Biomedical Engineering Reference
In-Depth Information
B
0
∗
.
SNR
(per unit sampling time)
∝
(15.6b)
Our experimental results showed approximately a four-fold SNR
gain in the in vivo
17
O MRS signal measured in the rat brain at 9.4
tesla compared to 4.7 tesla
(67)
. Moreover, the
* value deduced
from these experimental measurements and
Eq. (15.6B)
was
β
∼
2
(67)
, which is close to the theoretically predicted
* value of 7/4
(57)
. The significant sensitivity gain provides adequate SNR for
acquiring the 3 dimensional
17
O MRS imaging of the natural
abundance H
2
17
O from the small brain of rat with a temporal
resolution of
β
∼
10 seconds at 9.4T (see one example shown in
Fig. 15.2B
).
Clearly, the increase of
B
0
should benefit all of the in vivo
multinuclear MRS approaches. In vivo
17
O MRS probably ben-
efits the most from the substantial sensitivity gain at ultrahigh
fields. On the other hand, in vivo
31
P MRS can benefit from both
the moderate sensitivity gain and the significant spectral resolu-
tion improvement, which is crucial for resolving many overlap-
ping resonance peaks of different metabolites. All these benefits
provided by high field strength are essential for improving the
reliability of in vivo
17
Oand
31
P MRS and for accurately deter-
mining cerebral metabolic fluxes.
3. How to Apply In
Vivo
17
OMRSfor
Imaging CMRO
2
In vivo
17
O MRS methodology has two major applications for
studying brain function and cerebral bioenergetics through imag-
ing either cerebral blood flow (CBF) or CMRO
2
(51-55, 67,
69-87)
. Both applications rely upon the measurement of brain
H
2
17
O content and its dynamic change using in vivo
17
OMRS.
The CBF measurement is based on monitoring the washout rates
of inert H
2
17
O tracer in the brain tissue following an intravascular
bolus injection of
17
O-labeled water. The CMRO
2
measurement
is based on monitoring the dynamic changes of metabolically
generated H
2
17
O in the brain tissue from inhaled
17
O-labeled
oxygen gas
(55)
. There are two types of magnetic resonance
approaches for monitoring brain H
2
17
O in vivo: a direct approach
by using
17
O MRS detection, and an indirect approach by using
1
H magnetic resonance imaging (MRI) to measure the changes in
T
2
-orT
1
ρ
-weighted proton signals caused by the
17
O-
1
H scalar
coupling and proton chemical exchange (see recent review article
of
(55)
and the references cited therein). Both direct and indirect
approaches are suitable for CBF measurements. However, recent
studies indicated that the in vivo
17
O MRS approach at ultrahigh
fields seems to have more advantages for quantifying and imaging
CMRO
2
, which perhaps is the most important application of in
