NMR Even Echo Rephasing in Slow Laminar Flow

In nuclear magnetic resonance (NMR) imaging, flowing fluids possess several unusual properties not found in stationary materials. One of these is the strong signal emitted during slow flow by unsaturated protons just entering the imaging volume. Another is the observation that even echoes of a multiple spin-echo train have higher intensity than the odd. The two phenomena have not previously been distinguished in the NMR imaging literature and the term "paradoxical enhancement" has been applied to both. In this communication we consider the conditions under which such a phenomenon occurs, derive general mathematical relationships, and show clinical examples in which an understanding of spin-echo rephasing is especially useful.