A velocity k-space analysis of flow effects in echo-planar and spiral imaging.
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(kr); that is, its velocity-frequency (kv) position as a function of spatial-frequency (kr) position. In an echo-planar sequence, kv is discontinuous and asymmetric. However, in a spiral sequence, kv 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.
Nishimura
D G
DG
Department of Electrical Engineering, Stanford University, CA 94305, USA.
Irarrazabal
P
P
Meyer
C H
CH
D016477:Artifacts; D001783:Blood Flow Velocity; D003198:Computer Simulation; D017352:Echo-Planar Imaging / Q000379:methods*; D005583:Fourier Analysis; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D012815:Signal Processing, Computer-Assisted