B 1-gradient–based MRI using frequency-modulated Rabi-encoded echoes
Efraín Torres, Taylor Froelich, Paul Wang, Lance DelaBarre, Michael Mullen, Gregory Adriany, Daniel Cosmo Pizetta, Mateus José Martins, Edson Luiz Géa Vidoto, Alberto Tannús, Michael Garwood
Abstract
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.