental artifacts may cause a misunderstanding of the triple quantum filtered NMR spectra in experiments. The results suggest that one be aware of the instrumental artifacts when performing the triple quantum filtered NMR experiments.

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.

Double and zero quantum filtered (2)H NMR analysis of D2O in intervertebral disc tissue

The analysis of double and zero quantum filtered (2)H NMR spectra obtained from D2O perfused in the nucleus pulposus of human intervertebral disc tissue samples is reported. Fitting the spectra with a three-site model allows for residual quadrupolar couplings and T2 relaxation times to be measured. The analysis reveals changes in both the couplings and relaxation times as the tissue begins to show signs of degradation. The full analysis demonstrates that information about tissue hydration, water collagen interactions, and sample heterogeneity can be obtained and used to better understand the biochemical differences between healthy and degraded tissue.

Application of sodium triple-quantum coherence NMR spectroscopy for the study of growth dynamics in cartilage tissue engineering

We studied the tissue growth dynamics of tissue-engineered cartilage at an early growth stage after cell seeding for four weeks using sodium triple-quantum coherence NMR spectroscopy. The following tissue-engineering constructs were studied: 1) bovine chondrocytes cultured in alginate beads; 2) bovine chondrocytes cultured as pellets (scaffold-free chondrocyte pellets); and 3) human marrow stromal cells (HMSCs) seeded in collagen/chitosan based biomimetic scaffolds. We found that the sodium triple-quantum coherence spectroscopy could differentiate between different tissue-engineered constructs and native tissues based on the fast and slow components of relaxation rate as well as on the average quadrupolar coupling. Both fast (Tf ) and slow (Ts ) relaxation times were found to be longer in chondrocyte pellets and biomimetic scaffolds compared to chondrocytes suspended in alginate beads and human articular cartilage tissues. In all cases, it was found that relaxation rates and motion of sodium ions measured from correlation times were dependent on the amount of macromolecules, high cell density and anisotropy of the cartilage tissue-engineered constructs. Average quadrupolar couplings were found to be lower in the engineered tissue compared to native tissue, presumably due to the lack of order in collagen accumulated in the engineered tissue. These results support the use of sodium triple-quantum coherence spectroscopy as a tool to investigate anisotropy and growth dynamics of cartilage tissue-engineered constructs in a simple and reliable way.

Evaluation of B0-inhomogeneity correction for triple-quantum-filtered sodium MRI of the human brain at 4.7 T

Off-resonance can result in signal loss on triple-quantum-filtered (TQF) sodium images. Three correction methods have been proposed to mitigate this problem, but their effectiveness and necessity has not yet been evaluated for human brain. This evaluation is warranted given the doubling or quadrupling of scan length without the expected signal-to-noise ratio (SNR) benefit. First, simulations and agar gel experiments showed that the off-resonance effects on signal loss were asymmetric about on-resonance. Second, the two scan length doubling correction methods were tested for two sets of TQF acquisition parameters in 10 healthy volunteers at 4.7 Tesla. Using only manual shimming on the sodium signal and a 3-pulse TQF sequence with an optimal preparation time value of 6 ms, the majority of brain tissue voxels (87-94% depending on sequence parameters) experienced B0 inhomogeneity amounting to less than 10% signal losses. Relative signal intensities of 0.96 ± 0.04 and 0.98 ± 0.02 were measured in these voxels relative to on-resonant voxels for SNR-optimized and standard TQF parameters. The remaining brain voxels in regions with known susceptibility problems suffered more substantial signal losses, which were partially recovered with the correction methods. At field strengths below 4.7T, at similar ranges of offset frequencies at higher fields and in typical volunteers, B0 correction appears unnecessary for TQF analysis in most of the brain. In many cases where regions with known susceptibility issues are not of concern, a doubling of scan time may be better spent to either improve SNR or spatial resolution in the TQF sodium images.

Sodium quantification in the spinal cord at 3T

PURPOSE

Sodium channels are involved in neuronal function and therefore methods to assess tissue sodium concentration in vivo are exceptionally appealing. Recently there has been a renewed interest for brain sodium magnetic resonance imaging (MRI), thanks to higher magnetic field strength scanners. However, sodium measures in the spinal cord are lacking due to major technical challenges. Here we propose for the first time a clinically feasible non-invasive method for quantifying sodium in the spine using magnetic resonance spectroscopy.

METHODS

Sodium spectra from the cervical cord were collected using image selected in vivo spectroscopy (∼14 min scan time) and quantified using a reference phantom.

