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

Heterogeneous (bio)systems are often characterized by several water-containing compartments that differ in relaxation time values and diffusion constants. Because of the relatively small differences among these diffusion constants, nonoptimal measuring conditions easily lead to the conclusion that a single diffusion constant suffices to describe the water mobility in a heterogeneous (bio)system. This paper demonstrates that the combination of a T-2 measurement and diffusion measurements at various echo times (TE), based on the PFG MSE sequence, enables the accurate determination of diffusion constants which are less than a factor of 2 apart. This new method gives errors of the diffusion constant below 10% when two fractions are present, while the standard approach of a biexponential fit to the diffusion data in identical circumstances gives larger (>25%) errors. On application of this approach to water in apple parenchyma tissue, the diffusion constant of water in the 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 increased 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 investigate cell sizes is strongly dependent on the T-2 values of the fractions within the sample. Because relaxation times are used to discriminate the diffusion constants, we propose to name this approach diffusion analysis by relaxation-time-separated (DARTS) PFG NMR. (C) 1995 Academic Press, Inc.