Real-time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T

Purpose To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction. Methods Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, Delta B1-, (3) motion-induced static magnetic field inhomogeneities, Delta B-0, and (4) spatially varying transmit RF field fluctuations, Delta B1+. Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing Delta B1- was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective increment B-0 and -Delta B1+ corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTRasym). Results Dynamic Delta B1- correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTRasym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Delta omega = 1, 2, 3 +/- 0.25 ppm. The additional application of Delta B1- correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTRasym values were highly consistent between measurements with and without intended head rotations. Conclusion Temporal Delta B1- cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective Delta B1- correction significantly reduced motion-related artifacts on CEST-MRI.Peer reviewe

Differential functional benefits of ultra highfield MR systems within the language network

Several investigations have shown limitations of fMRI reliability with the current standard field strengths. Improvement is expected from ultra highfield systems but studies on possible benefits for cognitive networks are lacking. Here we provide an initial investigation on a prominent and clinically highly-relevant cognitive function: language processing in individual brains. 26 patients evaluated for presurgical language localization were investigated with a standardized overt language fMRI paradigm on both 3T and 7T MR scanners. During data acquisition and analysis we made particular efforts to minimize effects not related to static magnetic field strength differences. Six measures relevant for functional activation showed a large dissociation between essential language network nodes: although in Wernicke's area 5/6 measures indicated a benefit of ultra highfield, in Broca's area no comparison was significant. The most important reason for this discrepancy was identified as being an increase in susceptibility-related artifacts in inferior frontal brain areas at ultra high field. We conclude that functional UHF benefits are evident, however these depend crucially on the brain region investigated and the ability to control local artifacts

TEM and micromagnetic study of FePt ordered/disordered exchange spring media

Zsfassung in dt. SpracheIm Rahmen der folgenden Arbeit wurden "exchange-spring" Medien untersucht. Dies sind Materialen in welchen hart- und weichmagnetische Schichten stark miteinander austauschwechselwirken.Als Beispiel für ein solches Medium wurde eine Nanostruktur aus FePt L1_0/A1 Dünnenschichten analysiert. Diese Untersuchung ergab, dass sich zwischen der chemisch geordneten L1_0-Phase und der ungeordneten A1-Phase eine komplexe und raue Grenzfläche ausbildet. Aus den gemessenen Hysteresiskurven und der HRTEM Analyse lässt sich zudem schlußfolgern, dass die Rauigkeit der Grenzfläche einen Einfluss auf die Schalteigenschaften des Materials hat. Um herauszufinden, wie solch eine Grenzfläche das Ummagnetisierungsverhalten des Mediums beeinflusst, wurden mikromagnetische Simulationen durchgeführt. Für die Simulationen wurden markante Merkmale, so wie Obstakel der einen Phase eingebettet in der anderen Phase, übertragen in Finite Element Modelle.Zusätzlich wurde die Verteilungsfunktion der hartmagnetische Phase entlang der Probehöhe untersucht. Numerische Berechnungen ergaben, dass Position und Größe der Obstakeln einem entscheidenden Einfluss auf das Schaltverhalten des Materials haben. Je nach den Herstellungsparametern der Probe ließ sich die Funktion linear, logistisch oder quadratisch fitten. Für eine quadratische L1_0-Phase in einer 18nm breiten Grenzfläche ergaben die Berechnung als kleinstes mögliches Schaltfeld H_sw =0.9T.Die numerischen Resultate stimmen gut überein mit einer analytischen Formel für das Verankerungsfeld, welche hergeleitet wurde für den allgemeinen Fall räumlich veränderlicher Magnetisierung, Anisotropie und Austauschkonstante. Weiterführende Arbeit könnte sich mit der Wechselwirkung zwischen magnetischen Körnen sowie der Simulation von Lese-/Schreibprozessen in Bit-gemusterten Meterialen befassen. Abschließend ist hervorzuheben, dass sich die Resultate dieser Arbeit nicht nur auf FePt L1_0/A1 Dünnenschichten auwenden lassen, sondern auch interessant sein könnten für andere exchange-spring Materialen, z.B. CoPt L1_0/A1 oder FePt(L1_0)/FeAu(L1_0).In this work exchange spring media are discussed. Within such materials there exist magnetically hard and soft layers which are strongly exchange coupled. As an example a nanostructure of FePt L1_0/A1 thin films is analysed with electron microscopy. A complex rough interphase between the chemically ordered L1_0 and disordered A1 phase was discovered. This observation together with the experimental analysis of hysteresis loops lead to the conclusion, that a change of the interphase profile roughness by a few nanometers alters significantly the switching properties of FePt L1_0/A1 media. In order to find the physical processes responsible for this behaviour, micromagnetic simulations have been carried out. For the purpose of the simulations, observed features, such as obstacles of one phase embedded in the matrix of a second phase, were transferred into finite element models.Numerical calculations show that the position and size of the obstacles significantly alters the switching field of the media. Also the experimental distribution of the hard phase along the thickness of the medium was analysed. It was found to fit linear, logistic or quadratic functions in the interphase region, depending on the sample deposition conditions. The minimal switching field of H_sw =0.9T was calculated for a quadratic L1_0 distribution in a 18nm thick interphase region. The above mentioned numerical results agree well with an analytical formula for the pinning field of the exchange spring media, which is derived for the general case of spatially varying magnetization, anisotropy and exchange constant.In future work the analysis could be extended to a study of an interaction between magnetic grains and simulation of a read/write processes for a fully formulated FePt bit patterned media. It is emphasized that the applicability of the results is not limited to FePt L1_0/A1 films but could be interesting also for other exchange spring media, such as CoPt L1_0/A1 or FePt(L1_0)/FeAu(L1_0).6

