Taking, processing, and interpreting spin-echo data in porous media and tissues.

Measurements of T2 by the Carr-Purcell-Meiboom-Gill (CPMG) method without externally applied field gradients for fluids in porous media are affected by locally inhomogeneous fields from susceptibility differences. Diffusion leads to a substantial range of correlation times tau c for change of local field. Wide distributions often lead to the dependence of 1/T2 on tau (half pulse spacing) of the form 1/T2 approximately R1 + R2 tan-1 (R3 tau), while narrow distributions require the extra term (R2R3/R4)exp(-R4 tau). To determine tau-dependence, the effects of different ranges of data coverage can be minimized by using only data within the time range common to all measurements and by adding a virtual n = 0 point to each data set, where n is echo number and this point is determined by extrapolating echo amplitudes for the small tau values.