Biomedical Engineering Reference
In-Depth Information
Figure 3.14: Two-dimensional Phase contrast pulse sequence (left) and 3D
phase contrast pulse sequence (right) are shown with velocity-induced phase
shift to distinguish stationary and flowing spins. In both 2D/3D PC MRA, two or
more acquisitions with opposite polarity of the bipolar flow-encoding gradients
are subtracted to produce image of vasculture while these gradients are not
applied to all three axes simultaneously.
Phase (
φ
) and velocity (
ν
) are related by
1
/
2
γ
G
ν
t
2
φ
= ∫
ω
dt
= ∫
(
γ
G
ν
t
)
dt
=
(3.13)
Therefore, knowledge of the phase at any point in time allows us to calculate
the velocity. The most common method for PC MRA is the use of bipolar gradient
(see Fig. 3.14). This process is called flow encoding. Because the two lobes in
this bipolar gradient have equal areas, stationary tissues observe no net phase
change. However, flowing blood will experience a net phase shift proportional to
its velocity (assuming a constant flow velocity). This is how flow is distinguished
from stationary tissue in PC MRA (see Fig. 3.15).
PC MRA is illustrated for 2D PC and 3D PC MRA, respectively in the fol-
lowing section. At this point, it is important to describe “flow phase,” “velocity-
dephasing,” and the distinction between “magnitude” image and “phase” image.
Flow image results from phase changes in transverse magnetization of spins
moving along a magnetic field gradient. These phase shift effects can be used
to generate flow images to quantify flow velocities. These phase effects are also
present in stationary spins due to differences in their precession frequency.
Stationary tissues dephase over time in a spatially-dependent magnetic field
gradient. This dephasing can be exactly compensated to form an echo using a
