Iterative static ∆B0 field map estimation for off-resonance correction in non-Cartesian susceptibility weighted imaging

International audiencePatient-induced inhomogeneities in the magnetic field cause distortions and blurring during acquisitions with long readouts such as in susceptibility-weighted imaging (SWI). Most correction methods require collecting an additional ∆B 0 field map to remove these artifacts. The static ∆B0 field map can be approximated with an acceptable error directly from a single echo acquisition in SWI. The main component of the observed phase is linearly related to ∆B0 and the echo time TE , and the relative impact of non-∆B0 terms becomes insignificant with TE > 20ms at 3T for a well-tuned system. The main step is to combine and unfold the multi-channel phase maps wrapped many times, and several competing algorithms are compared for this purpose. Four in vivo brain data sets collected using the recently proposed 3D SPARKLING readouts are used to assess the proposed method. The estimated 3D field maps generated with a 0.6 mm isotropic spatial resolution provide overall similar off-resonance corrections compared to reference corrections based on an external ∆B0 acquisitions, and even improved for two out of four individuals. Although a small estimation error is expected, no aftermath was observed in the proposed corrections, while degradations were observed in the references. A static ∆B0 field map estimation method was proposed to take advantage of acquisitions with long echo times, and outperformed the reference technique based on an external field map. The difference can be attributed to an inherent robustness to mismatches between volumes and external ∆B0 maps, and diverse other sources investigated

Optimizing full 3D SPARKLING trajectories for high-resolution T2*-weighted Magnetic Resonance Imaging

International audienceThe Spreading Projection Algorithm for Rapid K-space samplING, or SPARKLING, is an optimization-driven method that has been recently introduced for accelerated 2D T2*-w MRI using compressed sensing. It has then been extended to address 3D imaging using either stacks of 2D sampling patterns or a local 3D strategy that optimizes a single sampling trajectory at a time. 2D SPARKLING actually performs variable density sampling (VDS) along a prescribed target density while maximizing sampling efficiency and meeting the gradient-based hardware constraints. However, 3D SPARKLING has remained limited in terms of acceleration factors along the third dimension if one wants to preserve a peaky point spread function (PSF) and thus good image quality.In this paper, in order to achieve higher acceleration factors in 3D imaging while preserving image quality, we propose a new efficient algorithm that performs optimization on full 3D SPARKLING. The proposed implementation based on fast multipole methods (FMM) allows us to design sampling patterns with up to 10^7 k-space samples, thus opening the door to 3D VDS. We compare multi-CPU and GPU implementations and demonstrate that the latter is optimal for 3D imaging in the high-resolution acquisition regime (600µm isotropic). Finally, we show that this novel optimization for full 3D SPARKLING outperforms stacking strategies or 3D twisted projection imaging through retrospective and prospective studies on NIST phantom and in vivo brain scans at 3 Tesla. Overall the proposed method allows for 2.5-3.75x shorter scan times compared to GRAPPA-4 parallel imaging acquisition at 3 Tesla without compromising image quality

Correction d'inhomogénéités de champs pour la SWI non-cartésienne par estimation des cartes de champs

International audiencePatient-induced inhomogeneities in the magnetic field cause distortions and blurring during acquisitions with long echo times, as in susceptibility-weighted imaging. Most correction methods require collecting an additional ΔB0 field map. To avoid that, we propose a method to approximate this field map using the single echo acquisition only. The main component of the observed phase is linearly related to ΔB0 and TE, and the relative impact of non-ΔB0 terms becomes insignificant with TE>20ms at 3T. The estimated 3D field maps, produced at 0.6 mm isotropic under 3 minutes, provide equivalent corrections to acquired ones.Les inhomogénéités de champs induites par les patients sont à l'origine de distorsions et de floutages durant les acquisitions à temps d'écho longs, comme pour l'imagerie pondérée en susceptibilité. La plupart des méthodes de correction nécessitent d'acquérir une carte de champ ΔB0 additionnelle. Pour éviter cela, nous proposons une méthode pour approximer cette carte de champs en utilisant seulement l'acquisition à écho unique. La composante principale de la phase observée est linéairement liée au ΔB0 et au TE, et l'impact relatif des termes indépendants du ΔB0 deviennent négligeables pour TE>20ms à 3T. Les cartes 3D estimées, produites à 0.6 mm isotrope en moins de 3 minutes, permettent d'obtenir une correction équivalente aux cartes acquises

Improving spreading projection algorithm for rapid k‐space sampling trajectories through minimized off‐resonance effects and gridding of low frequencies

International audiencePurpose Non‐Cartesian MRI with long arbitrary readout directions are susceptible to off‐resonance artifacts due to patient induced inhomogeneities. This results in degraded image quality with strong signal losses and blurring. Current solutions to address this issue involve correcting the off‐resonance artifacts during image reconstruction or reducing inhomogeneities through improved shimming. Theory The recently developed SPARKLING algorithm is extended to drastically diminish off‐resonance artifacts by generating temporally smooth k‐space sampling patterns. For doing so, the cost function which is optimized in SPARKLING is modified using a temporal weighting factor. Additionally, oversampling of the center of k‐space beyond the Nyquist criteria is prevented through the use of gridded sampling in the region, enforced with affine constraints. Methods Prospective k‐space data was acquired at 3 T on new trajectories, and we show robustness to inhomogeneities through in silico experiments by adding through artificial degradation of system shimming. Later on, in vivo experiments were carried out to optimize parameters of the new improvements and benchmark the gain in performance. Results The improved trajectories allowed for the recovery of signal dropouts observed on original SPARKLING acquisitions at larger field inhomogeneities. Furthermore, imposing gridded sampling at the center of k‐space provided improved reconstructed image quality with limited artifacts. Conclusion These advancements allowed us for nearly shorter scan time compared to GRAPPA‐p4x1, allowing us to reach 600 µm isotropic resolution in 3D ‐w imaging in just 3.3 min at 3 T with negligible degradation in image quality

