Biomedical Engineering Reference
In-Depth Information
Equation (13.4)
tells us a number of important things about
ASL. First, to cause a change in brain tissue magnetization related
to perfusion, one needs to label blood (i.e.,
(
t
) must be different
than zero). Second, the perfusion rate does not instantly change
brain tissue magnetization, but it does so with a time constant
given by
T
1app
. Third, because CBF is only on the order of 60
ml blood/100 g tissue-minute, CBF/
α
0.01 s
−
1
, the impact
of CBF on
T
1app
is much too small to allow CBF to be reli-
ably measured from changes in relaxation rates. Traditionally, ASL
techniques have been presented as belonging to one of two basic
implementation categories
(52, 57)
. In the first approach, arterial
water is continuously labeled proximally to the region of interest
in the brain
(58, 59)
. This approach is referred to as continuous
ASL (CASL). In the second approach, a single, yet large volume
of arterial blood is dynamically labeled proximally to the region
of interest and allowed to flow into the tissue prior to data col-
lection
(60-63)
. This approach is generally referred to as pulsed
ASL (PASL). A detailed comparison of CASL and PASL tech-
niques can be found in
(64)
, and is beyond the scope here.
The CASL approach is attractive for providing better sensitiv-
ity than PASL. In CASL, arterial water is continuously saturated
(
λ ≈
1. 0) proximally to the brain
for a period long enough to allow the establishment of a steady-
state in brain tissue magnetization. For a constant degree of label-
ing efficiency
α
0
=
α
(0)
≈
0. 5) or inverted (
α
0
≈
)
=
α
0
e
−
τ/
T
1
a
, where
α (
t
α
0
is the labeling efficiency
at the labeling site,
is the transit-time from the labeling site to
the detection site, and
T
1
a
is the longitudinal relaxation of arterial
water, a steady-state
M
label
b
τ
=
M
b
(
t
)
|
t
>
5
T
1
app
is reached in which:
CBF
λ
M
b
M
label
b
T
1
app
M
b
M
=
−
=
2
α
(13.6)
Thus, the CBF can be obtained from two images obtained with
(
α
=
0) and without (
α
=
0) labeling:
M
b
M
label
b
−
λ
CBF
=
T
1
app
·
(13.7)
α
M
b
2
Equation (13.7)
shows that CBF depends on the partition
coefficient
λ
and on three additional parameters: the longitudinal
tissue relaxation time
T
1
b
, the transit time
from the site of label-
ing to the site of interest in the tissue, and the difference in tissue
MR signal between the control and the labeled states of mag-
netization. Commonly, these parameters are obtained in separate
experiments. Furthermore, the parameters are measured usually
in single instances, precluding dynamic analysis of fast variations.
In many applications, as for example in functional MRI exper-
iments aimed at determining the hemodynamic response with
τ
