Pulse field gradient dipolar interaction

At the center of the echo all resonance offsets from interactions linear in the spin quantum number are canceled as long as these interactions operate for the full duration ofTE. Linear spin interactions include chemical shifts, heteronu-clear dipolar couplings, field inhomogeneity, field gradients, and transmitter frequency offsets but do not include quadru-polar and homonuclear dipolar couplings. There will however be a net phase evolution induced by an interaction to the extent its duration or intensity is not balanced with respect to the two halves of TE (that is, the balance with respect to amount of phase evolution on either side of the 180° pulse). 

Some care must be taken with fringe field methods if motional averaging of dipolar interactions is incomplete. Echoes then tend to be modulated by the so-called dipolar correlation effect [15, 16]. One can account for this phenomenon by dividing the (normalized) echo amplitudes recorded with and without gradient at the same Larmor frequency and at the same pulse intervals [11,12]. Any influence by relaxation and the dipolar correlation effect can be eliminated in this way. 

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