Echo‐planar imaging of the human head with 100 mT/m gradients and high‐order modeling of eddy current fields

Hennel, Franciszek; Wilm, Bertram J.; Roesler, Manuela Barbara; Weiger, Markus; Dietrich, Benjamin E.; Pruessmann, Klaas P.

doi:10.1002/mrm.28168

Cited by 8 publications

(19 citation statements)

References 30 publications

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“…The magnitude of the phase variation during the readout was around 10 radians within the imaging volume. Related eddy currents with time constants of a few milliseconds or less were previously shown to result in ghosting artifacts in EPI 27 . In the case spiral imaging, the related artifacts appear as blurring, which may seem less critical at first sight (Figure 5A).…”

Section: Discussionmentioning

confidence: 83%

“…From the combined data, a fifth‐order spherical harmonic model was fitted. This has previously been reported to sufficiently describe eddy currents generated by this gradient coil to allow for EPI imaging 27 . The choice of spherical harmonics as basic functions to describe magnetic field dynamics is motivated by magnetostatics.…”

Section: Methodsmentioning

confidence: 99%

“…This has previously been reported to sufficiently describe eddy currents generated by this gradient coil to allow for EPI imaging. 27 The choice of spherical harmonics as basic functions to describe magnetic field dynamics is motivated by magnetostatics. However, the effect of the concomitant fields on the magnetic field magnitude cannot be fully described by this basis, and therefore, may be modeled separately.…”

Section: Encoding Model Eddy Currents and Concomitant Fieldsmentioning

confidence: 99%

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Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system

Wilm

Hennel

Roesler

et al. 2020

Magnetic Resonance in Med

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Purpose Diffusion weighted imaging (DWI) is commonly limited by low signal‐to‐noise ratio (SNR) as well as motion artifacts. To address this limitation, a method that allows to maximize the achievable signal yield and increase the resolution in motion robust single‐shot DWI is presented. Methods DWI was performed on a 3T scanner equipped with a recently developed gradient insert (gradient strength: 200 mT/m, slew rate: 600 T/m/s). To further shorten the echo time, Stejskal‐Tanner diffusion encoding with a single‐shot spiral readout was implemented. To allow non‐Cartesian image reconstruction using such strong and fast gradients, the characterization of eddy current and concomitant field effects was performed based on field‐camera measurements. Results An echo time of only 19 ms was achieved for a b‐factor of 1000 s/mm2. An in‐plane resolution of 0.68 mm was encoded by a single‐shot spiral readout of 40.5 ms using 3‐fold undersampling. The resulting images did not suffer from off‐resonance artifacts and T2∗ blurring that are common to single‐shot images acquired with regular gradient systems. Conclusion Spiral diffusion imaging using a head gradient system, together with an accurate characterization of the encoding process allows for a substantial reduction of the echo time, and improves the achievable resolution in motion‐insensitive single‐shot DWI.

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Section: Discussionmentioning

confidence: 83%

Section: Methodsmentioning

confidence: 99%

Section: Encoding Model Eddy Currents and Concomitant Fieldsmentioning

confidence: 99%

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Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system

Wilm

Hennel

Roesler

et al. 2020

Magnetic Resonance in Med

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“…To facilitate the investigation of fast BOLD fluctuations, it was then natural to combine MREG with simultaneous EEG, whose high temporal resolution would provide useful baseline recordings of cerebral activity. While simultaneous EEG-fMRI had been previously almost exclusively used with EPI, it was quickly established that fast acquisitions and spiral trajectories did not cause additional safety issues [40,73], nor prevent an effective removal of MR gradient artifacts from the EEG [74,75]. As a starting point, we focused on the default-mode network (DMN), which is associated with the brain's baseline state, giving it a central role as a hub of global brain activity [76].…”

Section: Resting-state Networkmentioning

confidence: 99%

“…Most recently other groups have been working on ultrafast spiral imaging. A repetition time of 200 ms at 2.8 mm isotropic resolution has been reported using a T-Hex singleshot spiral trajectory [39] and 3.5 ms/slice could be achieved with 2D-spiral imaging using a dedicated gradient coil [40].…”