Iterative static Δ\DeltaB0 field map estimation for off-resonance correction in non-Cartesian susceptibility weighted imaging

International audiencePatient-induced inhomogeneities in the magnetic field cause distortions and blurring during acquisitions with long readouts such as in susceptibility-weighted imaging (SWI). Most correction methods require collecting an additional ∆B 0 field map to remove these artifacts. The static ∆B0 field map can be approximated with an acceptable error directly from a single echo acquisition in SWI. The main component of the observed phase is linearly related to ∆B0 and the echo time TE , and the relative impact of non-∆B0 terms becomes insignificant with TE > 20ms at 3T for a well-tuned system. The main step is to combine and unfold the multi-channel phase maps wrapped many times, and several competing algorithms are compared for this purpose. Four in vivo brain data sets collected using the recently proposed 3D SPARKLING readouts are used to assess the proposed method. The estimated 3D field maps generated with a 0.6 mm isotropic spatial resolution provide overall similar off-resonance corrections compared to reference corrections based on an external ∆B0 acquisitions, and even improved for two out of four individuals. Although a small estimation error is expected, no aftermath was observed in the proposed corrections, while degradations were observed in the references. A static ∆B0 field map estimation method was proposed to take advantage of acquisitions with long echo times, and outperformed the reference technique based on an external field map. The difference can be attributed to an inherent robustness to mismatches between volumes and external ∆B0 maps, and diverse other sources investigated

Optimizing full 3D SPARKLING trajectories for high-resolution T2*-weighted Magnetic Resonance Imaging

The Spreading Projection Algorithm for Rapid K-space samplING, or SPARKLING, is an optimization-driven method that has been recently introduced for accelerated 2D T2*-w MRI using compressed sensing. It has then been extended to address 3D imaging using either stacks of 2D sampling patterns or a local 3D strategy that optimizes a single sampling trajectory at a time. 2D SPARKLING actually performs variable density sampling (VDS) along a prescribed target density while maximizing sampling efficiency and meeting the gradient-based hardware constraints. However, 3D SPARKLING has remained limited in terms of acceleration factors along the third dimension if one wants to preserve a peaky point spread function (PSF) and thus good image quality.In this paper, in order to achieve higher acceleration factors in 3D imaging while preserving image quality, we propose a new efficient algorithm that performs optimization on full 3D SPARKLING. The proposed implementation based on fast multipole methods (FMM) allows us to design sampling patterns with up to 10^7 k-space samples, thus opening the door to 3D VDS. We compare multi-CPU and GPU implementations and demonstrate that the latter is optimal for 3D imaging in the high-resolution acquisition regime (600µm isotropic). Finally, we show that this novel optimization for full 3D SPARKLING outperforms stacking strategies or 3D twisted projection imaging through retrospective and prospective studies on NIST phantom and in vivo brain scans at 3 Tesla. Overall the proposed method allows for 2.5-3.75x shorter scan times compared to GRAPPA-4 parallel imaging acquisition at 3 Tesla without compromising image quality

Iterative static ∆B0 field map estimation for off-resonance correction in non-Cartesian susceptibility weighted imaging

International audiencePatient-induced inhomogeneities in the magnetic field cause distortions and blurring during acquisitions with long readouts such as in susceptibility-weighted imaging (SWI). Most correction methods require collecting an additional ∆B 0 field map to remove these artifacts. The static ∆B0 field map can be approximated with an acceptable error directly from a single echo acquisition in SWI. The main component of the observed phase is linearly related to ∆B0 and the echo time TE , and the relative impact of non-∆B0 terms becomes insignificant with TE > 20ms at 3T for a well-tuned system. The main step is to combine and unfold the multi-channel phase maps wrapped many times, and several competing algorithms are compared for this purpose. Four in vivo brain data sets collected using the recently proposed 3D SPARKLING readouts are used to assess the proposed method. The estimated 3D field maps generated with a 0.6 mm isotropic spatial resolution provide overall similar off-resonance corrections compared to reference corrections based on an external ∆B0 acquisitions, and even improved for two out of four individuals. Although a small estimation error is expected, no aftermath was observed in the proposed corrections, while degradations were observed in the references. A static ∆B0 field map estimation method was proposed to take advantage of acquisitions with long echo times, and outperformed the reference technique based on an external field map. The difference can be attributed to an inherent robustness to mismatches between volumes and external ∆B0 maps, and diverse other sources investigated

MORE-SPARKLING: non-cartesian trajectories with minimized off-resonance effects

International audienceWe augment the recently introduced SPARKLING algorithm and propose an improved mathematical formulation that takes the temporal dependence of the MR signal into account. This prevents the trajectories from sampling similar portions of k-space at different times, thereby reducing distortions and blurring induced by $B_0$ inhomogenieties. Overall, these trajectories present a smooth distribution over time in k-space and Minimized Off-Resonance Effects (MORE-SPARKLING), verified both retrospectively and prospectively with scans performed in vivo at 3T on a healthy volunteer

3D-SPARKLING for functional MRI: A pilot study for retinotopic mapping at 7T

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