A retrofit surface dielectric resonator for X-band EPR spectrometers enabling in vivo measurements.

Retrofitting existing X-band electron paramagnetic resonance (EPR) spectrometers for surface measurements of high-dielectric materials or for in vivo applications remains challenging, as conventional cavity resonators require small samples to be positioned within a confined resonant volume. We describe a retrofit surface dielectric resonator (X-SDR) that enables in vivo and surface EPR measurements on a standard X-band spectrometer without modification of the microwave bridge or console. The resonator comprises a cylindrical high-permittivity dielectric element enclosed within a conductive shield, with an aperture that allows the microwave magnetic field (B₁) to extend into the sample, thereby defining a localized detection region. The device is implemented as a plug-in resonator and operates with external magnetic field modulation, effectively converting conventional X-band systems into instruments capable of surface EPR measurements of high-dielectric materials. Electromagnetic simulations and experimental studies were used to characterize resonator coupling, B₁ field distribution, and spatial sensitivity. Perturbation and tissue-mimicking phantom experiments demonstrated detectable signal sensitivity to depths of up to 3 mm from the resonator surface. Measurements using lithium phthalocyanine (LiPc) and OxyChip oxygen sensors yielded signal-to-noise ratios comparable to those obtained with conventional surface loop-gap resonators at L-band frequencies. This retrofit resonator provides a practical and flexible approach for EPR measurements of solutions and biological samples, including in vivo applications.