Biomedical Engineering Reference
In-Depth Information
3.1.2.1 Slice-Transition Phenomenon
It is known as the time-of-flight effect. The time-of-flight effect occurs when-
ever a vessel takes course in part perpendicular to the imaging plane. During
this course, substantial amount of blood volume is replaced within the imaged
slice for several hundred milliseconds. Different pulse sequences capture these
events of blood volume turnover in selected slices and voxels of the circulatory
system. For example, images are captured in several hundred milliseconds by
spin echo (SE) pulse sequence and a few tens of milliseconds in GRE imaging.
Normally, the flow direction is selected perpendicular to the imaged slice axis
in the
xy
plane. The flow is chosen along the
z
direction. Time-of-flight depends
upon the slice thickness, position of slices in stack region of interest, multislice
acquisition order, flow velocity, and flow direction of spin isochromats. Assume
a volume element or voxel that moves with constant velocity
V
0
. If it moves a
distance
z
0
during time
t
, we can write
V
0
=
z
0
/
t
.
Two types of slice-transition phenomenon can occur. In the first case, blood
moves so fast that it leaves the slice between the 90
◦
nutation pulse and the
180
◦
refocusing pulse in an SE sequence. It results in signal loss and has been
termed as high-velocity signal loss. In the second case, the flow velocity is small
enough that only a small fraction of blood leaves the slice between the 90
◦
and
180
◦
pulses in the SE sequence, but a part of the blood is replaced by fully
magnetized blood from outside of the imaged volume during the repetition time
(TR). It leads to increased intravascular signal and has been termed flow-related
enhancement.
3.1.2.1.1 High-Velocity Loss.
Spin isochromats leave the imaged slice in an
SE sequence before the rephasing pulse for a given echo is applied. These spin
isochromats will not contribute to this echo. These spin isochromats continue
with their dephasing process or rephrasing. This dephasing process or rephras-
ing will not be experienced by spin isochromats. In general, the rephasing pulse
occurs at time
T
e
, which is equal to TE/2 for the first echo, or (
n
−
1
/
2)TE if
the
n
th
echo in a multiecho sequence with regular echo spacing is used. Here
T
e
is equal to
T
E1
+
(
T
E2
−
T
E1
)
/
2 if a sequence with two asymmetrical echoes
(at times
T
E1
and
T
E2
) is used. The maximum velocity (
V
z
) depends on slice
