Detection of sodium ions in anisotropic environments through spin‐lock NMR

Abstract

A new method for selectively detecting sodium ions in anisotropic environments is presented. A spin-lock (SL) sequence, followed by a coherence transfer pulse, generates rank-two zero-quantum coherences, and converts them into observable transverse magnetization. The quadrupolar polarization is only generated when there are residual quadrupolar couplings in the sample, and provided the SL field strength is comparable to these couplings. This filter has proved to be more efficient than a double-quantum magic-angle (DQ-MA) filter in generating observable signal from ions in anisotropic media in both a nasal bovine cartilage sample and a liquid crystalline DNA sample. In living tissues, sodium ions are distributed in a multitude of local environments (1,2). These environments can be loosely classified in three different categories: 1) The liquid-like environments, wherein the sodium ions are inside the extreme narrowing limit (ENL), yielding an NMR spectrum comprised of a single Lorentzian line.2) The more viscous, isotropic environments, wherein the quadrupolar interaction is still averaged to zero on a time scale less than the inverse of the Larmor frequency, but with the ions involved in slow molecular motion outside the ENL. The ions in such an environment yield a single NMR resonance, consisting of a superposition of two Lorentzians with different widths.3) The liquid-crystalline, or anisotropic environments, wherein the quadrupolar interaction is not entirely averaged out on a time scale less than the inverse of the Larmor frequency, yielding sodium spectra consisting of a central line and two satellites. The study of the distribution of sodium ions in these environments is very useful for understanding the pathophysiology of many diseases, since a redistribution of ions is associated with any pathological condition affecting normal cell function (3).Sodium single-quantum NMR is nonselective with regard to the motional properties of the ions in different environments (it contains contributions from all three types of environments mentioned above). A multitude of sequences have been developed to increase the selectivity of sodium NMR, and thereby improve its sensitivity to changes associated with disease. For example, the signal obtained with a triple-quantum (TQ) sequence does not have contributions from sodium ions inside the ENL, and has successfully been employed as a sensitive indicator for pathologic conditions in animal models and humans (4 -7). However, further selectivity may still be desired, since certain diseases, such as cartilage degenerative diseases, are known to correlate well with a loss of sodium-ion ordering in the affected tissue (8). A sequence that does not discern between sodium ions in more viscous, isotropic, and liquid-crystalline anisotropic environments (such as a TQ filter) produces an unnecessary baseline, which can eventually mask small changes taking place in the pool of ordered ions. Consequently, NMR pulse sequences that yield signal only from 23 Na ions in anisotropic enviro...

Multinuclear NMR and microscopic MRI studies of the articular cartilage nanostructure

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Thermal relaxation and coherence dynamics of spin 3/2. I. Static and fluctuating quadrupolar interactions in the multipole basis

“…T 21 ͑s, a͒ f 22 ͑1͒ ͑t͒ [36] for the inaccessible rank-two single-quantum coherences. The relaxation functions are given by Eqs.…”

“…This sequence generates spin polarization quadrupolar order and converts it into observable magnetization, when there are residual quadrupolar couplings in the sample and provided the spin-lock field strength is comparable with these couplings. The spin-lock filtering technique does not rely on a flip angle effect for the selection of the desired signal component and therefore might prove preferable in application studies (36).…”

Sodium and T1ρ MRI for molecular and diagnostic imaging of articular cartilage

“…The pronounced negative lobes of DQF spectra are due to the presence of (T 2 ±2 ) contribution in addition to T 3 ±2 . Other pulse sequences that are useful in studying ordered structures are based on the Jeener-Broekaert (108,110) and spin-locking sequences (84,111). In both these sequences, the signal is detected by filtering through the quadrupolar order T 2 0 .…”