Adiabatic sweep pulses for earth's field NMR with a surface coil

Adiabatic NMR sweep pulses are described for inversion and excitation in very low magnetic fields B₀ and with broad distribution of excitation field amplitude B₁. Two aspects distinguish the low field case: (1) when B₁ is comparable to or greater than B₀, the rotating field approximation fails and (2) inversion sweeps cannot extend to values well below the Larmor frequency because they would approach or pass through zero frequency. Three approaches to inversion are described. The first is a conventional tangent frequency sweep down to the Larmor frequency, a 180° phase shift, and a sweep back up to the starting frequency. The other two are combined frequency and amplitude sweeps covering a narrower frequency range; one is a symmetric sweep from above to below the Larmor frequency and the other uses a smooth decrease of B₁ immediately before and after the 180° phase shift. These two AM/FM sweeps show excellent inversion efficiencies over a wide range of B₁, a factor of 30 or more. We also demonstrate an excitation sweep that works well in the presence of the same wide range of B₁. We show that the primary effect of the counter-rotating field (i.e., at low B₀) is that the magnetization suffers large, periodic deviations from where it would be at large B₀. Thus, successful sweep pulses must avoid any sharp features in the amplitude, phase, or frequency.