Biomedical Engineering Reference
In-Depth Information
next slice before it is excited at
t
=
100 msec will contribute full magnetization to
the second slice. Such spin isochromats must move with a velocity
V
of at least 1
cm/(100
−
15 msec)
=
12 cm/sec, which is lower than 65 cm/sec (velocity needed
for total signal loss). Such spin isochromats contribute to signal enhancement
in the second slice. In fact, isochromats moving at approximately 24 cm/sec
and 36 cm/sec can reach the third and fourth slices, respectively, to contribute
to the signal enhancement. However, spin isochromats getting to slices farther
into the stack move progressively faster and these isochromats approach the
velocities at which high-velocity signal loss occurs. On this basis, it is possible
to detect a bright spot of signal with decreased diameter at multiple slices into
a stack. On the contrary, if the order of acquisition is reversed in another way
to minimize crosstalk between slices, signal enhancement may occur in slices
even deeper into the stack. Entry slice effects are principal causes for the high
signal intensity of blood vessels on gradient-recalled-echo (GRE) images. This
is a result of the fact that the short TR in GRE sequences does not permit the
z
magnetization to regrow to the values close to its maximum
M
z
0
. It is only a
small fraction when the next alpha (
α
) pulse is applied. If a substantial fraction
of blood is replaced during the TR of the sequence, entry slice effects can lead
to a very strong signal increase. It results in invisible high-velocity signal loss. It
is due to the fact that no slice-selective rephrasing pulse is applied. As a result,
intravascular signal will be very bright.
3.1.2.1.4 Slice Transition Effects.
These slice transition variations mea-
sure the flow in a vessel. The velocity of flowing spins depends upon the distance
traveled by the flowing spin isochromats and their travel time. Hence, their veloc-
ity may be calculated by dividing these two quantities. Velocity may be measured
as the number of excited spins present inside the voxel of interest as a function
of time. For this, one way is to apply a slice-selective 90
◦
pulse and then to apply
a 180
◦
rephasing pulse in the slice-displaced phase along the direction of flow at
some distance. Any signal measured in this second-slice duration will represent
spin isochromats that have been washed-in by the flow in the vessel. This type
of method of determination of the flow velocity is advantageous over the spin-
phase method. In this method, flow sensitization occurs by selecting a read-out
slice either proximal or distal to the tagging slice. However, the disadvantage
of this approach is that it measures the flow in positive, negative, or in both
directions. So, clinically this method is not acceptable.
