Method for selectively exciting nuclear spins in a radio- frequency field with predetermined strength

摘要

In an NMR spectrometer, nuclear spins are subjected to a radio- frequency (RF) pulse sequence consisting of repeating cycles, each having a first and a second pulse set. The first pulse set generates a first large-amplitude rotation of the spins in which the angle through which the spins rotate varies with RF field strength. The second pulse set generates a second rotation which has an axis orthogonal to the first rotation axis and the angle through which the spins rotate is independent of the RF field strength. When the nuclear spins are placed in a non- homogeneous RF field and the above pulse sequence is applied, those spins which are located in an area where the RF field strength causes the first pulse set to rotate the spins through an angle which is a multiple of 2 . pi., the second, uniform rotation accumulates with each repetition of the first and second pulse sets in order to selectively excite those spins.