Effect of PIN diode nonlinearity on decoupler circuits in magnetic resonance imaging surface coils

Decoupler circuits are the primary circuits used to maintain safety and image quality in switching magnetic resonance imaging (MRI) surface coils. Decoupler circuits predominantly employ PIN diodes as a switch and their performance is most commonly calculated on the bench at DC and low power RF conditions. The effects of high-power RF on PIN diode decoupler circuits are not usually measured. Experiments at high RF power levels reveal a decrease in the impedance of a typical decoupler as the PIN diode operates in the nonlinear region, effectively increasing the ON-resistance of the PIN diode. The constraints that dictate the start of nonlinearities are studied, and ways to control these nonlinearities are presented. Furthermore, this work is used as a basis to extend and improve upon previous work that established figure of merit (FOM) for PIN diode decouplers. This study is a comprehensive guide for MRI coil designers who face the task of designing decoupler circuits for surface coils and are looking for tools to accurately estimate the dynamic impedance of the circuit over the course of an MRI sequence.