We introduce an improved variant of the C7 pulse-sequence for efficient recoupling of spin-1/2 pair dipolar interactions in magic-angle spinning solid-state NMR spectroscopy. The tolerance of C7 toward isotropic as well as anisotropic chemical shift offsets and rf inhomogeneity is improved considerably by replacing the original basic element Cφ44̄=(2π)φ(2π)φ+π with the cyclically permuted element Cφ14̄3=(π/2)φ(2π)φ+π(3π/2)φ. The improved performance of this permutationally offset stabilized variant of C7 is analyzed by average Hamiltonian theory to fifth order, numerical simulations, and demonstrated by experiments on powder samples of doubly 13C-labeled barium oxalate hemihydrate and diammonium fumarate.
A novel approach to quadrupolar-echo (QE) NMR of
half-integer quadrupolar nuclei in static powders is
analyzed. By acquisition of the QE spectrum during a
Carr−Purcell−Meiboom−Gill (CPMG) train of selective
π pulses, the second-order quadrupolar line shape for the central
transition is split into a comb of sidebands
leading to a considerable increase in the sensitivity compared to a
conventional QE spectrum. The applicability
of the method for determination of magnitudes and relative orientation
of chemical shielding and quadrupolar
coupling tensors is examined. Through numerical simulation and
iterative fitting of experimental 87Rb
(RbClO4
and RbVO3) and 59Co spectra
(Co(NH3)5 Cl3), it is
demonstrated that the quadrupolar CPMG experiment
represents a useful method for studying half-integer quadrupolar nuclei
exhibiting large quadrupolar coupling
combined with anisotropic chemical shielding interactions.
Sensitivity enhancements by a factor of up to
about 30 are observed for the samples studied.
Due to its unique sensitivity to tissue microstructure, diffusion-weighted magnetic resonance imaging (MRI) has found many applications in clinical and fundamental science. With few exceptions, a more precise correspondence between physiological or biophysical properties and the obtained diffusion parameters remain uncertain due to lack of specificity. In this work, we address this problem by comparing diffusion parameters of a recently introduced model for water diffusion in brain matter to light microscopy and quantitative electron microscopy. Specifically, we compare diffusion model predictions of neurite density in rats to optical myelin staining intensity and stereological estimation of neurite volume fraction using electron microscopy. We find that the diffusion model describes data better and that its parameters show stronger correlation with optical and electron microscopy, and thus reflect myelinated neurite density better than the more frequently used diffusion tensor imaging (DTI) and cumulant expansion methods. Furthermore, the estimated neurite orientations capture dendritic architecture more faithfully than DTI diffusion ellipsoids.
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