Lawrence Berkeley National Laboratory

Magnetic resonance imaging (MRI) in very low fields is fundamentally limited by untruncated concomitant gradients which cause severe distortions in image acquisition and volume selection if the gradient fields are strong compared to the static field. In this paper, it is shown that gradient fields oscillating in quadrature can be used for spatial encoding in low fields and provide substantial improvements over conventional encoding methods using static gradients. In particular, cases where the gradient field is comparable to or higher than the external field, Gmax/B0 > 1, are examined. It is shown that undistorted slice selection and image encoding is possible because of smaller geometric phase errors introduced during cyclic motions of the Hamiltonian. In the low field limit (Gmax/B_0 -> infinity) slice selection is achieved with a combination of soft pulse segments and a coherent train of hard pulses to average out concomitant fields over the fast scale of the rf Hamiltonian.