Passive Decoupling Due to Low Q-Factors of Four-Channel Coils in 300-MHz Pulsed EPR Imaging

This article describes a four-channel coil array for in vivo time-domain electron paramagnetic resonance (EPR) imaging applications. Decoupling of the individual element coils was achieved with the low-quality factors required in pulsed EPR at radiofrequency to accommodate the imaging bandwidth. No active decoupling method was used in the coil array. Remote control of a coaxial switch connected to the four-channel coil array was performed with voltage signals from a personal computer. To verify the operation of this four-channel coil array, a phantom tube filled with a solution of paramagnetic triarylmethyl radicals (OX063) was visualized by pulsed EPR imaging based on the single-point method involving pure phase encoding. In an in vivo experiment, OX063 in a mouse tumor-bearing right hind leg was visualized by pulsed EPR imaging. Even without active decoupling circuits, there was no shift in resonances for individual coils when those were tuned to the same frequency due to low quality factors and low coupling coefficients with neighboring coils. The coil assembly allowed us to study an object that was larger than an individual coil element, enabling the imaging of larger subjects with reduced overall specific absorption rate. Such strategies should be useful for studying larger objects by EPR imaging.