Methods

We designed, constructed, and tested a parallel-plate waveguide continuous-wave (CW) EPR spectrometer as shown in Figure 1. Two 26 Gauge brass sheets of size 9.4 × 4.7 cm² were used to create a PPWG. Nylon plastic screws and bolts were used to separate the plates by 6 cm. Using a monopole antenna to realize a TW excitation, simulation results demonstrated the creation of a uniform magnetic field along the length of the PPWG (Figure 1(B)) across a large volume. The excitation probe was optimized to provide an input impedance of 50Ω ensuring that maximum power is transferred from the source to the waveguide and any reflections towards the source are minimized. The opposite end of the transmission line was resistively terminated at the characteristic impedance of the waveguide to ensure that the wave launched by the antenna along the transmission line was indeed a traveling wave and reflections at the far end did not create a standing wave pattern and distort the uniformity of the magnetic field. To perform a proof-of-concept traveling wave experiment, an EPR spectrometer similar to a previous design utilizing a solenoid electromagnet and lock-in detection [2] was modified to (1) replace the RF bridge with a much simpler direct feed of the monopole antenna (e.g., no hybrid junction), (2) employ a new modulation coil that was printed and constructed to fit inside the waveguide, and (3) utilize the existing resonator (shown in Figure 2) as only the receiver in a two-port configuration where port 1 (transmitter) is connected to the input of the monopole antenna and port 2 is connected to the receive resonator/coil. RF bridge of the TW EPR spectrometer is shown in Figure 3. This implements a transmission type measurement system where the RF magnetic field required for EPR excitation is created by the transmission line and the monopole antenna. A conventional RF coil is used for signal reception. Figure 4(A) Shows the constructed prototype PPWG placed inside a solenoid electromagnet. Figure 4(B) shows the inside view of the waveguide showing the parallel resonator and modulation coil.

Results

A spectrum of 2,2-diphenyl-1-picrylhydrazyl (DPPH) measured with the TW EPR spectrometer is shown in Figure 5. Note that the spectrum in Figure 5 does not represent the first derivative of the absorption signal seen in the conventional reflection-based CW spectrometers because in the two-port system, the lock-in detection does not measure the signal around a null point (S₁₁), rather a local maximum (S₂₁). By eliminating the directional coupler used in reflective-type EPR systems (with finite isolation in the 30-40 dB range), this operating mode is also expected to improve the sensitivity of the proposed EPR system compared to those of the reflection-mode designs.

Discussion and Conclusion

In this abstract, we constructed a traveling-wave EPR spectrometer using a parallel plate transmission line for excitation. We tested the system by measuring the EPR spectra of DPPH powder. In the TW approach, RF signal excitation is performed using traveling waves that propagate from a remote antenna inside of a waveguide through subject [3]. In comparison to the conventional reactive near field approach, uniform magnetic fields across a much larger volume can be realized. EPRI operates at a very low frequency compared to MRI (~660x lower) allowing the use of non-superconducting electromagnetics to substantially reduce instrumentation cost. The development of TW EPRI is a promising technique to sufficiently high magnetic field strength toward instrumentation capable of imaging the human body.

Acknowledgements

The project described was supported by the Clinical and Translational Science Award (CTSA) program, through the NIH National Center for Advancing Translational Sciences (NCATS), grant UL1TR000427. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.