NMR imaging, NMR diffraction and applications of pulsed gradient spin echoes in porous media.

Diffraction ideas were first introduced to NMR by Mansfield and Grannell. Here we compare "Mansfield (k-space) diffraction" with the "diffusive (q-space) diffraction," noting their similarities and differences. The diffusive-diffraction concept is extremely helpful in elucidating the Pulsed Gradient Spin Echo NMR experiment for fluid molecules in porous media and has been applied successfully to the model isolated pore (the "single slit" case) and the orientationally disordered interconnected pore glass (the "powder grating" case). This picture is based on the idea that the PGSE experiment has a scattering analogy in which a wavevector (q) is conjugate to the displacement of the spins over the time, D, between the gradient pulses. Recently, the theory has been extended to allow for both wall relaxation effects and finite gradient pulse width effects.