Parameter maps of 1H residual dipolar couplings in tendon under mechanical load

Fourier and Laplace-like low-field NMR spectroscopy: The perspectives of multivariate and artificial neural networks analyses

Low field Nuclear Magnetic Resonance (LF-NMR) is a rich source of information for a wide range of samples types. These can be hard or soft solids, such as plastics or elastomers; bulk liquids or liquids absorbed in porous materials, and can come from biomaterials, biological tissues, archaeological artifacts, cultural heritage objects. LF-NMR instruments present a significant advance especially for in situ, ex situ and in vivo measurement of relaxation and diffusion. Moreover, high resolution 1D and 2D spectroscopy, as well as magnetic resonance (MR) imaging are available in these fields. In this work we discuss the advanced analysis of the data measured in LF-NMR from the perspectives of tertiary level that implies the analysis on principal components (PCA), and on the quaternary analysis that uses an artificial neural network (ANN). The principles of PCA and ANN are largely discussed. For the PCA analysis, a series of 52 spectra were analyzed, having been recorded in vivo by LF-NMR. Of these spectra, 38 were generated from normal uterus, 7 by uterus tissue with endometrial cancer, and another 7 were obtained from tissues of women with uterine cervical cancer. The PC1 vs PC2 plot was further analyzed using an artificial neural network, and the results are presented as 2D maps of probability. Furthermore, the perspectives of applying an ANN to solve the problem of Laplace-like inversion are discussed. An example of such ANN was presented and the performance was discussed. Finally, a model of complex ANN, capable to sequentially solve this kind of problems specific to LF-NMR is proposed and discussed.

2Q NMR of (2)H2O ordering at solid interfaces

Solvent ordering at an interface can be studied by multiple-quantum NMR. Quantitative studies of (2)H2O ordering require clean double-quantum (2Q) filtration and an analysis of 2Q buildup curves that accounts for relaxation and, if randomly oriented samples are used, the distribution of residual couplings. A pulse sequence with absorption mode detection is extended for separating coherences by order and measuring relaxation times such as the 2Q filtered T2. Coherence separation is used to verify 2Q filtration and the 2Q filtered T2 is required to extract the coupling from the 2Q buildup curve when it is unresolved. With our analysis, the coupling extracted from the buildup curve in (2)H2O hydrated collagen was equivalent to the resolved coupling measured in the usual 1D experiment and the 2Q to 1Q signal ratio was in accord with theory. Application to buildup curves from (2)H2O hydrated elastin, which has an unresolved coupling, revealed a large increase in the 2Q signal upon mechanical stretch that is due to an increase in the ordered water fraction while changes in the residual coupling and T2 are small.

DQF-MT MRI of connective tissues: application to tendon and muscle

OBJECT

The sequence combining DQF (double quantum filtering) with magnetisation transfer (DQF-MT) was tested as an alternative to the DQF sequence for characterising tendon and muscle by MR imaging.

MATERIALS AND METHODS

DQF-MT images of tendon-muscle phantoms were obtained at 4.7 T using ultra-short time to echo (UTE) methods in order to alleviate the loss of SNR due to the short T2 of the tissues. Two different sampling schemes of the k-space, Cartesian or radial, were employed. In vivo images of the human ankle on a clinical 1.5 T scanner are also presented. Parameters providing optimal tendon signal as well as optimal contrast between this tissue and muscle were determined.

RESULTS

Two sets of parameters resulting in different contrasts between the tissues were found. For the first set (short creation time τ = 10 μs and magnetisation exchange time t LM = 100 ms), DQF-MT signals in muscle and tendon were detected, with that of the tendon being the larger one. For the second set (long creation time τ = 750 μs and magnetisation exchange time 10 μs < t LM < 100 ms), the DQF-MT signal was detected only in the tendon, and the decay of the double quantum coherence was slower than that observed for the first one, which allowed us to acquire DQF-MT MR images on a clinical 1.5 T MR scanner with minimal software interventions. In favourable conditions, the DQF-MT signal in the tendon could represent up to 10 % of the single-quantum signal.

CONCLUSION

Dipolar interaction within macromolecules such as collagen and myosin is at the origin of the DQF-MT signal observed in the first parameter set. This should enable the detection of muscle fibrosis.

Structure and dynamics of water in native and tanned collagen fibers: Effect of crosslinking

The influence of crosslinking on the hydration structure of collagen has been investigated. Nuclear magnetic resonance, dielectric relaxation and thermoporometry were used to investigate water structure in native and crosslinked collagen fibers on both wet and dried specimen. Measurements reveal the influence of different chemical treatments on the transverse relaxation time and polarization of the collagen fibers. The frequency dependence of dielectric constant of collagen fibers displays an induction behavior on low frequencies. Bound water constrained in collagen fibers seems to provide signatures for changes induced by crosslinking agents on the pore diameter and distribution in collagen fibers. A correlation of transverse relaxation time of water in dry and wet states presented in this study presents an experimental tool for examining the differences in efficacy of crosslinking agents. Changes in the dielectric relaxation, dynamics of water structure and hydroporometric structure of collagen are dependent on the nature of crosslinking material.

Water in tendon: orientational analysis of the free induction decay

The orientation dependence of the free induction decay (FID) of 1H NMR water signal in ex vivo bovine digital flexor tendon at the native level of hydration is reported. Residual dipolar coupling due to the overall tissue anisotropy produces a 6:1 change in the signal intensity as an angle between the long axis of a specimen and the external magnetic field is changed from the "magic angle" of 54.7 degrees to 0 degrees. The strength of residual dipolar interactions between water protons was estimated by orientational analysis of the signal intensity to be equal to 780 Hz. Apparent signal maxima are observed at orientations 8-13 degrees away from 54.7 degrees due to an inhomogeneous contribution to the decay. A small fraction of total water in tendon is detectable at all orientations and exhibits a shift in the precession frequency. It is hypothesized that this water fraction resides in the interconnecting gaps at the ends of collagen molecules. The gaps have a disordered environment that allows for a zero time average of dipolar interactions. Measured frequency and phase shifts are interpreted as signatures of the bulk magnetic susceptibility effect due to geometry of the cavity formed by adjacent gaps at the ends of the collagen molecules. The multiexponentiality of the FID decay is hypothesized to be due to the exchange between orientationally restricted water structured along the length of the collagen molecule and disordered water in the cavity.