Biomedical Engineering Reference
In-Depth Information
low sensitivity is even more manifest for the nuclear spins (e.g.,
31
Pand
17
O) with a relatively low magnetogyric ratio (
)com-
paredtoinvivo
1
H MRS. This significantly limits the reliability,
applicability, spatial and temporal resolutions of in vivo
31
Pand
17
O MRS for general application, as well as clinical studies. One
common trade off is to reduce the spatial and/or temporal reso-
lution of in vivo MRS, thereby gaining detection sensitivity. How-
ever, in order to determine the nonuniform distribution of cere-
bral metabolites and chemical reaction fluxes in different brain
regions, there is a high demand on improving the spatial resolu-
tion of in vivo MRS. Moreover, the dynamic changes caused by
physiological perturbation (e.g., sensory stimulation) may occur
in a relatively short time and the magnitudes of the changes are
usually subtle compared to those observed under pathological
states. Therefore, both reasonably high spatial and temporal res-
olutions are desired and they rely heavily on the achievable detec-
tion sensitivity, which poses the major challenge for in vivo het-
eronuclear MRS. One way to overcome this challenge is the use
of high/ultrahigh field MRS. Besides the sensitivity gain at high
fields, the spectral resolution of in vivo MRS is also significantly
improved and many overlapped resonance peaks from different
metabolites as observed at low fields become resolvable at ultra-
high fields as demonstrated in
Fig. 15.2A
.
γ
2.2. Sensitivity
Improvement of In
Vivo Heteronuclear
MRS at High Field
One of the most important advantages at ultrahigh fields is the
potential gain in detection sensitivity. This is particularly crucial
for the low
nuclei MRS such as in vivo
17
Oand
31
PMRS.
However, it is not so straightforward to evaluate and compare
detection sensitivity (or signal-to-noise ratio, SNR) at different
magnetic field strengths (B
0
). The apparent SNR achievable at a
given field strength relies not only on the B
0
but also on other B
0
-
dependent parameters, such as the longitudinal relaxation time
(T
1
), the apparent transverse relaxation time (T
2
*) and the repe-
tition time (TR) for acquiring MRS signal. It is, in general, more
useful to determine and evaluate the SNR of signal detected in
a given unit sampling time under optimal acquisition condition,
which means that the excitation flip angle (
γ
α
) satisfies the Ernst
equation of cos(
α
opt
) = exp(-TR/T
1
)
(56)
. For a simple case with
the single pulse and data acquisition scheme, such an averaged
SNR can be quantified by
Eq. (15.3)
(56-60)
,
2.5
B
0
Q
1
/
2
T
2
1
/
2
E
2
)
1
/
2
G
(
x
)
∝
γ
−
SNR
(per unit sampling time)
C
(1
(15.3)
T
1
where
2
1
/
2
e
−
x
1
−
TR
T
1
and
E
2
=
T
2
),
G
(
x
)
=
x
=
exp (
−
2
·
a
t
/
(15.4)
e
−
x
)
x
(1
+
