Biomedical Engineering Reference
In-Depth Information
9.5.3.1
Temperature Compensation
The NMR frequency f
0
is the parameter that requires most frequent adjustments and
affects the measured NMR signals most significantly. The frequency f
0
changes as
the magnetic field (B
0
) from the permanent magnet drifts with temperature [
58
].
For example, with a 1
ı
C increase in temperature, B
0
field from a NdFeB magnet
will drop
0:1% from its initial value, and f
0
will proportionally decrease by
0.1 %; when the initial f
0
is 20 MHz (B
D
0:47 T), the frequency change is then
20 kHz. Such changes can place down-converted NMR signals near or beyond
the low-frequency cutoff in the amplification stage, distorting the measured signal.
Commercial benchtop NMR systems address the problem by housing the entire
magnet block inside a heated container. This solution, however, significantly under-
mines the portability of the system due to the use of bulky and power-consuming
parts. In the new NMR system, we employed a dynamic control approach. Namely,
programmable hardware in the NMR electronics and temperature compensation
engine in the NMR software are designed to track and compensate for temperature
dependency of the system. These implementations ensure optimal measurement
settings for reliable and robust performance.
Figure
9.9
a and b show the algorithm for temperature compensation. The feed-
back loop tracks the Larmor frequencyf
0
and reconfigures the frequencyf of NMR
excitation. Coarse-tuning mode starts with initial NMR excitation frequency f
i
and
increases f by f . When the spectral power (P/ of NMR spin echo reaches a
predefined threshold P
th
, a fine-tuning mode takes over to measure the frequency
offset f
d
.
/. The fine tuning iterates until f
d
reaches a target value. The
target f
d
value is carefully selected to keep down-converted NMR signal within the
passband of the low-pass filter. Once the new NMR excitation frequency f has been
established, CPMG pulse sequence is used to measure the T
2
relaxation time of the
sample.
Figure
9.9
c demonstrates the effectiveness of the developed temperature com-
pensation method. When f
0
was allowed to drift but the RF frequency .f / for
sample excitation was fixed, T
2
values varied up to 200 % relative to its starting value
with typical fluctuation of room temperature .T
Dj
f
f
0
j
2
ı
C/. When the temperature
compensation engine was activated, however, T
2
variations were significantly
reduced to <1%. We further tested the system in environmental settings with a wide
range of temperature differences (4-50
ı
C). To determine the measurement accuracy,
the linear dependence of T
2
on temperature was utilized. Figure
9.9
dshowsthe
T
2
values of an MNP solution monitored at different temperature. For a given
environment setting, the NMR system was operated with the temperature tracking
activated to compensate for minute temperature variations (
1
ı
C). The results
show a linear relationship (R
2
>98%) as theoretically predicted, demonstrating the
capacity for reliable T
2
measurements in various settings.
