Abstract
A microwave Surface Resonator Array (SRA) structure is described for use in Electron Paramagnetic Resonance (EPR) spectroscopy. The SRA has a series of anti-parallel transmission line modes that provides a region of sensitivity equal to the cross-sectional area times its depth sensitivity, which is approximately half the distance between the transmission line centers. It is shown that the quarter-wave twin-lead transmission line can be a useful element for design of microwave resonators at frequencies as high as 10 GHz. The SRA geometry is presented as a novel resonator for use in surface spectroscopy where the region of interest is either surrounded by lossy material, or the spectroscopist wishes to minimize signal from surrounding materials. One such application is in vivo spectroscopy of human finger-nails at X-band (9.5 GHz) to measure ionizing radiation dosages. In order to reduce losses associated with tissues beneath the nail that yield no EPR signal, the SRA structure is designed to limit depth sensitivity to the thickness of the fingernail. Another application, due to the resonator geometry and limited depth penetration, is surface spectroscopy in coating or material science. To test this application, a spectrum of 1.44 μM of Mg(2+) doped polystyrene 1.1 mm thick on an aluminum surface is obtained. Modeling, design, and simulations were performed using Wolfram Mathematica (Champaign, IL; v. 9.0) and Ansys High Frequency Structure Simulator (HFSS; Canonsburg, PA; v. 15.0). A micro-strip coupling circuit is designed to suppress unwanted modes and provide a balanced impedance transformation to a 50 Ω coaxial input. Agreement between simulated and experimental results is shown.
Highlights
One fundamental issue of surface magnetic resonance spectroscopy, in vivo, is limiting the depth sensitivity to the volume of interest
The Bruker cavity is relatively uniform in the volume of the sample, while the microwave field of the Surface Resonator Array (SRA) is uniform in a plane and diminishes exponentially
This paper is concerned with limiting depth sensitivity by tailoring the geometry for acquisition of Electron Paramagnetic Resonance (EPR) signals from surfaces with a known region of interest
Summary
One fundamental issue of surface magnetic resonance spectroscopy, in vivo, is limiting the depth sensitivity to the volume of interest. Ikeya et al designed an aperture resonator for Electron Paramagnetic Resonance (EPR) X-band in vivo tooth dosimetry by cutting an iris, either a hole or slot, in a rectangular TE102 cavity.. Aperture resonator geometries use the cavity resonator as a coupler to an evanescent wave at a localized region. In Magnetic Resonance Imaging (MRI), Song et al used a small 1 cm surface coil at 1.5 T to increase the magnetic field at the surface of the skin to image the cutaneous surface.. The vector reception field of a small surface coil will minimize coupling to deep tissue regions. Ikeya et al and Song et al use the size of their probes to minimize losses and coupling to unwanted volumes surrounding the sample
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