phase and velocity

[cbertoglio]

Hi everybody

I am curious to understand a bit better what the formula that it is used in PCMRI for relating the change of phase and the velocity assume ...

As far I have unterstand that it is assumed the phase of a moving particle can be measured at two time instants, am I right? Does this not assume we can "track" the particle? Does this not "contradict" the fact that we can only compare phases that we measure at the same fixed place?

Thanks,

C.