Abstract
Fast ROtary Nonlinear Spatial ACquisition (FRONSAC) was recently introduced as a new strategy that applies nonlinear gradients as a small perturbation to improve image quality in highly undersampled MRI. In addition to experimentally showing the previously simulated improvement to image quality, this work introduces the insight that Cartesian-FRONSAC retains many desirable features of Cartesian imaging. Cartesian-FRONSAC preserves the existing linear gradient waveforms of the Cartesian sequence while adding oscillating nonlinear gradient waveforms. Experiments show that performance is essentially identical to Cartesian imaging in terms of (1) resilience to experimental imperfections, like timing errors or off-resonance spins, (2) accommodating scan geometry changes without the need for recalibration or additional field mapping, (3) contrast generation, as in turbo spin echo. Despite these similarities to Cartesian imaging, which provides poor parallel imaging performance, Cartesian-FRONSAC consistently shows reduced undersampling artifacts and better response to advanced reconstruction techniques. A final experiment shows that hardware requirements are also flexible. Cartesian-FRONSAC improves accelerated imaging while retaining the robustness and flexibility critical to real clinical use.
Highlights
MRI is one of the safest and most informative imaging modalities available to modern medicine, but its overall applicability is limited by long imaging times
In this work we demonstrate that Cartesian Fast ROtary Nonlinear Spatial ACquisition (FRONSAC), which is one particular instance of the FRONSAC approach where oscillating nonlinear gradients are added to a Cartesian sequence, provides excellent accelerated imaging, but it exhibits a number of features critical to real world clinical imaging
Artifacts are mitigated from the addition of FRONSAC waveforms, despite the fact that these waveforms have not been optimized for frequency, phase, or amplitude and use only 10% of the available gradient amplitude
Summary
MRI is one of the safest and most informative imaging modalities available to modern medicine, but its overall applicability is limited by long imaging times. Nonlinear gradient encoding samples a distribution of k-space rather than a single point, whether used alone or in combination with receiver arrays. NLG encodings create a sampling distribution in k-space that can be varied dynamically as the linear gradients sweep the sampling distribution across k-space (Supplementary Videos S4 and S5)[23]. NLGs in conjunction with receiver arrays generate sets of dynamic sampling distributions that are simultaneously sampled at each timepoint This additional degree of freedom can be used to design trajectories that more efficiently measure the gaps created by the “skipped” parts of k-space, which are sampled only by the wings of these distributions. The linear gradients of a traditional trajectory translate these distributions across k-space until the entire space is sufficiently sampled
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