Figure 43

Blood flow aliasing. Phase contrast velocity map and velocity profile from a transverse slice showing flow in the great vessels above the heart. a) Aliased velocities are seen in the ascending aorta. b) Velocity measurement corrected by “offsetting” the velocity window post-scan. The offset shifts the range of velocities corresponding to black and white pixels, in this example upwards so that the fast forward velocity does not exceed the top of the range. Although this “offset” correction usually suffices in simple cases (like this one) where pixels of wrapped velocity are surrounded by non-wrapped pixels, problems can occur in other situations where the wraparound has occurred in pixels at the edge of a flow. This results in “uncorrectable” aliasing where no amount of offset produces a sensible value for the wrapped pixels. This correction is not generally available on scanners, but is available in post image-processing software.