Double PGSE NMR with stimulated echoes: Phase cycles for the selection of desired encoding

When two pairs of position-encoding pulses are used in a pulsed gradient sp in echo (PGSE) NMR experiment, it is possible to examine velocity fluctuati ons. The one-dimensional version of double PGSE NMR uses identical pulse pa irs whose amplitudes are stepped simultaneously. In the two-dimensional ver sion (VEXSY) the pulse pairs are stepped independently, resulting in a velo city exchange spectrum. A key limitation in such experiments is transverse relaxation, so that stimulated echoes are often used as the method of choic e. It is shown here that the use of stimulated echoes results in a superpos ition of signals arising from different magnetization pathways such that th e spin phases may reflect both the sum and difference of displacements over the pulse pair encoding times, as well as the displacement over the exchan ge time between the pulse pairs. A phase cycle scheme that selects desired encodings as required is demonstrated. (C) 2001 Academic Press.