Advances in Magnetic Resonance Angiography and Physical Principles - Biomedical Image Analysis - page 157

3.2.5.2 Aliasing in Phase Images
When peak velocity in a vessel is equal to the VENC value, the bipolar gradients
give either a 180 ◦ or 180 ◦ phase shift, depending on the direction of flow. When
velocity exceeds the VENC value and the phase shift exceeds 180 ◦ , it becomes
indistinguishable from the phase shift produced by flow in the opposite direction.
The result is phase aliasing. Here aliasing flow seems to change direction, since
the + 190 ◦ phase shift is equivalent to a − 170 ◦ phase shift (see Fig. 3.22). For this
reason, aliasing in individual flow-axis images is often recognized by adjacent
white and black pixels. In addition, the measured phase shift increases with
velocity up to a value of 180 ◦ , at which point it is aliased with an equal negative
velocity. This sets a limit on the usable degree of flow encoding for quantitative
Figure 3.22: Phase plot shows the effect of a gradient on transverse magneti-
zation at three different locations along the frequency axis. The gradient echo
is formed by first dephasing the transverse magnetization along the frequency-
encoding axis. The first half of the read-out gradient refocuses the magnetization,
producing an echo at time TE.