Biomedical Engineering Reference
In-Depth Information
Figure 3.21: Three-Dimensional PC angiogram at multiple velocity encoding
(VENC) shows the effect of high velocity encoding (cm/sec) at 80
◦
(left panel),
40
◦
(right panel) on top row and 20
◦
(left panel), 10
◦
(right panel) on bottom
row to emphasize the better venous anatomical appearance with clear spheno-
parietal sinus at low VENC.
3.2.5.1 Aliasing in Speed Images
When the velocity-encoding set below the peak velocities is encountered within
the vessel lumen, the higher velocities will be aliased and appear as lower sig-
nal intensities from the lower velocities. Since the highest velocities are usually
present at the center of the vessel, aliasing may result in a decrease in signal
intensity within the center of the vessel. If a very low velocity encoding (VENC
=
20 cm/sec) is used, the higher flow velocities will be aliased and the slower ve-
locities will have greater signal intensity. The advantage of aliasing in magnitude
and velocity images is also noticeable to bring out slower flow along the walls of
arteries, structures, or to emphasize venous anatomy. VENC may be set lower
than the peak velocity. Aliasing artifacts makes the flow information at the cen-
ter of the artery meaningless but this part of the vessel is often not seen in the
MIP projection images.
