Four-dimensional 1H and 23Na 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-T2 species. For spatial localization, steady-state sinusoidal gradient waveforms are used to sample a ball in k space. 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 considering T2*, off-resonance effects, and RF excitation options. Experimental phantom, in vivo, and in vitro 1H and 23Na images collected at 2.35 T are presented. The 1H images were acquired with isotropic spatial resolution ranging from 0.03 to 0.27 cm3 and gradient-oscillation frequencies ranging from 600 to 700 Hz, thus allowing for the separation of water and lipid signals within a voxel. The 23Na images, acquired with 500 and 800 Hz gradient waveforms and 0.70 cm3 isotropic resolution, were resolved in the time domain, yielding spatially localized FIDs.