Even‐echo rephasing and constant velocity flow

It has been shown previously that for constant magnetic field gradients, constant velocity flow leads to even‐echo rephasing for all echo delay times. We show that for flow which is not pluglike, even‐echo rephasing also occurs for the pulsed readout gradients used in magnetic resonance imaging if and only if the gradients begin at the time the 90° pulse is applied. We also show for these gradients that even‐echo rephasing for all echo delay times in the case of nonpluglike flow implies a constant flow velocity at the point considered. Furthermore, it suffices to assume the vanishing of the spin‐echo phase for any even echo, since the vanishing of any even echo for all echo delay times implies that all other even echoes also vanish identically. The odd echoes are then all equal to each other and proportional to the flow velocity. If acceleration is present, it may then be seen that for nonpluglike flow, even‐echo rephasing may only be present for some but not all echo delay times. However, for the typicall slice selection gradients used in magnetic resonance imaging or for usual readout gradients starting after the 90° pulse is applied, it is shown that for constant velocity flow the even‐echo phases do not vanish identically. Hence, rephasing cannot always ocur for nonpluglike flow in either of these situations. Furthermore, the spin‐echo phases are proportional to the flow in velocity. The converse is also showh; namely, if the phase of any even echo at given point is proportional to the velocity at the point considered for all echo delay times, then the flow velocity is constant at the point. Furthermore, the phases of all even echoes are also equal to each other and proportional to the flow velocity at that point. In addition, the odd echoes are then also equal to each other and proportional to the flow velocity, but with a proportionally constant different from that of the even echoes. © 1987 Academie Press, Inc.