High-Power T/R Switches with 350 ns Rise Time for Zero Echo Time Imaging

Introduction: High-power transmit-receive (T/R) switches are a core component of pulsed NMR and MRI spectrometers. The usually employed PIN diodes offer low losses and high power handling capabilities, but are inherently slow semiconductors and let switches usually transit within 5-20 μs from transmission to reception. For liquid state NMR methods, this dead-time is mostly not a big caveat, however with the advent of novel techniques for imaging of ultra-short T2 * species such as UTE[1], SWIFT[2], ZTE[3] and others the need for switching in a microsecond and less has been set. This however necessitates the diode biasing being itself an actual high-bandwidth (few MHz) RF design with considerable power to swiftly drive the diodes. But the strong switching transients must be well isolated from the RF signal paths of the NMR signal in order not to hamper the high-gain, low-noise amplification chain joining the receive (RX) port of the switch which is very difficult in most traditional topologies. In this work we will present an approach to design high power RF T/R switches with RF signal rise times in the realm of 300 ns with low glitches (video leakage) from the switching bias at the outputs and demonstrate an implementation for 7 T proton ultra –short T2 * imaging (f0=298 MHz) with zero echo time (ZTE) and up to 650 kHz bandwidth.