Digital Signal Processing Reference
In-Depth Information
2.8.4 T
1
MRI Image
We understand that there is the natural relaxation in the transverse component of
the resultant magnetic moment due to spin-spin interaction. This is known as
T
2
relaxation. We exploit the different relaxation time and different proton-density
(refer Sects.
2.8.1
-
2.8.3
) for the different tissues to obtainMRI image (
T
2
and proton-
density respectively). In the same way, due to spin-lattice interaction, the longintu-
idanal components gradually increases. This is the another natural relaxation known
as
T
1
relaxation (which is independent of
T
2
relaxation). Different tissues have dif-
ferent
T
1
relaxation time. This helps to obtain the another type of MRI image known
as
T
1
MRI image. Usually
T
1
is very larger when compared with the corresponding
T
2
. Exploiting this property, the following trick is used to obtain the
T
1
MRI image.
1. Steps 1-3 in Sect.
2.8.1
is performed.
2. Wait for the time to get almost zero transverse-component. But by this time,
longituidal component gradually increases (not reached maximum).
3. At this moment, flip the longituidanal component by an angle
(usually 90
◦
)to
obtain the transverse component. Sample the transverse component immediate
after obtaining the transverse component (i.e at
t
α
=
0 during read-out phase) to
obtain the K-space of the
T
1
image for the particular
G
x
and
G
y
. The operation
is repeated for the required range of
G
x
and
G
y
values. Apply 2D-IDFT to obtain
the
T
1
image. Instead of sampling the transverse component at time instant
t
0,
gradient echo can also be used to measure the transverse component as described
in the Sect.
2.8.1
.
4. Note that
T
1
image is obtained by measuring the transverse component. But the
measured transverse component ismainly due to longituidanal relaxation obtained
from spin-lattice interaction. The trick is to flip the longituidal component, which
are in relaxation state to obtain the transverse component. The magnitude of the
transverse component describes the
T
1
characteristics of the tissue. Thus the image
=
Search WWH ::
Custom Search