At the same time diffusion and flow have to be discriminated. These goals can best be obtained by the use of pulsed magnetic field gradient (PFG) techniques (for some background, see Van
As and Windt 2008). In this experiment, a sequence of two magnetic field gradient pulses of duration δ and equal magnitude G but opposite sign (or equal sign but separated by an 180 rf pulse) label the protons as a function of their position. If the spins remain at exactly the same position #selleck inhibitor randurls[1|1|,|CHEM1|]# the effect of the gradient pulses compensate each other. However, as soon as translation (displacement) motion occurs, the gradients do not exactly compensate each other anymore, resulting in https://www.selleckchem.com/products/AZD6244.html attenuation of the signal amplitude. The amount of this attenuation is determined by the length and amplitude of the gradient pulses, and by the mean translation distance traveled during the interval Δ between the two gradient pulses. In order to be able to discern flowing water from randomly diffusing water, Δ is typically varied from 15 ms for fast flowing xylem water, to 200 ms for slow moving phloem water (Scheenen et al. 2001). Linear displacement can be measured by stepping G of the pulsed field gradients –G max to +G max, as described previously by Scheenen et al. (2000a). After Fourier transformation of the signal
as a function of G, the complete distribution of displacements (i.e., flow profile) within Δ in the direction of the gradient is obtained for every pixel of an image. Such a displacement distribution is called a propagator. Making use of the fact that non-flowing (only diffusion) water results in a propagator that is symmetrical around zero, the signal in the non-flow direction can be mirrored around the displacement axis and subtracted from the signal in the flow direction to produce the flow profile of the flowing as well as the stationary water. The resulting flow profiles can then be used to see more calculate per pixel or in any selected area in
an image: the flow conducting area, the average velocity of the flowing water, and by taking the integral of the propagator of the flowing water, the volume flow (cf Fig. 3). Fig. 3 Example of combined water content (MSE) in one of the storage pools and flow measurements (PFG-TSE) in the stem of a 4 years old oak during a developing drought period, followed by rewatering (indicated by the line). Water content of the bark as (represented by the relative amplitude, the fraction of signal intensity with respect to that of pure water, averaged over all pixels in the mask of the bark as highlighted in the inserted image of the stem), the flow conducting area and volume flow in the active xylem (the xylem ring just inside the bark and the cambial zone).