B-1-gradient-based MRI using frequency-modulated Rabi-encoded echoes

Purpose Reduce expense and increase accessibility of MRI by eliminating pulsed field (B-0) gradient hardware. Methods A radiofrequency imaging method is described that enables spatial encoding without B-0 gradients. This method, herein referred to as frequency-modulated Rabi-encoded echoes (FREE), utilizes adiabatic full passage pulses and a gradient in the RF field (B-1) to produce spatially dependent phase modulation, equivalent to conventional phase encoding. In this work, Cartesian phase encoding was accomplished using FREE in a multi-shot double spin-echo sequence. Theoretical analysis and computer simulations investigated the influence of resonance offset and B-1-gradient steepness and magnitude on reconstruction quality, which limit other radiofrequency imaging methodologies. Experimentally, FREE was compared to conventional phase-encoded MRI on human visual cortex using a simple surface transceiver coil. Results Image distortions occurred in FREE when using nonlinear B-1 fields where the phase dependence becomes nonlinear, but with minimal change in signal intensity. Resonance offset effects were minimal for Larmor frequencies within the adiabatic full-passage pulse bandwidth. Conclusion For the first time, FREE enabled slice-selective 2D imaging of the human brain without a B-0 gradient in the y-direction. FREE achieved high resolution in regions where the B-1 gradient was steepest, whereas images were distorted in regions where nonlinearity in the B-1 gradient was significant. Given that FREE experiences no significant signal loss due to B-1 nonlinearities and resonance offset, image distortions shown in this work might be corrected in the future based on B-1 and B-0 maps.