Automatic coupling control system for radio frequency in vivo electron paramagnetic resonance based on a piezoelectric controlled capacitor

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Abstract
Electron paramagnetic resonance (EPR) experiments on whole animals present a number of challenging problems. With the animal placed inside the resonant structure, respiratory and cardiac motion cause the resonant frequency and coupling to change dynamically. We have developed an automatic coupling control (ACC) system designed to reduce motion artifacts using an electronically controlled variable capacitor based on a piezoelectric actuator. The capacitor is varied between 11 and 21 pF. We have also developed a novel method for detecting coupling mismatches. The ACC system operates by applying a small modulation signal to the piezo-capacitor control voltage, which in turn modulates the coupling and the reflected signal from the resonator. The phase of the modulation signal, at the output of the diode detector, depends on whether the resonator is under- or overcoupled. We have tested the system with a phantom designed to simulate respiratory motion. The results reported here show that the use of the ACC system in combination with an automatic frequency control can dramatically reduce motion-induced artifacts on the EPR spectrum.