EXTRACTING DIFFUSION CONSTANTS FROM ECHO-TIME-DEPENDENT PFG NMR DATA USING RELAXATION-TIME INFORMATION

Heterogeneous (bio)systems are often characterized by several water-co ntaining compartments that differ in relaxation time values and diffus ion constants. Because of the relatively small differences among these diffusion constants, nonoptimal measuring conditions easily lead to t he conclusion that a single diffusion constant suffices to describe th e water mobility in a heterogeneous (bio)system. This paper demonstrat es that the combination of a T-2 measurement and diffusion measurement s at various echo times (TE), based on the PFG MSE sequence, enables t he accurate determination of diffusion constants which are less than a factor of 2 apart. This new method gives errors of the diffusion cons tant below 10% when two fractions are present, while the standard appr oach of a biexponential fit to the diffusion data in identical circums tances gives larger (>25%) errors. On application of this approach to water in apple parenchyma tissue, the diffusion constant of water in t he vacuole of the cells (D = 1.7 X 10(-9) m(2)/s) can be distinguished from that of the cytoplasm (D = 1.0 X 10(-9) m(2)/s). Also, for mung bean seedlings, the cell size determined by PFG MSE measurements incre ased from 65 to 100 mu m when the echo time increased from 150 to 900 ms, demonstrating that the interpretation of PFG SE data used to inves tigate cell sizes is strongly dependent on the T-2 values of the fract ions within the sample. Because relaxation times are used to discrimin ate the diffusion constants, we propose to name this approach diffusio n analysis by relaxation-time-separated (DARTS) PFG NMR. (C) 1995 Acad emic Press, Inc.