Traveling-Wave Excitation for Electron Paramagnetic Resonance Imaging
Bahareh Behzadnezhad1,2, Nader Behdad1, and Alan B. McMillan2,3

1Electrical Engineering, University of Wisconsin-Madison, Madison, WI, United States, 2Radiology, University of Wisconsin-Madison, Madison, WI, United States, 3Medical Physics, University of Wisconsin-Madison, Madison, WI, United States


One primary factor limiting the extension of electron paramagnetic resonance imaging (EPRI) beyond small animal applications is the challenge of create uniform RF fields at the higher RF frequencies needed to achieve the desired sensitivity. In this abstract, we discuss the development of a traveling-wave spectrometer operating in continuous-wave mode at 115 MHz. The spectrometer uses a parallel-plate waveguide supporting transverse electromagnetic waves to create a uniform transmit field and uses a conventional receive coil. We demonstrate the feasibility of a traveling wave system and its potential to be developed into an EPR imaging system.


Electron paramagnetic resonance imaging (EPRI) is a low cost and highly specific molecular imaging modality. Compelling applications of biomedical EPR have been demonstrated to non-invasively measure in-vivo tissue properties such as oxygenation, pH, redox status, and exposure to ionizing radiation [1]. One of the primary factors limiting the extension of EPRI beyond small animal (mice) imaging is the need for using higher RF frequencies to achieve the desired sensitivity, which presents fundamental challenges in terms of creating a uniform magnetic field. We demonstrate and discuss the development of a continuous and traveling-wave (TW) spectrometer operating at 115 MHz using a parallel-plate waveguide (PPWG) supporting transverse electromagnetic (TEM) waves for transmit.


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.


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.


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.


[1] J. A. (John A. Weil, J. R. Bolton, and Wiley InterScience (Online service), Electron paramagnetic resonance : elementary theory and practical applications. Wiley-Interscience, 2007.

[2] Behzadnezhad B, Dong J, Behdad N, McMillan A. A Very Low-cost EPR Spectrometer Using 3D Design and Manufacturing. Proc ISMRM 25th Annu Meet Exhib. 2017.

[3] D. O. Brunner, N. De Zanche, J. Fröhlich, J. Paska, and K. P. Pruessmann, “Travelling-wave nuclear magnetic resonance,” Nature, vol. 457, no. 7232, pp. 994–998, Feb. 2009.


Figure 1. (A) Proof of concept results with a parallel plate waveguide (PPWG). (B) Design of a parallel plate waveguide fed by a monopole antenna.

Figure 2. (A) fabricated parallel loop resonator. (B) Simulation of the parallel plate loop resonator

Figure 3. RF bridge of the traveling-wave EPR spectrometer

Figure 4. (A) Constructed prototype PPWG placed inside a solenoid electromagnet. (B) Inside view of the waveguide showing the parallel resonator and modulation coil.

Figure 5. Measured spectrum (absorbance) of 1,1-diphenyl-2-picrylhydrazyl (DPPH) on the homebuilt TW spectrometer.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)