RESULTS

The sodium magnetic resonance spectroscopy measures provided in vivo concentration estimates of 31.2±2.4 mM. Repeat scans showed good reproducibility with a coefficient of variation of <6%.

CONCLUSION

Proposed here for the first time is a fast non-invasive technique to quantify total sodium in the spinal cord in vivo. This newly proposed technique has a great potential for translation into clinic, thanks to its simplicity.

Sodium MR imaging of the lumbar intervertebral disk at 7 T: correlation with T2 mapping and modified Pfirrmann score at 3 T--preliminary results

PURPOSE

To compare sodium imaging of lumbar intervertebral disks in asymptomatic volunteers at 7-T magnetic resonance (MR) imaging with quantitative T2 mapping and morphologic scoring at 3 T.

MATERIALS AND METHODS

Following ethical board approval and informed consent, the L2-3 to L5-S1 disks were examined in 10 asymptomatic volunteers (nine men, one woman; mean age, 30 years; range, 23-43 years). At 7 T, normalized sodium signal-to-noise ratios were calculated, by using region-of-interest analysis. At 3 T, T2 mapping was performed with a multiecho spin-echo sequence (repetition time msec/echo times msec, 1500/24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156). T2 values were calculated over the nucleus, with a pixelwise, monoexponential nonnegative least-squares-fit analysis. Morphologic grading according to a modified Pfirrmann score was assessed independently by three experienced musculoskeletal radiologists, and Pearson correlation analysis of the covariates was performed.

RESULTS

The mean normalized sodium signal intensity was 275.5±115.4 (standard deviation). The T2 mapping showed a mean value of 89.8 msec±19.34. The median modified Pfirrmann score was 2b (90% had score≤3c). The Pearson correlation coefficient showed a cubic function between sodium imaging and the modified Pfirrmann score, a moderate inverse correlation between T2 mapping and the modified Pfirrmann score (r=-0.62), and no correlation between sodium imaging and T2 mapping (r=0.06).

CONCLUSION

The results suggest that MR imaging of the intervertebral disk, using sodium imaging and T2 mapping, can help characterize different component changes and that both of these methods are to some degree related to the Pfirrmann score.

Assessment of glycosaminoglycan distribution in human lumbar intervertebral discs using chemical exchange saturation transfer at 3 T: feasibility and initial experience

Recent studies have proposed that glycosaminoglycan chemical exchange saturation transfer (gagCEST) is associated with a loss of glycosaminoglycans (GAGs), which may be an initiating factor in intervertebral disc (IVD) degeneration. Despite its promising potential, this application has not been reported in human in vivo IVD studies because of the challenges of B(0) magnetic field inhomogeneity in gagCEST. This study aimed to evaluate the feasibility of quantifying CEST values in IVDs of healthy volunteers using a clinical 3 T scanner. A single-slice turbo spin echo sequence was used to quantify the CEST effect in various GAG phantoms and in IVDs of 12 volunteers. The phantom results indicated high correlation between gagCEST and GAG concentrations (R(2)  = 0.95). With optimal B(0) inhomogeneity correction, in vivo CEST maps of IVDs showed robust contrast between the nucleus pulposus (NP) and the annulus fibrosus (AF) (p < 0.01), as well as higher signal in the central relative to the peripheral NP. In addition, a trend of decreasing CEST values from upper to lower disc levels was evident in NP. Our results demonstrate that in vivo gagCEST quantification in human lumbar IVDs is feasible at 3 T in combination with successful B(0) inhomogeneity correction, but without significant hardware modifications. Our initial findings suggest that it would be worthwhile to perform direct correlation studies between CEST and GAGs using cadaver samples, and to extend this novel technique to studies on patients with degenerative discs to better understand its distinct imaging features relative to conventional techniques.

Triple-quantum-filtered sodium imaging of the human brain at 4.7 T

The limited signal-to-noise ratio of triple-quantum-filtered MRI of sodium is a major hurdle for its application clinically. Although it has been shown that short 90° radiofrequency pulses in combination with sufficiently long repetition time for full T(1) recovery (labelled "standard" parameters) produce the maximum signal through the triple-quantum-filter, and in this work, simulation and images of agar phantoms and human brain demonstrate that the use of longer radiofrequency pulses and reduced repetition time (optimized parameters to accommodate more averages for a constant specific absorption rate, reducing noise variance for a given scan length) results in signal-to-noise ratio improvement (22 ± 5% in brain tissue of five healthy volunteers--images created in 11 min with nominal resolution of 8.4 mm isotropic). However, residual intensity was observed in the ventricular space on triple-quantum-filtered images acquired with either optimized or standard parameters, contrary to the expectation of complete single-quantum signal suppression. Further simulation and experimentation suggest that this is likely due to the combination of triple-quantum-passed signal from surrounding brain tissue being spatially smeared into the ventricular space and single-quantum-signal breakthrough from sodium nuclei in the fluid space. It is shown that the latter can be eliminated with judicious first flip angle selection.

