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

Abstract

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.

Keywords: adiabatic full passage; frequency-modulated; portable MRI; radiofrequency imaging; silent MRI; surface coil.