“One‐shot” velocity microscopy: NMR imaging of motion using a single phase‐encoding step

The use of the pulsed gradient spin‐echo sequence in NMR microscopy enables the measurement of molecular translational motion and simultaneous construction of velocity and self‐diffusion images, a technique that has been termed dynamic NMR microscopy. In this method the PGSE contrast gradient is stepped in a fourth dimension ( q space) and so is inherently inefficient. Provided that one is prepared to sacrifice some of the additional information provided by the multiple PGSE gradient approach, it is possible to construct a velocity image alone by means of a single PGSE phase‐encoding step. We illustrate applications of this method in which a signal from the stationary spins is nulled by the use of both gradient phase cycling and a final “z‐storage” rf pulse. The limits to velocity resolution are around 10μm s −1 in free water but can be considerably smaller for molecules with a low self‐diffusion coefficient. We demonstrate this method in a study of water capillary flow at 12 μm transverse pixel resolution, extending the velocity range by employing a four‐quadrant analysis method. This method is also used to measure vascular transport in a living plant and find a flow rate of around 45μm s −1 . © 1992 Academic Press, Inc.