PyMR: a framework for programming magnetic resonance systems

DelaBarre

et al. 2023

Purpose To expand on the previously developed B1+$$ {\mathrm{B}}_1^{+} $$‐encoding technique, frequency‐modulated Rabi‐encoded echoes (FREE), to perform accelerated image acquisition by collecting multiple lines of k‐space in an echo train. Methods FREE uses adiabatic full‐passage pulses and a spatially varying RF field to encode unique spatial information without the use of traditional B0 gradients. The original implementation relied on acquiring single lines of k‐space, leading to long acquisitions. In this work, an acceleration scheme is presented in which multiple echoes are acquired in a single shot, analogous to conventional fast spin‐echo sequences. Theoretical analysis and computer simulations investigated the feasibility of this approach and presented a framework to analyze important imaging parameters of FREE‐based sequences. Experimentally, the multi‐echo approach was compared with conventional phase‐encoded images of the human visual cortex using a simple surface transceiver coil. Finally, different contrasts demonstrated the clinical versatility of the new accelerated sequence. Results Images were acquired with an acceleration factor of 3.9, compared with the previous implementation of FREE, without exceeding specific absorption rate limits. Different contrasts can easily be acquired without major modifications, including inversion recovery–type images. Conclusion FREE initially illustrated the feasibility of performing slice‐selective 2D imaging of the human brain without the need for a B0 gradient along the y‐direction. The multi‐echo version maintains the advantages that B1+$$ {\mathrm{B}}_1^{+} $$ encoding provides but represents an important step toward improving the clinical feasibility of such sequences. Additional acceleration and more advanced reconstruction techniques could further improve the clinical viability of FREE‐based techniques.

Section: Methodsmentioning

confidence: 99%

Section: In Vivo Validation Of the Me-free Sequencementioning

confidence: 99%

Fast spin‐echo approach for accelerated B₁ gradient–based MRI

DelaBarre

et al. 2023

“…Phantom and in vivo human brain images were acquired as previously described by Torres et al 10 using a CIERMag digital MR spectrometer (DMRS) [19][20][21] interfaced to a 1.5T, 90-cm magnet (Oxford Magnet Technology) and a clinical gradient system (model SC72; Siemens). The magnet was designed for operation at 4T, but was ramped down to 1.5T without passive shim adjustment or higher order shimming, resulting in a relatively inhomogeneous B 0 .…”

Section: Methodsmentioning

confidence: 99%

Correcting image distortions from a nonlinear B1+$$ {\boldsymbol{B}}_{\mathbf{1}}^{+} $$‐gradient field in frequency‐modulated Rabi‐encoded echoes

Torres

et al. 2022

To correct image distortions that result from nonlinear spatial variation in the transmit RF field amplitude (B + 1 ) when performing spatial encoding with the method called frequency-modulated Rabi encoded echoes (FREE).Theory and Methods: An algorithm developed to correct image distortion resulting from the use of nonlinear static field (B 0 ) gradients in standard MRI is adapted herein to correct image distortion arising from a nonlinear B + 1 -gradient field in FREE. From a B + 1 -map, the algorithm performs linear interpolation and intensity scaling to correct the image. The quality of the distortion correction is evaluated in 1.5T images of a grid phantom and human occipital lobe.Results: An expanded theoretical description of FREE revealed the symmetry between this B + 1 -gradient field spatial-encoding and standard B 0 -gradient field spatial-encoding. The adapted distortion-correction algorithm substantially reduced image distortions arising in the spatial dimension that was encoded by the nonlinear B + 1 gradient of a circular surface coil. Conclusion: Image processing based on straightforward linear interpolation and intensity scaling, as previously applied in conventional MRI, can effectively reduce distortions in FREE images acquired with nonlinear B + 1 -gradient fields.

“…Each Tx channel is capable of performing AM modulation in a 4‐quadrants approach, with amplitude dynamics of 16 bits, along with FM (24 bits‐0.373 Hz resolution) and phase‐modulated (14 bits‐0.088° resolution) capabilities. This digital MR spectrometer was interfaced to a 1.5 T, 90‐cm magnet (Oxford Magnet Technology, Oxfordshire, UK) with a clinical gradient system (model SC72, Siemens, Erlangen, Germany), using the software created and controlled by Python MR framework, using Python v3.8 19‐22 , a specialized integrated development environment for creating MR sequences, 19,22 and Tomografo de Ressonância Magnética‐Console 19,23 . The magnet was designed to operate at 4 T but was ramped down to 1.5 T without adjusting the passive shims, leaving a relatively nonuniform B 0 .…”

Section: Methodsmentioning

confidence: 99%

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

Torres

et al. 2021

Self Cite

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.