B1‐gradient–based MRI using frequency‐modulated Rabi‐encoded echoes. Issue 2 (9th September 2021)

• Record Type: Journal Article
• Authors: Torres, Efraín; Froelich, Taylor; Wang, Paul; DelaBarre, Lance; Mullen, Michael; Adriany, Gregory; Pizetta, Daniel Cosmo; Martins, Mateus José; Vidoto, Edson Luiz Géa; Tannús, Alberto; Garwood, Michael
• Is Part Of: Magnetic resonance in medicine. Volume 87:Issue 2(2022)
• Page Start: 674
• Page End: 685
• Publication Date: 2021-09-09
• Subjects: adiabatic full passage -- frequency‐modulated -- portable MRI -- radiofrequency imaging -- silent MRI -- surface coil; Nuclear magnetic resonance -- Periodicals; Electron paramagnetic resonance -- Periodicals; 616.07548
• DOI: 10.1002/mrm.29002
• Languages: English
• ISSNs: 0740-3194

Abstract

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.