Dielectric-Backed Aperture Resonators for X-band Depth-Limited in Vivo EPR Nail Dosimetry

A new resonant geometry for X-band in vivo electron paramagnetic resonance (EPR) nail dosimetry has been developed, fabricated, and tested. The dielectric-backed aperture resonator (DAR) was specifically designed for depth-limited surface spectroscopy. The DAR improves EPR sensitivity of surface samples with sub-millimeter thicknesses by at least a factor of 20 compared to other aperture resonator designs. The increase in EPR sensitivity was achieved using a non-resonant dielectric slab which is placed on the aperture inside the cavity. The dielectric slab provides an increased microwave magnetic field, while minimizing the problematic resonance conditions of the aperture. It has been shown that the DAR provides sufficient sensitivity to make biologically-relevant measurements both in vitro and in vivo. This work demonstrates that in vivo tests with an equivalent dosimetry sensitivity of approximately 1.4 Gy are feasible. Plausible ways to further increase the sensitivity are discussed, such as, the introduction and simulations of a DAR based on a semi-spherical TE 011 geometry.