Compact 1 GHz electron paramagnetic resonance spectrometer and imager

A 1 GHz preclinical electron paramagnetic resonance spectrometer and imager has been developed to study low concentrations of radicals using rapid-scan detection. The design focuses on minimizing the losses between the resonator and the first-stage low-noise amplifier. The adjustable-frequency source with the lowest available noise was selected and the output was amplified to provide sufficient power for experiments with relatively rapidly relaxing nitroxide radicals. For resonators with similar geometries and efficiencies, improved signal to noise was demonstrated relative to our previous instrument that operates at 700 MHz, consistent with predictions for frequency dependence. System performance was demonstrated for solutions of nitroxide radicals in resonators that contain 8 mm or 25 mm diameter cylindrical tubes. For 15N-d16 tempone in aqueous solution and a signal averaging time of 3.8 s, a signal-to-noise of 3 required 2 × 1014 spins in the 8 mm resonator or 4.5 × 1015 spins in the 25 mm resonator. 3D spatial imaging was demonstrated with a phantom consisting of three tubes containing solutions of a nitroxide radical.