Biomedical Engineering Reference
In-Depth Information
As mentioned earlier, both in vivo
31
Pand
17
OMRSmeth-
ods can provide noninvasive tools for studying brain metabolism
and bioenergetics. Each of these methods has unique features for
probing the metabolic functions in mitochondria. Specifically, the
in vivo
17
O MRS imaging approach is useful for mapping the
absolute CMRO
2
within a relatively short
17
O
2
inhalation time.
The CMRO
2
images should reflect the total metabolic rate of
oxygen utilization (i.e.,
R1
in
Fig. 15.1
) and its spatial distri-
bution in the brain. In contrast, the in vivo
31
PMTapproach
is powerful for measuring all the cerebral ATP metabolic fluxes
associated with four ATP reactions (i.e.,
R2
to
R5
in
Fig. 15.1
)
occurring in different subcellular compartments. All of these mea-
sured physiological parameters can be linked to brain ATP energy
and ATP transportation. Therefore, the combination of in vivo
31
Pand
17
O MRS methods may provide an essential in vivo MR
tool being able to studying the crucial roles of both oxygen and
ATP metabolisms in cerebral bioenergetics and brain functions.
One relevant question in the brain function and neuroimag-
ing research fields is how much extra brain energy is needed dur-
ing the brain stimulation and/or task performance. This topic is
still in intense debate, especially regarding how much CMRO
2
is
induced by brain stimulation
(7, 8, 91, 124-136)
.Amoreimpor-
tant question, perhaps, is how many more ATP molecules are con-
sumed during brain activation as compared to the resting brain
because of a close relation between the ATP utilization and energy
consumption
(3, 7, 8)
. The answers to these questions are cen-
tral for understanding the mechanisms underlying most modern
neuroimaging techniques including two most popular methods of
the functional MRI (fMRI) based on the blood oxygenation level
dependent (BOLD) contrast
(137-142)
and PET
(7, 8, 143)
.One
major hurdle for addressing the questions is the lack of sophis-
ticated and robust neuroimaging modalities for directly assess-
ing the absolute CMRO
2
and CMR
ATP
, which is equivalent to
F
f
ATPase
in this article, and their changes elevated by brain acti-
vation. Therefore, the in vivo
31
Pand
17
O MRS approaches as
described here could fill this methodological gap and make sig-
nificant contribution in understanding the relation between brain
bioenergetics and function. This notion is convincingly supported
by several lines of evidence provided by recent research pro-
gresses. One example of these progresses was to apply the in vivo
17
O MRS imaging approach for successfully mapping CMRO
2
images in the cat brain under resting and visual stimulation con-
ditions
(144)
. A significant CMRO
2
increase was observed in the
activated cat visual cortex. This initial observation indicates the
possible central role of oxygen metabolism for supporting the ele-
vated brain activity during activation.
Another significant progress is related to the in vivo
31
P
MT application in the rat brain for studying the quantitative
5.2. What Is the
Possible Role that in
vivo
31
P and
17
OMRS
can Play for Studying
Brain Activation and
Function?
