Biomedical Engineering Reference
In-Depth Information
i.e.,
M
a
,
M
b
and
M
c
) are required by the MSS MT approach for
the measurements
(50)
.
The in vivo
31
P MT measurements can significantly benefit from
the high-field advantages of improved MR detection sensitivity
and spectral resolution. We have conducted extensive measure-
ments at 7T for validating the in vivo
31
P MSS MT approach,
and ultimately, determining the entire chemical exchange of
PCr
4.2. Application of in
vivo
31
PMSSMT
Approach in Human
Brain at 7T
↔
ATP
↔
Pi
in the human occipital lobe
(20, 50)
.
Figure 15.10A
illustrates the progressive saturation transfer
measurements and related in vivo
4.2.1. Progressive
Saturation for Measuring
Intrinsic T
1
,ForwardRate
Constants And Fluxes
31
P spectra when the
γ
-ATP
resonance peak was completely saturated (i.e., Step 1 measure-
ment used in the in vivo
31
P MSS MT approach). It shows a
gradual decrease of Pi signal when the
-ATP saturation time
increases because of the chemical exchange between
γ
γ
-ATP
and Pi (i.e.,
ATP
Pi
) and the increased magnetization trans-
fer effect
(20)
. This MT effect was observed on the chemical-
exchangeable Pi resonance but not for those adjacent and non-
chemical-exchangeable phosphate metabolites such as PDE and
PME groups. Moreover, the MT effect on Pi disappeared when
the RF saturation frequency was moved to the opposite side of the
in vivo
31
P spectrum with the same chemical shift difference as
shown in
Figure 15.10B. Figure 15.10C
plots the quantitative
relation between the normalized Pi signal intensity (i.e.,
M
(
t
)
↔
M
a
/
a
in
Eq. (15.13a)
) under the
-ATP saturation with the saturation
time of
t
. This plot can be used to calculate the intrinsic
T
1
of
Pi (i.e.,
T
1
a
) and the forward ATP
ase
reaction rate constant (
k
1
)
by the regression fitting according to
Eq. (15.13a)
. Then, the
forward ATP
ase
reaction flux can be determined according to
Eq.
(15.13e)
. The same in vivo
31
P MT spectral data can be applied
to quantify the MT effect on the PCr resonance as a function of
the
γ
-ATP saturation time, ultimately, to determine the intrinsic
T
1
of PCr (i.e.,
T
1
c
) and the forward CK reaction rate constant
and flux according to
Eqs. (15.13b)
and
(15.13e)
.
γ
One technical limitation of progressive saturation approach is the
requirement of a number of saturated in vivo
31
P spectra with var-
ied saturation time (e.g.,
Fig. 15.10
). One alternative but more
robust approach is the use of steady-state saturation if the intrinsic
T
1
values have been determined at a given
B
0
(see details in the
discussion of Step 1 measurement above).
Figure 15.11
demon-
strates such a measurement (i.e., Step 1) and the in vivo
31
PMT
spectra acquired in the absence (control spectrum;
Fig. 15.11A
)
and presence (saturated spectrum;
Fig. 15.11B
) of sufficiently
long RF saturation for ensuring complete saturation of the
4.2.2. Steady-State
Saturation for Measuring
Forward Rate Constants
And Fluxes
γ
-ATP
resonance peak and approaching steady-state magnetizations for
