| Abstract: | The sensitivities of phase-difference (PD) and complex-difference (CD) processing strategies to in-plane motion were examined theoretically and experimentally. Errors in velocity and volume flow rate (VFR) estimates were attributed to (a) motion between different velocity encodings and, in the case of segmented k-space acquisition strategies, (b) motion over the segment duration. PD estimates were found to be insensitive to in-plane motion between velocity encodings, whereas CD VFR estimates were found to be sensitive to this motion. PD estimates, however, were affected by partial volume effects. A corrected CD (CD') scheme was developed that minimizes both partial-volume and in-plane motion effects. Segmented k-space acquisitions with sequential offset and sequential interleaved offset (or centric) phase-encoding schemes were studied. Images obtained using these techniques were found to include both blurring and replication artifacts. The amount of artifact generally increased with the number of views per segment (vps) and the in-plane velocity. PD, CD, and CD' VFR estimates were found to be degraded by these artifacts. The sequential offset phase-encoding scheme generally had acceptable VFR errors (at 4 vps, a CD' VFR error of 7.0%) when averaged over the physiologic range of myocardial motion (>12 cm second?1); however, larger errors were observed outside this range. VFR estimates obtained using the sequential interleaved phase-encoding scheme at 4 vps were unacceptable. More accurate VFR measurements were obtained using a revised segmented PC strategy, which reversed the order in which the velocity and phase encodings were interleaved. The weighted average CD' VFR error obtained using the revised strategy was 24.5% (for 4 vps). Using displacement information obtained from the two velocity-encoded images, an estimate of the in-plane velocity was obtained and used to correct the acquired data. This decreased the VFR error (weighted average CD' error at 4 vps decreased from 24.5% to ?6.3%); however, the implemented correction algorithm could potentially introduced other artifacts in the images. |
