Biomedical Engineering Reference
In-Depth Information
correlation between the CMR
ATP
and the varied brain activity
from mild anesthesia state to the isoelectric state
(123)
.Inthis
study, we observed a strong correlation between the Pi
ATP flux
measured by the in vivo
31
P MT approach and the brain activity
quantified by EEG signal
(123)
. This result indicates a tight cor-
relation between the ATP synthesis rate and brain bioenergetic
demand under varied brain activity states. Therefore, the mea-
surement of Pi
→
→
ATP flux should provide a sensitive energy index
for quantifying the brain bioenergetics. In contrast, the steady-
state HEP concentrations (e.g., [ATP], [PCr] and [Pi]) and pH
are relatively stable in a wide range of brain activity level
(123)
.
These results support the view that one of the major functions
of the
PCr
Pi
exchange process is to sustain the balance
between the ATP supply and demand in the brain resulting in
a stable ATP concentration under the normal physiological con-
dition. Such a balance is likely maintained by the rapid adjust-
ments of both forward and reverse metabolic fluxes involved in
the
PCr
↔
ATP
↔
Pi
exchange according to the change of brain
energy. We conclude that the ATP metabolic flux measurements
using the in vivo
31
P MT approach should be more useful for
studying the brain bioenergetic change in response to brain acti-
vation as compared to the steady-state HEP concentration mea-
surements using conventional in vivo
31
P MRS. This is evident
from two high-field in vivo
31
P MRS studies showing a few per-
cents of decrease in the PCr signal in contrast to a
↔
ATP
↔
30% increase
in the CK forward flux in the human visual cortex during visual
stimulation
(108, 145)
. It is conceivable that the in vivo
31
PMT
approach, as a sole in vivo tool being able to directly and quan-
titatively measure the cerebral ATP metabolic fluxes, will play a
vital role for studying the brain bioenergetics associated with brain
function and activation. One particularly interesting question pos-
sibly to be addressed by this in vivo
31
P MT approach is whether
more ATP utilization is required by the elevated neuronal activ-
ity, and if yes, how much more is needed? The answers should
advance our understanding regarding the central roles of cerebral
oxidative phosphorylation and mitochondria in supporting brain
function.
>
5.3. What are the
Potential Applications
of in vivo
31
P and
17
O
MRS for Clinical
Research and
Diagnosis?
In vivo MRS methodology can provide detailed metabolite fin-
gerprints in the living organs, and it is perhaps one of the most
classic molecular imaging modalities. Many metabolites and their
abnormal changes, detectable by in vivo MRS, have been proved
to be tightly linked to various brain diseases and disease progres-
sion.
Both of the in vivo
17
OMRSand
31
P MRS MT approaches
as discussed here are especially useful for studying the brain
oxidative metabolism and the mitochondrial metabolic function.
They would provide opportunities for a variety of clinical brain
