Biomedical Engineering Reference
In-Depth Information
images, signal intensity is proportional to blood flow velocity for each phase of
the cardiac cycle. The phase images also display the direction of blood flow;
a bright vascular structure represents flow in the same direction as the flow-
encoding gradient, while dark vessels indicate flow in the opposite direction.
Scan information may be acquired in a multislice or projection/slab format. This
technique allows the assessment of alterations in hemodynamic conditions dur-
ing the cardiac cycle. Phase contrast angiograms may also be obtained using a
projection technique, to create a single-projection angiogram through a selected
volume of interest.
3.2.4 3D Phase Contrast Angiography
3D PC MRA technique relies on velocity-induced phase shifts to distinguish
flowing blood from surrounding tissue. The phase contrast approach can be
made sensitive to slow flow in small vessels. The reason for this can be attributed
to the fact that the contrast between flowing blood and stationary-tissues is
related to blood velocity, rather than stationary-tissue T1. PC angiography also
permits excellent cancellation of stationary tissues and offers the potential for
quantitative measurements of blood velocity.
3.2.4.1 Image Acquisition
To understand the mechanisms of image acquisition during phase contrast vascu-
lar imaging, conventional, rotating-frame vector of spin magnetization provides
a simple picture. This is described in the following section. After a 90
◦
pulse
has been applied to the equilibrium magnetization, the spin is rotated into the
transverse plane, on an axis perpendicular to the static magnetic field
B
. Here,
it precesses at the Larmor frequency; at any moment in time, the magnetiza-
tion can be described by the length of the magnetization vector and its phase
in relation to reference positions. As this precession continues, the phase of
the magnetization vector or the phase angle varies. It is primarily because of
the effect of the main magnetic field, although smaller contributions may also
change the rate of phase evolution. For example, if the local magnetic field of
spin is changed by a magnetic field gradient, the spin's Larmor frequency will
be slightly different and the rate of change of the phase will be altered. To de-
tect flow, phase contrast angiography uses a bipolar gradient to encode a spin's
