In pulsed electron paramagnetic resonance (EPR), free-induction decay (FID) or spin echo (SE) signals of unpaired electrons are recorded in the time-domain. In both methods, electromagnetic waves play an important role in the detection of unpaired electrons in EPR spectroscopy. The resonator generates and senses electromagnetic waves and therefore serves as a critical interface between unpaired electrons and the transmit/receive systems of an EPR spectrometer. Since a resonator is a sensitive electrical circuit that can amplify voltages and currents when the electrical circuit of the resonator is on resonance, the resonator is an essential component for EPR detection in continuous wave and pulsed EPR. Without the resonator, EPR signals cannot be detected with sufficient sensitivity. In this chapter, the basics of resonators and some examples of resonators used in preclinical studies with small animals and human subjects are explained.
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