Biomedical Engineering Reference
In-Depth Information
read-out direction are used to determine flow velocity. In quantitative terms,
the time difference
t
is the time between the phase-encoding and the read-
out events. The distance
A
is the measurement by which the signal is dis-
placed outside a vessel. Measurement of the angle
C
is the angle between
the vessel and the read-out direction, which permits determination of the flow
velocity
V
:
V
=
A
/
(
t
cos
C
)
(3.15)
3.2.5 Velocity-Encoding and Aliasing
Flow encoding in a vessel can be called velocity encoding (VENC). It is a pa-
rameter that is selected by the MR operator when using PC MRA. VENC is the
maximum velocity present in the imaging volume. Any velocity greater than
VENC will be aliased according to the following formula: aliased velocity
=
VENC
−
actual velocity. A small VENC is always more sensitive to slow flow
(venous flow) and to smaller branches, but it causes more rapid (arterial) flow
to get aliased. A larger VENC is more appropriate for arterial flow. So, small and
large VENC are important for imaging all flow components. This method has
several advantages. PC MRA is capable of generating magnitude and phase im-
ages with superior background suppression. VENC is less sensitive to intravoxel
dephasing or saturation effects. On the other hand, this method suffers from
several disadvantages such as long scan time, sensitivity to signal losses due to
turbulence and dephasing on vessel turns (carotid siphon), and dependence on
maximum flow velocity in order to select an optimum VENC. To provide quan-
titative information regarding velocity in PC angiography, the VENC should be
selected to encompass the highest velocities that are likely to be encountered
within the area of interest (see Fig. 3.21). The normal maximal flow velocities
are likely to be encountered within the vessel region of interest. The normal
maximal flow velocities of intracranial arteries do not exceed 80 cm/sec. So,
the VENC of 80 cm/sec would encompass all flow velocities up to and including
80 cm/sec. When a velocity encoding is selected, the amplitude of the bipolar
flow-encoding gradients is adjusted so that all velocities including the selected
value can be imaged without aliasing. Aliasing in phase contrast occurs when
high flow velocities are incorrectly represented in the velocity image as lower
flow velocities.
