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.
Bibliographical noteFunding Information:
This research was supported by the Minnesota Lions Diabetes Foundation, Schott Family, and Malcolm B. Hanson Endowed Chair in Radiology. Additional support was provided by the National Institutes of Health (grants U01 EB025153, RF1 MH123698, and P41 EB027601). E. Torres is grateful to be supported by the National Science Foundation Graduate Research Fellowship Program and the University of Minnesota’s Creative Inclusive Cohort Fellowship.
This research was supported by the Minnesota Lions Diabetes Foundation, Schott Family, and Malcolm B. Hanson Endowed Chair in Radiology. Additional support was provided by the National Institutes of Health (NIH), grants U01 EB025153, RF1 MH123698, and P41 EB027061. Support also provided by the National Science Foundation (NSF) Graduate Research Fellowship Program (GRFP) and the University of Minnesota’s Creative Inclusive Cohort (CIC) Fellowship ( e.t .)
© 2021 International Society for Magnetic Resonance in Medicine
- adiabatic full passage
- portable MRI
- radiofrequency imaging
- silent MRI
- surface coil
PubMed: MeSH publication types
- Journal Article