Grad sandw.m

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Computes the effect of a gradient sandwich on the sample average density matrix using Edwards formalism. It is assumed that the effect of diffusion is negligible, that the gradients are linear, and that they are antisymmetric about the middle of the sample. Syntax:

    rho=grad_sandw(spin_system,L,rho,P,g_amps,s_len,g_durs,s_facs)

Arguments:

             rho   - spin system state vector
               L   - system Liouvillian
               P   - total propagator for all events happening
                     between the two gradients
          g_amps   - row vector containing the amplitudes of
                     the two gradients, Gauss/cm
           s_len   - sample length, cm
          g_durs   - row vector containing the durations of
                     the two gradients, seconds
          s_facs   - shape factors of the two gradients, use
                     [1 1] for square gradient pulses

Note: the function integrates over sample coordinates - subsequent gradient pulses would not refocus the magnetization that it has left defocused. More information on the subject is available in Luke's paper (http://dx.doi.org/10.1016/j.jmr.2014.01.011).