Biomedical Engineering Reference
In-Depth Information
vivo
17
O MRS. Here, the in vivo
17
O MRS approach for imaging
CMRO
2
is described in detail.
In general, there is a similarity between in vivo
17
OMRS
approach and positron emission tomography (PET) approach
(88-91)
for measuring CMRO
2
through the use of isotropic
labeled oxygen gas (
17
O
2
for MRS and
15
O
2
for PET). After
gas exchange in the lung, the inhaled and labeled
O
2
molecules
quickly bind to hemoglobin (Hb) in the blood, forming
O
2
-
hemoglobin complexes (HbO
2
)(see
Fig. 15.1
). Through the
feeding arteries and arterioles, the HbO
2
complex enters the brain
capillaries. The O
2
molecules are dissociated from hemoglobin in
the capillaries, then cross the brain blood barrier (BBB) in the
form of free gas, diffuse into the brain tissue (intra- and extra-
cellular space), and finally enter mitochondria where they are
metabolized and produce the water with the isotope label fol-
lowing the chemical reaction
(3)
:
3.1. Theory and
Quantification of
CMRO
2
Based on In
Vivo
17
OMRS
Approach
2H
2
O
(15.7)
According to this reaction (equivalent to
R1
in
Fig. 15.1
), one
oxygen molecule produces two isotopic labeled water molecules
(H
2
17
Ofor
17
OMRSandH
2
15
O for PET), which can move
out of the mitochondria (traversing the opposite pathway as O
2
entry) and finally be washed out from the brain through venules
and veins. The dynamic change of the isotope labeled water in
the brain is tightly linked to the cerebral oxygen utilization rate.
It provides the vital signal source for determining CMRO
2
for
both
15
OPETand
17
OMRS.
Though the principle underlying the
17
O MRS approach
for measuring CMRO
2
was historically adopted from the well-
established
15
O PET approach, there is significant distinction
between these two neuroimaging approaches. For instance, one
complication in PET is that it is unable to distinguish the
15
O
signal contribution from
15
O
2
molecules versus the metaboli-
cally generated H
1
2
O molecules. This limits the robustness of
PET technique for imaging CMRO
2
(55, 90)
. In contrast, the
17
O MRS approach specifically detects the metabolically gener-
ated H
2
17
O without confounding signals from
17
O
2
molecules
because of the “invisibility” of
17
O
2
in an in vivo
17
O spec-
trum
(55)
. This unique magnetic resonance specificity leads to a
great advantage which significantly simplifies the in vivo
17
OMRS
methodology for measuring and quantifying CMRO
2
(55, 87)
.
Nevertheless, the dynamic change of the metabolically generated
H
2
17
O concentration in the brain during an
17
O
2
inhalation is
interplayed by three parallel processes (see
Fig. 15.3
): (i) Cere-
bral oxygen utilization for generating the metabolic H
2
17
O in
the brain tissue, (ii) Cerebral blood perfusion resulting in H
2
17
O
Carbon substrate
+
O
2
→
Oxidized carbon substrate
+
