Biomedical Engineering Reference
In-Depth Information
at 7T as compare to 4T. Moreover, the
value derived from the
experimental measurements and
Eq. (15.5)
in this study was 1.4
(60)
. This value is in an excellent agreement with the theoretically
predicted
β
value of 1.5 as described in the literature
(57)
.
Another interesting observation of this study is that the
linewidth broadening of the phosphate resonance peak with
increased field strength is not linearly correlated with the field
strength and, thus, at higher field strength, there is a small
increase in the line width of phosphate signals
(60)
. Moreover,
higher field strength increases the chemical shift dispersion (i.e.,
in Hz unit) of in vivo
31
P MRS linearly with field strength. The
combination of these two factors (i.e., large increase in chemical
shift dispersion and relatively small increase in linewidth broaden-
ing) improves the
31
P spectral resolution at 7T (see
Fig. 15.2A
).
This improvement makes it possible to resolve many adjacent
phosphate metabolite resonance peaks (e.g.,
β
-ATP versus NAD)
and it is especially helpful for resolving the Pi peak from other
adjacent resonance peaks. The well-resolved Pi peak is crucial
for measuring the ATP production flux and will be discussed
later.
α
17
O is a stable oxygen isotope with a spin quantum number
of 5/2 and detectable by
17
O MRS. It possesses an electric
quadrupolar moment and its natural abundance is only 0.037%,
which is almost 30 times lower than that of
13
C and 2700 times
lower than that of
1
H. Moreover, the
2.2.2. Field Dependence
of in vivo
17
OMRS
Sensitivity
value of
17
O is 7.4 times
lower than that of
1
H. These factors attribute to the low inherent
17
O sensitivity that might be the major obstacle for the develop-
ment and application of in vivo
17
O MRS approaches.
The relaxation mechanism of
17
O spin is distinct from
31
P
spin. The
17
O quadrupolar moment can interact with local elec-
tric field gradients and the temporal fluctuations of this interac-
tion induced by molecular motion dominate the
17
O relaxation
processes and determine both T
1
and T
2
of H
2
17
O
(66)
, where
T
2
is the transverse relaxation time. Theoretically, the T
1
,T
2
and
T
2
*valuesof
17
O water spins are expected to be insensitive to B
0
(55, 66)
. We have experimentally investigated and compared the
17
O relaxation times of the natural abundance H
2
17
O in rat brain
at both 4.7 and 9.4 tesla. We confirmed that the
17
O relaxation
times are B
0
-independent
(67)
. Recent experimental evidence has
indicated that the B
0
-independence of
17
O relaxivity can further
extend to a much higher magnetic field, such as 17.6 tesla
(68)
.
The field independence of
17
O relaxivity reveals that
17
Osensi-
tivity gain at higher fields is not compromised by the relaxation
times, and this could lead to a substantial sensitivity gain when
the field strength increases according to
Eq. (15.6B)
if the same
acquisition parameters (e.g., TR and
a
t
) are applied at different
field strengths,
γ