2H double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in Nafion

Multiple quantum 2H NMR spectroscopy is used to study the structure and dynamics of D2O in Nafion membranes as a function of membrane hydration. By employing both double- and zero-quantum filtered experiments, residual quadrupolar coupling constants and T2 relaxation values are obtained. The residual couplings vary from 240 to 20 Hz and the T2 values range from 20 to 180 ms, with the high hydration values having smaller couplings and longer T2 values. Analysis of the data using a water-exchange model suggests that the changes in parameters arise from a change in the fraction of time water spends in the anisotropic environments and not from changes in the order parameters that characterize the anisotropic sites. It has been found that a two-site model is needed to accurately fit the spectra above a hydration level of 10 D2O per sulfonate, with the second site having a negligible residual quadrupolar coupling. The data supports a model with two different hydration layers at high hydration and can be understood in terms of the recently proposed parallel-channel model for Nafion hydration.

Sodium MRI

Sodium ((23)Na) imaging has a place somewhere between (1)H-MRI and MR spectroscopy (MRS). Like MRS it potentially provides information on metabolic processes, but only one single resonance of ionic (23)Na is observed. Therefore pulse sequences do not need to code for a chemical shift dimension, allowing (23)Na images to be obtained at high resolutions as compared to MRS. In this chapter the biological significance of sodium in the brain will be discussed, as well as methods for observing it with (23)Na-MRI. Many vital cellular processes and interactions in excitable tissues depend on the maintenance of a low intracellular and high extracellular sodium concentration. Healthy cells maintain this concentration gradient at the cost of energy. Leaky cell membranes or an impaired energy metabolism immediately leads to an increase in cytosolic total tissue sodium. This makes sodium a biomarker for ischemia, cancer, excessive tissue activation, or tissue damage as might be caused by ablation therapy. Special techniques allow quantification of tissue sodium for the monitoring of disease or therapy in longitudinal studies or preferential observation of the intracellular component of the tissue sodium. New methods and high-field magnet technology provide new opportunities for (23)Na-MRI in clinical and biomedical research.

Quadrupolar-coupling-specific binomial pulse sequences for in vivo 23Na NMR and MRI

Aimed at selective detection of (23)Na with specific quadrupolar couplings for in vitro NMR and MRI, we present a series of quadrupolar binomial pulse sequences offering high specificity with respect to the quadrupolar couplings of the excited species. It is demonstrated that pulse sequences with an increasing number of elements, e.g., 11, 121, 1331, 14641, and 15101051, with the units representing flip angles smaller than the 90 degrees pulses typically encountered in binomial spin-1/2 solvent suppression experiments, and different phase combinations may provide a high degree of flexibility with respect to quadrupolar coupling selectivity and robustness towards rf inhomogeneity. This may facilitate efficient separation of, for example, intra and extracellular (23)Na in tissues with efficient control of the excitation (or suppression) of central as well as satellite transitions through on- and off-resonance irradiation. The pulse sequences are described in terms of their analogy to binomial liquid-state NMR solvent suppression experiments and demonstrated numerically and experimentally through NMR and MRI experiments on a 7 T horizontal small-bore animal magnet system.

Anisotropy of spin relaxation of water protons in cartilage and tendon

Transverse spin relaxation rates of water protons in articular cartilage and tendon depend on the orientation of the tissue relative to the applied static magnetic field. This complicates the interpretation of magnetic resonance images of these tissues. At the same time, relaxation data can provide information about their organisation and microstructure. We present a theoretical analysis of the anisotropy of spin relaxation of water protons observed in fully hydrated cartilage. We demonstrate that the anisotropy of transverse relaxation is due almost entirely to intramolecular dipolar coupling modulated by a specific mode of slow molecular motion: the diffusion of water molecules in the hydration shell of a collagen fibre around the fibre, such that the molecular director remains perpendicular to the fibre. The theoretical anisotropy arising from this mechanism follows the 'magic-angle' dependence observed in magnetic-resonance measurements of cartilage and tendon and is in good agreement with the available experimental results. We discuss the implications of the theoretical findings for MRI of ordered collagenous tissues.