Biomedical Engineering Reference
In-Depth Information
thickness (
s
). The maximum velocity of blood (
V
z
) above which blood leaves
the slice will be more than
s
/
T
e
. For lower velocities, only a fraction of blood
leaves the voxel up to the rephrasing pulse time
T
e
. The fraction of blood (
g
)
will be dependent on maximum velocity and slice thickness as:
g
=
V
z
/
s
.
Thus, the recorded magnetization decreases linearly with the velocity down to
zero. The blood flow may be observed to be very slow. As a result, intravascular
signal is seen on first echo, but no longer on second echo. It indicates blood
has moved out of the slice during the time interval of TE/2 and 3TE/2. If we
know the slice thickness, we can calculate blood flow velocity. The signal loss
on the first echo at the rim of the vessel is commonly observed due to spin-
phase effect in SE sequences. On the contrary, in GRE sequences, slice-selective
rephrasing pulse is not applied. So, slice-transition effects are not observed and
do not produce any intravascular signal loss. At low blood velocities, only few
out of all spin isochromats leave the slice during the time
T
e
. Hence, during the
repetition time TR
T
e
applied, some or all spin isochromats can be replaced
by still fully magnetized spin isochromats. These spin isochromats represent
from outside the imaged volume. In this region, these spin isochromats have
not undergone repetitive 90
◦
pulse in SE or alpha pulse in GRE sequences. This
gives rise to 'entry slice phenomenon'. We shall discuss entry slice phenomenon
in the following section.
3.1.2.1.2 Entry Slice Phenomenon.
If T1
TR for blood, spin isochromats
cannot fully recover their magnetization along the
z
axis. If spin isochromats in
blood move perpendicular to a stack of slices with velocity
v
, they are subjected
to 90
◦
in SE imaging and to an alpha pulse in GRE pulse sequence. Due to flow,
they are partly replaced by spin isochromats from outside imaging volume. This
outside volume has not been subject to such pulse. So, net magnetization
M
available for the next 90
◦
pulse or alpha pulse is the sum of the magnetization
of the remaining spin isochromats. It leads to increased intravascular signal in-
tensity. The temporal dependence of
M
over time will represent an increase in
M
in such a way that the first linear increase will be due to incomplete magneti-
zation recovery. The later phase in the increase in
M
as a plateau will represent
flow-related enhancement. The magnitude of this effect is again dependent on
the flow velocity (
v
), the slice thicknes, and TR. The fraction of blood
g
in the
