Efficient adiabatic fast passage for NMR population inversion in the presence of radiofrequency field inhomogeneity and frequency offsets

Abstract

Abstract Adiabatic fast passage is a powerful tool for uniformly inverting magnetization in the presence of rf magnetic field inhomogeneity and frequency offsets. Long pulses or large rf powers are necessary for this technique to be effective, however, when the usual linear frequency sweep is used. A sweep of the form ω(t) − ω0 = ω1tan(αω1t), on the other hand, is shown to be an order of magnitude more efficient than the linear sweep. Here ω(t) is the rf field frequency, ω0 is the Larmor frequency, w1 = -γB1 (gyromagnetic ratio times rf field strength), and α is a constant determined by w1, the total sweep time, and the range of the frequency sweep. This tangential frequency sweep can produce substantially better inversion than composite pulses of the same amplitude.