Biomedical Engineering Reference
In-Depth Information
⎨
⎩
⎬
⎭
⎡
⎣
⎤
⎦
mnCBF
λ
mnCBF
λ
2
mn
2
CBF
(
mnCBF
Af
2
λ
te
−
t
e
−
t
C
b
(
t
)
−
+
−
1)
Af
2
λ
t
n
λ
−
mnCBF
λ
e
−
t
1
−
CMRO
2
(
t
)
=
.
(15.9)
f
1
mnCBF
2
αλ
According to this equation, the CMRO
2
value at
each
data
point measured at different inhalation time (
t
) can be calculated
by using the experimentally measured CBF,
A
,
n
,
C
b
(t)
values
and other known constants (
f
1
,
f
2
,
m
,
). The quantifica-
tion approach based on
Eq. (15.9)
is a complete model which
accounts for all required parameters for precisely determining
CMRO
2
(54, 55, 87)
.
α
and
λ
3.2. Measurement
and Imaging of
CMRO
2
using In Vivo
17
O MRS Approach
The complete model as described by
Eq. (15.9)
requires multiple
experimental measurements of four variables (A,
n
and
C
b
(t)
and
CBF) in order to calculate CMRO
2
.
Figure 15.4
illustrates the
procedures for performing these in vivo
17
O MRS measurements
and the results of a rat brain study with
α
-chloralose anesthesia at
9.4 T
(54, 55)
.
The CBF measurement was performed via bolus injection of a
small amount of
17
O-enriched water into one internal carotid
artery and monitoring the washout process of the H
2
17
O tracer in
the brain using 3 dimensional
17
O chemical shift imaging (CSI)
(67)
.
Figure 15.4A
demonstrates the stacked plots of H
2
17
O
spectra acquired from a single voxel (as indicated in the brain
anatomy image) of 3 dimensional
17
O CSI data set in a represen-
tative rat before and after the H
2
17
O bolus injection. The peak
height of the H
2
17
O spectra shows an exponential decay and its
decay rate determines the CBF value in the CSI voxel
(54,55,67)
.
3.2.1. Imaging CBF
The crucial step for CMRO
2
measurements is to monitor and
image the dynamic changes of the metabolic H
2
17
O content
in the brain (i.e.,
C
b
(t)
) during an inhalation of
17
O
2
gas.
Figure 15.4B
illustrates the stacked plots of
17
O spectra of cere-
bral H
2
17
O from the CSI voxel acquired before, during and after
a 2-minute inhalation of
17
O
2
(54)
. It indicates excellent
17
O
sensitivity for detecting the cerebral H
2
17
O signal and its change
during the inhalation; and the approximately linear increase of
H
2
17
O during a short
17
O
2
inhalation is evident, and the slope is
tightly coupled to CMRO
2
.
One practical challenge for most in vivo MRS approaches
is the difficulty for measuring the absolute concentrations
of metabolites of interest. Nevertheless, the natural abun-
dance H
2
17
O signal which can be accurately measured in the
3.2.2. Imaging
C
b
(t)
