Biomedical Engineering Reference
In-Depth Information
C
is the concentration of the nuclear spin under observation;
a
t
is
the spectrum acquisition time;
Q
is the radio frequency (RF) coil
quality factor.
C
and
γ
are B
0
-independent and can be treated as
constants.
Equation (15.3)
accounts for most B
0
-dependent parame-
ters that have influences in the apparent SNR, such as T
2
∗
signal
loss and the partial saturation effect on signal due to a relatively
short TR of
5T
1
. For in vivo
31
P MRS applications, the term of
E
2
)
1
/
2
G
(
TR
(1
−
/
T
1
) approximates to 1.0
(60)
, thus
Eq. (15.5)
is a simplification
B
0
Q
1
/
2
T
2
1
/
2
SNR
(per unit sampling time)
∝
,
(15.5)
T
1
or
T
2
T
1
1
/
2
B
β
∗
0
SNR
(per unit sampling time)
∝
,
(15.6a)
B
1
/
2
0
β
∗
if one applies the approximation of
Q
∝
resulting
=
β
∗
and
β
based on the theoretical
predictions are 1.75 and 1.5 respectively
(57)
.
For precisely quantifying and understanding the sensitivity of
in vivo
31
P MRS as a function of magnetic field strength, it is
necessary to quantify all B
0
-dependent parameters used in
Eq.
(15.6A)
.
+1/4
(57, 60)
. The values for
β
It has been demonstrated that both SNR and the quality of in
vivo
31
P MRS acquired in human brain are improved at higher
fields
(24, 61-63)
. Recently, we have quantitatively measured and
compared the values of T
1
,T
2
* and SNR of PCr resonance peak
acquired from the human occipitallobe at 4 tesla and 7 tesla
(60)
.
We found that not only T
2
* of PCr, but also T
1
of PCr (i.e.
T
1,PCr
) were shortened at 7 tesla compared to 4 tesla. This obser-
vation indicates that the field dependence of T
1,PCr
is likely deter-
mined by two competing relaxation mechanisms,
i.e.
the chemical
shift anisotropy (CSA) and the dipolar interaction that simultane-
ously influence the
31
P longitudinal relaxation time
(60,64,65)
.In
the case with dominant CSA contribution, the T
1
value decreases
with increasing field strength following the relation of 1/T
1
∝
2.2.1. Field Dependence
of in vivo
31
PMRS
Sensitivity
B
0
;
in contrast, the T
1
value increases with increasing field strength
when the dipolar interaction mechanism dominates. The oppo-
site trends between these two relaxation mechanisms lead to a
decrease in T
1,PCr
at higher field and offer an advantage for in
vivo
31
P MRS as a shorter repetition time can be employed which
allows more signal averaging within the same sampling time; as
a result, improving SNR and leading to a total SNR gain of 56%
