Symmetrically biased T/R switches for NMR and MRI with microsecond dead time

Abstract

For direct NMR detection and imaging of compounds with very short coherence life times the dead time between radio-frequency (RF) pulse and reception of the free induction decay (FID) is a major limiting factor. It is typically dominated by the transient and recovery times of currently available transmit–receive (T/R) switches and amplification chains.A novel PIN diode-based T/R switch topology is introduced allowing for fast switching by high bias transient currents but nevertheless producing a very low video leakage signal and insertion loss (0.5dB). The low transient spike level in conjunction with the high isolation (75dB) prevent saturation of the preamplifier entirely which consequently does not require time for recovery.Switching between transmission and reception is demonstrated within less than 1μs in bench tests as well as in acquisitions of FIDs and zero echo time (ZTE) images with bandwidths up to 500kHz at 7T. Thereby the 2kW switch exhibited a rise-time of 350ns (10–99%) producing however a total video leakage of below 20mV peak-to-peak and less than −89dBm in-band.The achieved switching time renders the RF pulse itself the dominant contribution to the dead time in which a coherence cannot be observed, thus making pulsed NMR experiments almost time-optimal even for compounds with very short signal life times.