High-resolution NMR of static samples by rotation of the magnetic field

Mechanical rotation of a sample at 54.7 degrees with respect to the static magnetic field, so-called magic-angle spinning (MAS), is currently a routine procedure in nuclear magnetic resonance (NMR). The technique enhances the spectral resolution by averaging away anisotropic spin interactions thereby producing isotropic-like spectra with resolved chemical shifts and scalar couplings. It should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied, e.g., magic-angle rotation of the B0 field (B0-MAS). Here, this principle is experimentally demonstrated in a static sample of solid hyperpolarized xenon at approximately 3.4 mT. By extension to moderately high fields, it is possible to foresee interesting applications in situations where physical manipulation of the sample is inconvenient or impossible. Such situations are expected to arise in many cases from materials to biomedicine and are particularly relevant to the novel approach of ex situ NMR spectroscopy and imaging.     

Four-DimensionalH andNa Imaging Using Continuously Oscillating Gradients

A class of fast magnetic spectroscopic imaging methods using continuously oscillating gradients for four-dimensional (three spatial and one spectral) localization is introduced. Sampling may start immediately following the application of an RF excitation pulse, thus enabling measurement of spin density, chemical shift, and relaxation rates of short-T₂species. For spatial localization, steady-state sinusoidal gradient waveforms are used to sample a ball inkspace. The two types of trajectories presented include: (1) continuously oscillating gradients with continuously rotating direction used for steady-state free-precession imaging and (2) continuously oscillating gradients followed by a spoiler directed along discrete projections. Design criteria are given and spatial–spectral and spatial–temporal reconstruction methods are developed. Theoretical point-spread functions and signal-to-noise ratios are derived while consideringT₂^*, off-resonance effects, and RF excitation options. Experimental phantom,in vivo,andin vitro¹H and²³Na images collected at 2.35 T are presented. The¹H images were acquired with isotropic spatial resolution ranging from 0.03 to 0.27 cm³and gradient-oscillation frequencies ranging from 600 to 700 Hz, thus allowing for the separation of water and lipid signals within a voxel. The²³Na images, acquired with 500 and 800 Hz gradient waveforms and 0.70 cm³isotropic resolution, were resolved in the time domain, yielding spatially localized FIDs.