Fig. 4.

Velocity effects on the phase of MR signals. Stationary spins can be refocused with a pair of gradient pulses of equal area and opposite polarity, which is a basis of gradient echo. Spins moving along the gradient direction acquire additional phase due to the fact that dephasing and refocusing occur at different physical locations where the same gradient indices different frequency shifts, leading to different amounts of the phase acquired during dephasing and refocusing periods. Greyed segments on the corresponding phase circles mark the amount of phase acquired by the moving spins during the last gradient pulse.