Comparison of cocurrent and countercurrent flow-related enhancement in MR imaging

Patterns of flow-related signal loss and enhancement have been observed in fluid moving perpendicular to the imaging plane on multisection-acquisition magnetic resonance images. These patterns depend on whether the flow is in the same direction (cocurrent) or opposite (countercurrent) to the direction of sequential section excitation. For a given velocity, flow-related enhancement penetrates deeper into the imaging volume with countercurrent flow than with cocurrent flow. The signal intensity of inner sections is generally greater with countercurrent flow than with cocurrent flow and depends on the velocity. These effects were investigated quantitatively with phantom studies and correlated with theoretical predictions. It was found that the limited penetration of flow-related enhancement with cocurrent flow is a consequence of recently saturated upstream protons flowing into the section to be excited. The flow-related enhancement penetrates further with countercurrent flow, because inflowing spins have had a greater period of time to recover from the saturating effect of the radio-frequency pulse. Because different laminae have different time intervals to recover magnetization, this also accounts for the observation of concentric rings representing laminar flow in the countercurrent direction.