Thermal modeling of Q-spoil switching elements for MRI coils

Thermal models for simulating heating and cooling of PIN diode and FET-based MRI coil blocking circuits are presented. The temperature dependent parameters in the two basic circuit elements as well as the related SPICE-compatible circuit models are discussed. Impedance versus temperature measurements for both PIN diode and MOSFET are presented that confirm the validity of model usage. A MRI coil blocking application is presented, showing significant device heating during the transmit pulse prior to application of the blocking control signal, with significant blocking as well as device cooling occurring rapidly. The models discussed should assist MRI circuit designers in determining potential failure of the device due to heating under the application of high power MRI pulses.