| Abstract: | A velocity k-space formalism facilitates the analysis of flow effects for imaging sequences involving time-varying gradients such as echo-planar and spiral. For each sequence, the velocity k-space trajectory can be represented by kv (k)r; that is, its velocity-frequency (kr) position as a function of spatial-frequency (kr) position. In an echo-planar sequence, kr is discontinuous and asymmetric. However, in a spiral sequence, kr is smoothly varying, circularly symmetric, and small near the kr origin. To compare the effects of these trajectory differences, simulated images were generated by computing the k-space values for an in-plane vessel with parabolic flow. Whereas the resulting echo-planar images demonstrate distortions and ghosting that depend on the vessel orientation, the spiral images exhibit minimal artifacts. |
