Biomedical Engineering Reference
In-Depth Information
DASL technique is attractive in improving the temporal resolu-
tion of ASL, because data can be acquired with a high sampling
rate while the magnetization state of the brain oscillates between
the control and the steady-state conditions induced by the peri-
odic labeling function.
In order to obtain high temporal resolution, short ASL RF
pulses are used interleaved to an ultra-fast imaging sequence, such
as echo-planar or spiral imaging
(67)
. A compromise must exist
between using ASL RF pulses that are short enough to allow for
high temporal resolution, but long compared to the imaging time
so that high labeling duty-cycles (and thus labeling efficiency) can
be maintained. Specifically, between the acquisition of two con-
secutive images with a short repetition time (
TR
), ASL is achieved
by a labeling RF pulse of length
TL
. According to this scheme,
labeling of the arterial spins can be done with a duty-cycle
k
=
TL
/
TR
.If
TL
is not too short compared to
TR
, the duty-cycle
k
is high enough to preserve the sensitivity of the CASL technique.
The proper determination of quantitative CBF maps requires a
correct measure of the degree of arterial spin labeling,
α
(
t
). The
new definition of
α
(
t
) to account for the labeling duty-cycle
k
is
given by:
k
e
−
τ/
T
1
a
·
α
0
·
,
0
≤
t
≤
α (
t
)
=
(13.11)
0
,
≤
t
≤
2
Equation (13.11)
shows that the effect of using the DASL
scheme as described above is to decrease the efficiency of labeling
by the duty-cycle factor
k
. For example, ASL RF pulses of 78 ms
duration were implemented in conjunction with a 30 ms read-
out EPI sequence
(27, 67)
. Under these conditions, CBF images
could be formed every 108 ms, however, with a decreased label-
ing efficiency due to the lower duty-cycle
k
=
0. 72. Similar DASL
data, acquired using
TL
=
200 ms and
TR
=
250 ms, are shown
in
Fig. 13.4.
In this case,
k
0. 8.
To demonstrate the usefulness of DASL in probing the
CBF response to functional brain activation, DASL experiments
were performed during bilateral somatosensory stimulation in
α
=
-chloralose anesthetized rats at 7 T. A labeling frequency of
0.0167 Hz was used corresponding to a half-period
30 s.
The functional paradigm was defined so that 3 s-long stimulus was
presented during the stationary period of the DASL cycles. The
stimulus parameters were: current amplitude
=
=
2 mA, pulse width
=
s and repetition rate 3 Hz.
Fig. 13.4a
shows a t-score
map of the BOLD functional response. Robust activation regions
were present in both left and right primary somatosensory cor-
tices.
Fig. 13.4b
shows the combined DASL-fMRI time-course.
The functional hemodynamic response can be easily noticed on
top of the DASL experiment. The control phase of the DASL
333
μ
