Image Processing Reference
In-Depth Information
Aletras et al. [36] used this method to measure myocardial displacement, and
coined the term: displacement encoding with stimulated echoes (DENSE).
Although displacement could be encoded at each pixel in the image, displacement
over a period of only ~100 msec was able to be imaged, due to the use of crusher
gradients in the slice-select direction to destroy the relaxed component. Later
[37], an inversion recovery sequence was employed to ameliorate signal due to
T1 relaxation. However, due to the specific TI (inversion time), displacement to
only one time in the cardiac cycle could be encoded for each imaging sequence.
10.6.2
CSPAMM DENSE (C
INE
-DENSE)
Kim et al. [38] exploited the similarity between DENSE and SPAMM to develop
an imaging sequence which acquires displacement over all frames in the cardiac
cycle (cine-DENSE). They noted that the initial displacement-encoding pulse
sequence is the same as the tagging pulse sequence applied in 1-1 SPAMM.
This is followed (at time d in
Figure 10.11
) by an imaging sequence, which
includes a displacement-decoding gradient of the same magnitude as the encod-
ing gradient. A complementary tag pattern can be created by inverting the second
90
°
RF (as in
Figure 10.5
), allowing subtraction of the two complementary
images to eliminate the signal due to T1 relaxation.
As in CSPAMM HARP, by time d in Figure 10.11, the signal consists of a
tag-modulated component together with a component due to longitudinal (T1)
relaxation (Equation 17). Between d and f the longitudinal magnetization is
transferred into the transverse plane, and a decoding pulse of the same duration
as the encoding pulse is applied. This adds a phase to the signal proportional to
the current position
x
:
Mt
( , )
x
=
{(
M
cos(
k
⋅
X
)
−
M
) exp(
−
t T M
/
1
)
+
}exp(
−−⋅
j
e
kx
)
(10.21)
z
0
e
0
0
According to the shift theorem of the FT, the effect of the decoding gradient is
to shift the spectral peaks by
k
e
in k-space. Employing the complex exponential
form for the cosine and rearranging terms, the signal can be seen to arise from
three components:
M
Mt
( , )
x
=
0
exp(
−
tT
/
1
) exp(
−
j
k
⋅
(
x
−
X
))
xy
e
2
+
M
(
1
−
exp(
−
tT
/
1
)) exp(
−
j
kx
⋅
)
(10.22)
0
e
M
0
+
exp(
−
tT
/
1
) exp(
−
j
kkxX
e
⋅ +
(
))
2
The first component is the desired DENSE echo, whose phase is directly
proportional to the tissue displacement
u
. The second term arises out of the T1
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