Chemistry Reference
In-Depth Information
used to provide adequate inflowing gas to the facemask and the respiration signal
was monitored using custom hardware and software (SA Instruments Inc., Stony
Brook, NY).
1.
Imaging Sequence
Due to the higher gradient capability, it was possible to perform diffusion imaging
with increasing gradient duration on the Bruker system using a spin-echo sequence.
Spin-echo sequences have the advantage of reducing the effects of static field
inhomogeneities. Using a much higher gradient strength,
b
-values
>
2000 s mm
−
2
could be achieved while maintaining a very short echo time of 20.2 ms. These
factors resulted in the appearance of significantly higher image quality in the
animal experiments.
Diffusion weighting was achieved using a Stejskal-Tanner spin-echo prepa-
ration with two trapezoidal gradients, one on each side of the 180
◦
refocusing
pulse. Seven separate diffusion weighted acquisitions were performed, each with
an increasing
b
-value. The strength of the diffusion gradients was maintained at
90 mT m
−
1
, while the duration of the gradients was increased from 0 to6sin
steps of 1s. This created an effective range of
b
-values of0-2177smm
−
2
. The
acquisition was performed with a repetition time of 1.2s,anecho time of 20.2 ms,
and an rf flip angle of 90
◦
. The gradients were separated by 9.5 ms. The diffusion
weighting was applied in all three directions at once. A 3
×
3-cm field of view
was chosen, and six 1.5-cm slices were acquired.
The IDl implementation of the Levenberg-Marquardt algorithm was also used
for the fitting of these acquisitions to the fractional spin-echo diffusion equation.
VI.
RESULTS OF EXPERIMENTAL INVESTIGATION
A.
Water Phantom Assessment
The assessment of the performance of the fractional expression in the analysis
of monoexponential behavior was carried out using the images acquired with the
spherical water phantom. Each of the customized imaging sequences were applied,
and subsequently five regions of interest were chosen. The appropriate version of
the anomalous diffusion expression were used to fit the diffusion decay
ln
S
(
t
)
S
0
t
α
+
2
2
γ
2
G
2
D
=−
(Rectangular)
(181)
(2
+
α
)
(1
+
α
)
ln
S
(
t
)
S
0
Dγ
2
G
2
(2
δ
)
2
−
α
α
(
α
+
3)
=−
(Ramped)
(182)
(5
+
α
)
ln
S
(
t
)
S
0
Dγ
2
G
2
δ
2
+
α
8
(5
α
2
))
=−
(3
+
α
)(12
+
α
(24
+
9
α
+
(Triangular)
+
α
)
(183)
