(PC)
Phase contrast sequences are the basis of
MRA techniques utilizing the change in the
phase shifts of the
flowing protons in the
region of interest to create an image. Spins that are moving along the direction of a
magnetic field gradient receive a
phase shift proportional to their
velocity.
In a
phase contrast sequence two data sets with a different amount of
flow sensitivity are acquired. This is usually accomplished by applying
gradient pairs, which sequentially dephase and then rephase spins during the sequence. Both 2D and 3D acquisition techniques can be applied with
phase contrast MRA.
The first data set is acquired with a
flow compensated sequence, i. e. without
flow sensitivity. The
second data set is acquired with a
flow sensitive sequence. The amount of
flow sensitivity is controlled by the strength of the
bipolar gradient pulse pair, which is incorporated into the sequence. Stationary tissue undergoes no effective
phase change after the application of the two gradients. Caused by the different spatial localization of
flowing
blood to stationary tissue, it experiences a different size of the
second bipolar
gradient compared to the first. The result is a
phase shift.
The
raw data from the two data sets are subtracted. By comparing the
phase of signals from each location in the two
sequences the exact amount of motion induced
phase change can be determined to have a map where
pixel brightness is proportional to spatial
velocity.
Phase contrast images represent the
signal intensity of the
velocity of spins at each point within the
field of view. Regions that are stationary remain black while moving regions are represented as grey to white.
The
phase shift is proportional to the spin's
velocity, and this allows the quantitative assessment of
flow velocities.
The difference
MRI signal has a maximum value for opposite directions. This
velocity is typically referred to as
venc, and depends on the pulse
amplitude and distance between the
gradient pulse pair. For velocities larger than venc the difference signal is decreased constantly until it gets zero. Therefore, in a
phase contrast angiography it is important to correctly set the venc of the sequence to the maximum
flow velocity which is expected during the measurement. High venc factors of the PC angiogram (more than 40 cm/sec) will selectively image the arteries (
PCA - arteriography), whereas a venc factor of 20 cm/sec will perform the veins and sinuses (PCV or MRV - venography).
See also
Flow Quantification,
Contrast Enhanced MR Venography,
Time of Flight Angiography,
Time Resolved Imaging of Contrast Kinetics.