Biomedical Engineering Reference
In-Depth Information
voxel that is replaced is given by
g
1
=
v
T
/
s
(3.11)
while other one remaining is given by 1
−
g
2
. The sum of the two magnetization
components in the vessel is thus
M
(TR)
=
M
{
(1
−
g
1
)[1
−
exp(
−
TR
/
T1)]
+
g
2
}{
(1
−
g
1
)[1
−
exp(
−
TR
/
T1)]
}
(3.12)
It represents previous voxel in slice and
g
2
represents fresh voxel. The flow-
related enhancement is prominent when a significant fraction of blood in a
slice is replaced during the time TR. With optimized values for slice thickness,
s
, and repetition time, TR, in a sequence, such flow velocities,
v
, are of the
order of a few centimeters per second. At TR
=
500 msec,
s
=
0
.
5 cm, the
flow velocity will measure 1 cm/sec. At higher velocities the combination of
higher velocity signal loss and flow-related enhancement tends to reduce the
intravascular signal intensity.
3.1.2.1.3 Flow-Related Enhancement.
During multislice acquisition, it
can be operative in several slices of a stack. The spin isochromats moving at
the center of a vessel are generally faster than those close to the vessel wall.
Therefore, centrally located spin isochromats move deeper onto the stack dur-
ing the repetition time (TR) than peripherally located ones. Suppose the planes
of entry into different slices are separated by a distance
q
(
q
slice thickness),
then fully magnetized spin isochromats moving with a velocity
v
will enter the
j
th slice after a time,
t
=
j
·
q
/v
after entering the first slice. Spin isochromats
that move a distance
j
·
q
after the
j
th slice and before the (
j
+
1)th slice are
irradiated with RF pulses. These contribute their full magnetization to the signal
measured in the (
j
+
1)th slice from outside the stack without being disturbed
by RF irradiation. Here slices are acquired in the sequence parallel to flow. The
deeper slice in the stack indicates that faster blood flow enhances the signal
in that slice. The fast flow causes high-velocity signal loss in SE images. Thus
flow-related enhancement cannot be observed in all slices of a stack. Suppose
q
=
10 mm, TE
=
30 msec, and adjacent slices are excited 100 msec apart, the
total signal loss will occur for velocities of approximately 65 cm/sec and above
according to
V
=
K
·
s
/
TE where
K
and
s
are slice thickness. Blood moving
through the first slice between the refocusing pulse at T
=
15 msec and into the
