  We consider the behaviour of precessional angle (phase) carried by molecules
of a diffusing specimen under magnetic fields typical of magnetic resonance
experiments. An evolution equation for the ensemble of particles is
constructed, which treats the phase as well as the position of the molecules as
random variables. This "position-phase (probability) density" (PPD) is shown to
encode solutions to a family of Bloch-Torrey equations (BTE) for transverse
magnetization density, which is because the PPD is a more fundamental quantity
than magnetization density; the latter emerges from the former upon averaging.
The present paradigm represents a conceptual advantage, since the PPD is a true
probability density subject to Markovian dynamics, rather than an aggregate
magnetization density whose evolution is less intuitive. We also work out the
analytical solution for suitable special cases.
