Ringing effects eliminated spin echo in solids.

Two types of ringing effects eliminated spin echo sequences have been introduced. To achieve the task, two additional 90° pulses with proper phase cycles are placed at the beginning of the pulse sequences. The spin echo time is calculated with the perturbation method to the first order, i.e. taking into account only the dipolar secular term. The non-secular term causes an imaginary part of the FID, leading to an unsymmetrical NMR spectrum. This effect, according to a symmetry of NMR sequences under phase inversion, can be compensated by inverting all the x and -x or y and -y phases. The properties of the symmetry are derived based on the theory of density matrix. In addition, the non-secular term also results in a small drop (several per cent) of the echo amplitude, but it nearly does not affect the echo time. With these pulse sequences we are able to get a spectrum with an echo delay only 1.1μs without distortion using a Bruker AVANCE III NMR instrument.