The effects of coupled B1 fields in B1 encoded TRASE MRI - A simulation study

Bohidar, Pallavi; Sun, Hui; Sharp, Jonathan C.; Sarty, Gordon E.

doi:10.1016/j.mri.2020.09.003

Cited by 5 publications

(2 citation statements)

References 15 publications

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“…26 The process by which FREE encodes spatial information is equivalent to conventional phase encoding; thus, many of the approaches already available to MRI for distortion-correction, parameter optimization, and postprocessing can be reasonably applied here to tackle the fractional pixel distortion seen at lower B 1 values. 25,[27][28][29] Although for some applications FREE may be limited by the linearity of B 1 (r), this limitation might be overcome in the future. Currently, to maximize reconstruction quality, RF coils producing a nearly linear B 1 gradient should be used.…”

Section: Discussionmentioning

confidence: 99%

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

Torres

Froelich

Wang

et al. 2021

Magnetic Resonance in Med

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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.

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Section: Discussionmentioning

confidence: 99%

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

Torres

Froelich

Wang

et al. 2021

Magnetic Resonance in Med

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“…Therefore, TRASE removes the need for the entire B0 gradient system and, additionally, relaxes the main magnet homogeneity requirement, potentially increasing the compact and portable nature of current low-cost LF MRI systems. Recent developments of TRASE hardware enabled sub-millimeter 1D resolution (0.33 mm/pixel) in ~ 8.5 minutes using a conventional 0.2 T bi-planar permanent magnet with a twisted-solenoid RF coil set [33,34], a custom-made multi-channel RF amplifier [35], improved image reconstruction techniques [36][37][38] and a custom MRI console [39]. As seen above, the development of MRI technology enables new magnet designs.…”

Section: Introductionmentioning

confidence: 99%

A Short and Light, Sparse Dipolar Halbach Magnet for MRI

Purchase

Vidarsson²,

Wachowicz

et al. 2021

IEEE Access

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Recently designed dipolar Halbach magnets used in portable MRI systems are much lighter and more compact than standard permanent or superconductive magnets. However, improved designs and manufacturing techniques aiming at lower weight and smaller external size are an area of continual interest especially for application to space flight. Most Halbach magnet design techniques aim to optimize homogeneity suitable for MRI over a diameter-spherical volume (DSV) that requires the aspect ratio (length/inner diameter) to be larger than 1.5:1. Furthermore, current magnet construction techniques often use low-coercivity magnetic pieces and imperfect formers that produce a mismatch in the intended designs. As a result, Halbach magnets require complex shimming methods to improve the magnetic field homogeneity, causing further size and weight increase. Here, we propose to reduce the weight and the aspect ratio of the Halbach magnet by optimizing homogeneity over a cylindrical region of interest (ROI) rather than a DSV, applying a genetic algorithm, high-coercivity ferromagnets (N40UH) and a robust construction technique. The assembled 67 mT magnet, with aspect ratio ~ 1:1, produces almost identical homogeneity (11152 ppm) as simulations (11451 ppm) within a 12.7 cm diameter, 1 cm long cylinder ROI. The magnet structure was 3D printed ring-by-ring and assembled coaxially. The magnet can be disassembled for transportation.INDEX TERMS MRI, magnets, magnetic field homogeneity, permanent magnet array

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