Time-interleaved radiation damping feedback for increased steady-state signal response

INTRODUCTION Radiation damping (RD) describes a second-order effect where the signal-induced current in the receiver coil acts back onto the primary spin system [1]. According to Lenz’s law, the RD field acts in a way to oppose its original cause. In that sense RD can be understood as a 1) self-generating and 2) selfregulating flip-back pulse, which causes the transverse magnetization to return to equilibrium more rapidly than it otherwise would [2,3]. The natural RD effect is characterized by the RD damping time constant according to τRD = 2 / (γ μ Mt η Q), with γ the gyromagnetic ratio, μ the magnetic susceptibility, Mt the transverse magnetization, η the coil filling factor and Q the quality factor. Accordingly, natural radiation damping can only be observed with very high Q and/or high filling factor arrangements, but typically not under normal MRI conditions. Recently, RD feedback loops have been introduced into the transmit-receive (Tx/Rx) RF signal path to artificially boost the natural RD effect. Using such feedback circuits, it was demonstrated that the natural RD could be amplified in a way to become effective as a signal recovering mechanism [2,3]. While previous RD circuits were limited in terms of RD feedback gain, here we present a new feedback circuit, which principally circumvents this problem via time separation of RD receive and transmit.