B₁-gradient–based MRI using frequency-modulated Rabi-encoded echoes

Purpose: Reduce expense and increase accessibility of MRI by eliminating pulsed field (B₀) gradient hardware. Methods: A radiofrequency imaging method is described that enables spatial encoding without B₀ 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 (B₁) 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 B₁-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 B₁ 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 B₀ gradient in the y-direction. FREE achieved high resolution in regions where the B₁ gradient was steepest, whereas images were distorted in regions where nonlinearity in the B₁ gradient was significant. Given that FREE experiences no significant signal loss due to B₁ nonlinearities and resonance offset, image distortions shown in this work might be corrected in the future based on B₁ and B₀ maps.