Balanced Steady-State Feedback Radiation Damping: Balanced SSFR

INTRODUCTION The SNR efficiency in MR experiments is intrinsically limited by longitudinal-, and transverse spin relaxation mechanisms. This becomes particularly limiting for unbalanced, short repetition time (TR) sequences, where incomplete T1-relaxation results in a steady-state signal that is only a small fraction of the available thermal equilibrium magnetization. Radiation damping (RD) is a second-order effect where the signal-induced current in the receiver coil is strong enough to act back on the signalgenerating spins [1]. According to Lenz’s law, the RD field acts in a way to oppose its original cause. In that sense it can be understood as a selfregulating flip-back pulse causing the transverse magnetization to return to equilibrium more rapidly than it otherwise would. Generally, signals are sufficiently strong to produce RD only in very high-Q coils with high filling factors, such as in high-resolution NMR spectrometers. Recently, active feedback loops have been introduced into the transmit-receive RF signal path as a means to either cancel [2-3], or amplify [4-6] the RD effect. For instance, Huang et al [6] used RD feedback to actively control the recovery of longitudinal magnetization immediately following a nonselective saturation pulse. Here we investigate RD feedback as a means to achieve increased SNR efficiency for short-TR, gradient-echo (GRE) sequences.