Analysis of flow effects in echo-planar imaging.

The effects on the phase of spins moving during echo-planar imaging (EPI) acquisition were studied. Standard single-shot and interleaved multishot blipped EPI acquisitions were considered, assuming either high gradient strength and slew rates or standard gradient strength and slew rates. A spiral k-space trajectory was also considered. Flow components in the section-select and phase- and frequency-encoding directions were analyzed separately. While the effect of flow in the section-select direction is identical to that in a standard two-dimensional Fourier transform (2DFT) acquisition, flow in the phase- or frequency-encoding directions can have substantial effects on the image, different from that in 2DFT imaging. The magnitude of these effects, which include displacement, distortion, and/or ghosting of vascular structures, is analyzed and predicted for a given velocity and direction of flow, the specific acquisition sequence, and the strength and slew rate of the gradients. For example, 50-cm/sec flow along the phase-encoding direction can cause a blurring of 1.25 cm full width at half maximum for blipped EPI with high-strength gradients, assuming a 40-cm field of view and 64 x 64 matrix.