An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

Phase imaging benefits from strong susceptibility effects at very high field and the high signal-to-noise ratio (SNR) afforded by multi-channel coils. Combining the information from coils is not trivial, however, as the phase that originates in local field effects (the source of interesting contrast) is modified by the inhomogeneous sensitivity of each coil. This has historically been addressed by referencing individual coil sensitivities to that of a volume coil, but alternative approaches are required for ultra-high field systems in which no such coil is available. An additional challenge in phase imaging is that the phase that develops up to the echo time is " wrapped" into a range of 2p radians. Phase wraps need to be removed in order to reveal the underlying phase distribution of interest. Beginning with a coil combination using a homogeneous reference volume coil -the Roemer approach -which can be applied at 3 T and lower field strengths, we review alternative methods for combining single-echo and multi-echo phase images where no such reference coil is available. These are applied to high-resolution data acquired at 7 T and their effectiveness assessed via an index of agreement between phase values over channels and the contrast-to-noise ratio in combined images. The virtual receiver coil and COMPOSER approaches were both found to be computationally efficient and effective. The main features of spatial and temporal phase unwrapping methods are reviewed, placing particular emphasis on recent developments in temporal phase unwrapping and Laplacian approaches. The features and performance of these are illustrated in application to simulated and high-resolution in vivo data. Temporal unwrapping was the fastest of the methods tested and the Laplacian the most robust in images with low SNR. (C) 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd

A method for the dynamic correction of B0-related distortions in single-echo EPI at 7T

We propose a method to calculate field maps from the phase of each EPI in an fMRI time series. These field maps can be used to correct the corresponding magnitude images for distortion caused by inhomogeneity in the static magnetic field. In contrast to conventional static distortion correction, in which one 'snapshot' field map is applied to all subsequent fMRI time points, our method also captures dynamic changes to B-0 which arise due to motion and respiration. The approach is based on the assumption that the non-B-0-related contribution to the phase measured by each radio-frequency coil, which is dominated by the coil sensitivity, is stable over time and can therefore be removed to yield a field map from EPI. Our solution addresses imaging with multi-channel coils at ultra-high field (7T), where phase offsets vary rapidly in space, phase processing is non-trivial and distortions are comparatively large. We propose using dual-echo gradient echo reference scan for the phase offset calculation, which yields estimates with high signal-to-noise ratio. An extrapolation method is proposed which yields reliable estimates for phase offsets even where motion is large and a tailored phase unwrapping procedure for EPI is suggested which gives robust results in regions with disconnected tissue or strong signal decay. Phase offsets are shown to be stable during long measurements (40min) and for large head motions. The dynamic distortion correction proposed here is found to work accurately in the presence of large motion (up to 8.1 degrees), whereas a conventional method based on single field map fails to correct or even introduces distortions (up to 11.2mm). Finally, we show that dynamic unwarping increases the temporal stability of EPI in the presence of motion. Our approach can be applied to any EPI measurements without the need for sequence modification. (C) 2018 The Authors. Published by Elsevier Inc

Computationally efficient combination of multi-channel phase data from multi-echo acquisitions (ASPIRE)

Purpose: To develop a simple method for combining multi-echo phase information from a number of coils in an array that requires no volume coil or additional scans and yields signal-to-noise ratio-optimal images that reflect only ΔB -related phase. Theory and Methods: Two SNR optimal coil combination methods were developed which retrieve the ΔB -related phase by determining the coil-dependent phase offsets. The first variant, MCPC-3D-S, requires the unwrapping of one phase image; the second variant, ASPIRE, allows unwrapping to be avoided if two echoes j and k satisfy the echo time relation m.T =(m+1).T , where m is an integer, making this a particularly fast and robust approach. Both developed methods constitute improvements over a prior method, MCPC-3D, in terms of robustness and computational expense. Results: In the brain at 7 T, phase matching and contrast-to-noise ratio were higher with MCPC-3D-S and ASPIRE than with phase difference reconstruction, and similar to the reference coil–dependent Roemer combination. Unlike the Roemer and virtual reference coil methods, the proposed approaches also eliminated all non–ΔB -related phase. Conclusion: MCPC-3D-S is an improvement over prior multi-echo methods, which is useful if the ASPIRE echo time condition cannot be fulfilled. ASPIRE is a particularly fast and robust approach that runs on the scanner's reconstructor in a small fraction of the acquisition time. Magn Reson Med 79:2996–3006, 2018