Section: Introductionmentioning

confidence: 99%

15 Years MR-encephalography

Hennig

Kiviniemi

Riemenschneider

et al. 2020

Magn Reson Mater Phy

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Objective This review article gives an account of the development of the MR-encephalography (MREG) method, which started as a mere ‘Gedankenexperiment’ in 2005 and gradually developed into a method for ultrafast measurement of physiological activities in the brain. After going through different approaches covering k-space with radial, rosette, and concentric shell trajectories we have settled on a stack-of-spiral trajectory, which allows full brain coverage with (nominal) 3 mm isotropic resolution in 100 ms. The very high acceleration factor is facilitated by the near-isotropic k-space coverage, which allows high acceleration in all three spatial dimensions. Methods The methodological section covers the basic sequence design as well as recent advances in image reconstruction including the targeted reconstruction, which allows real-time feedback applications, and—most recently—the time-domain principal component reconstruction (tPCR), which applies a principal component analysis of the acquired time domain data as a sparsifying transformation to improve reconstruction speed as well as quality. Applications Although the BOLD-response is rather slow, the high speed acquisition of MREG allows separation of BOLD-effects from cardiac and breathing related pulsatility. The increased sensitivity enables direct detection of the dynamic variability of resting state networks as well as localization of single interictal events in epilepsy patients. A separate and highly intriguing application is aimed at the investigation of the glymphatic system by assessment of the spatiotemporal patterns of cardiac and breathing related pulsatility. Discussion MREG has been developed to push the speed limits of fMRI. Compared to multiband-EPI this allows considerably faster acquisition at the cost of reduced image quality and spatial resolution.

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Evaluating diffusion dispersion across an extended range of b‐values and frequencies: Exploiting gap‐filled OGSE shapes, strong gradients, and spiral readouts

Michael

Hennel

Pruessmann

2022

Magnetic Resonance in Med

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To address the long echo times and relatively weak diffusion sensitization that typically limit oscillating gradient spin-echo (OGSE) experiments, an OGSE implementation combining spiral readouts, gap-filled oscillating gradient shapes providing stronger diffusion encoding, and a high-performance gradient system is developed here and utilized to investigate the tradeoff between b-value and maximum OGSE frequency in measurements of diffusion dispersion (i.e., the frequency dependence of diffusivity) in the in vivo human brain. In addition, to assess the effects of the marginal flow sensitivity introduced by these OGSE waveforms, flow-compensated variants are devised for experimental comparison.Methods: Using DTI sequences, OGSE acquisitions were performed on three volunteers at b-values of 300, 500, and 1000 s/mm 2 and frequencies up to 125, 100, and 75 Hz, respectively; scans were performed for gap-filled oscillating gradient shapes with and without flow sensitivity. Pulsed gradient spin-echo DTI acquisitions were also performed at each b-value. Upon reconstruction, mean diffusivity (MD) maps and maps of the diffusion dispersion rate were computed. Results:The power law diffusion dispersion model was found to fit best to MD measurements acquired at b = 1000 s/mm 2 despite the associated reduction of the spectral range; this observation was consistent with Monte Carlo simulations.Furthermore, diffusion dispersion rates without flow sensitivity were slightly higher than flow-sensitive measurements. Conclusion:The presented OGSE implementation provided an improved depiction of diffusion dispersion and demonstrated the advantages of measuring dispersion at higher b-values rather than higher frequencies within the regimes employed in this study.

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Echo‐planar imaging of the human head with 100 mT/m gradients and high‐order modeling of eddy current fields

Cited by 8 publications

References 30 publications

Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system

Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system

15 Years MR-encephalography

Evaluating diffusion dispersion across an extended range of b‐values and frequencies: Exploiting gap‐filled OGSE shapes, strong gradients, and spiral readouts

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