Ayano Enomoto1, Gadisetti V. R. Chandramouli2, Alan P Koretsky3, Chunqi Qian4, Murali K Cherukuri1, and Nallathamby Devasahayam1
1Radiation Biology Branch, National Cancer Institute, Bethesda, MD, United States, 2GenEpria Consulting Inc., Columbia, MD, United States, 3National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, United States, 4Department of Radiology, Michigan State university, East Lansing, MI, United States
Synopsis
Sensitivity enhancement
is required to detect the weak signals with Fourier transform Electron
Paramagnetic Resonance (FT-EPR). In the
proposed method, a small amount of sample was placed at a distance less than
half the diameter of the receiving surface coil. The signal was enhanced by a
wirelessly pumped coil. Presently, we used the TCNQ for our studies to study
signal enhancement. Here, we achieved 7-fold of improvement in signal intensity
in compared with conventional FT-EPR acquisition. We will show the results of in vivo oximetry using oxygen sensing
solids LiPc and LiNc in in vivo
applications to measure tissue oxygenation.Purpose
The purpose of
this study is to investigate the feasibility of signal enhancement using the
wireless coil in Fourier transform electron paramagnetic resonance (FT-EPR) system
for high sensitive repeated oximetry measurements. The knowledge of temporal change
in partial pressure of oxygen (pO
2) in tissue is important for understanding
the pathology of hypoxia related diseases. The EPR oximetry is the one of the
strong methods for repeated monitoring pO
2 over time
1,2. For repeated
EPR oximetry, the signal from the reagents implanted in such as lithium
phthalocyanine (LiPc) or lithium naphthalocyanine (LiNC)
3 is so weak in the
presence of oxygen that it is difficult to detect with 300 MHz FT-EPR. Therefore,
improvements in sensitivity of detection is required to achieve high sensitive
oximetry in 300 MHz FT-EPR. In magnetic resonance imaging (MRI), there are some
reports to detect and enhance signals from deep lying internal organs by using a
wireless coil which enables weak signal to amplify
4,5. We applied the same principle
using the wireless coil to 300 MHz FT-EPR and attempted to increase the
detection sensitivity of FT-EPR.
Methods
A wireless coil is a detector using parametric amplification,
which can amplify the weak signal by applying some power (pumping power)
through the inductive coupling with external coil (pumping coil). The detailed theory
and construction of the wireless coil has been reported earlier5.
To
evaluate the effect of using wireless coil, we obtained spectra using a homebuilt
300 MHz FT-EPR for the all measurements in this study. The parameters of
measurements were as follows: repetition time, 10 µs; 90 degree pulse width, 80
ns; number of average, 100000 times; transmit power, 64 W. The surface coil resonator
with 22 mm diameter coil was used as transmitter and receiver. The quality
factor of the resonator was set to around 30 to reduce dead time. The resonant
frequency of the wireless coil was set to 300 MHz for idler signal and
amplified signal, and 600 MHz for pumping signal. The diameter of the pumping
coil for pumping the wireless coil was also 22 mm. The sample was a cylinder (2
mm inner diameter and 1.5 mm height) filled with solid N-methyl pyridinium
tetra-cyanoquinodimethane (TCNQ). Figure 1 shows the configuration of the
measurements and the pictures of the coils.The
EPR measurements were repeated under different conditions: a) without the
wireless coil (normal EPR measurement); b) with wireless coil and no pumping power; and c) with wireless coil and pumping power.
To determine the peak pumping
power accurately, we performed experiments under the slightly different
magnetic field from original zero magnetic field (10.7 mT). That enabled us to
distinguish the signal generated by electron spin from the signal due to the
oscillation signal of wireless coil. The pumping power at which we got the highest peak related
to 300 MHz was the peaking power. Then the pumping power was reduced step by step
until the zero frequency disappeared. It was found at that time the enhancement
of signal due to pumping frequency was high. Then the magnetic field was changed
to zero magnetic field and the measurements were made.
Results and Discussion
Figure
2 shows the results of the FT-EPR spectra with and without the wireless coil. Without
the wireless coil and with the wireless coil but no pumping power, the signal
intensity was 0.53 and 0.49, respectively. When the proper pumping power (in
this case 1.46 dBm) was applied to the wireless coil, the signal intensity was
increased to 3.82. The line width got narrow when the wireless coil was placed.
However, since the calibration curve will be constructed before EPR oximetry, the
change of line width does not affect to measurements of oximetry. Figure 3 shows
the relation between the signal level and applied pumping power. When the
pumping power was decreased, the signal intensity was also decreased. When
pumping power was more than the proper pumping power (1.46 dBm), the signal
from the electron spin
was hidden by the oscillation
signal generated by the wireless coil itself.
Conclusion
We
have demonstrated the feasibility of enhancement of sensitivity by using the
wireless coil to FT-EPR. We succeeded to increase the signal intensity 7 times
compared with conventional FT-EPR measurement. This shows it is possible to
detect the small signal with FT-EPR using wireless coil. Therefore we will
apply the wireless coil to
in vivo
oximetry in future applications.
Acknowledgements
No acknowledgement found.References
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