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

Abstract

Measurements of T 2 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 τ c for change of local field. Wide distributions often lead to the dependence of 1 T 2 on τ (half pulse spacing) of the form 1 T 2 ≈ R 1 + R 2 tan −1 (R 3τ) , while narrow distributions require the extra term ( R 2R 3 R 4 ) exp(−R 4τ) . To determine τ-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 τ values.