Nichtinvasive Magnetresonanz-Perfusionsmessung des Gehirns mittelsMagnetischer Blutbolusmarkierung(Spin-Labeling)

Die magnetische Blutbolusmarkierung (Spin-Labeling) ermöglicht die nichtinvasive quantitative Messung des Blutflusses im Gewebe. Beim Spin-Labeling wird arterielles Blut durch Radiofrequenzpulse magnetisch markiert und der Transport der Markierung MR-tomographisch gemessen. Am Modell einer unter physiologischen Bedingungen perfundierten extrakorporalen Schweineniere konnte die Quantifizierbarkeit der Messmethode nachgewiesen werden. In einer Studie an 36 Hirntumorpatienten wurde das Verfahren mit der kontrastmittelbasierten First-Pass-Bolus-Methode zur nicht-quantitativen Perfusionsmessung verglichen. Es zeigte sich eine sehr gute Übereinstimmung zwischen beiden Methoden, der lineare Korrelationskoeffizient des relativen Blutflusses in der Tumorregion lag bei R=0,83. Die mittels Spin-Labeling ermittelten Absolutwerte des Blutflusses spielen bei der Beurteilung des Tumorgrades eine untergeordnete Rolle, da die mittlere Perfusion individuell sehr verschieden ist. Ein zweiter Anwendungsbereich für das Spin-Labeling ist die Darstellung großer Arterien. Spin-Labeling ermöglicht die nichtinvasive dynamische Angiographie (Dynamische Spin-Labeling-Angiographie - DSLA). Analog zur digitalen Subtraktionsangiographie kann damit der Einstromvorgang des Blutes in den Gefäßbaum zeitaufgelöst gemessen werden, jedoch mit wesentlich höherer zeitlicher Auflösung und frei wählbarer Projektionsrichtung. In einer Studie an 18 Patienten mit einseitigen Carotisstenosen wurden die Zeitdifferenzen der Anflutung der zerebralen Gefäße zwischen der betroffenen und der nicht stenosierten Seite bestimmt. Die im Carotis-Siphon gemessenen Zeitdifferenzen korrelieren signifikant mit dem Stenosegrad, steigen aber erst ab einer Lumeneinengung oberhalb von 80 Prozent deutlich an. Im Vergleich zu den etablierten Methoden werden die Möglichkeiten und Grenzen der DSLA dargestellt.Arterial spin labeling methods allow to determine quantitative tissue blood flow values noninvasively. Arterial blood is labelled by an inversion pulse and the distribution of this intrinsic tracer is measured using magnetic resonance imaging. Experiments using an extra corporal in-vitro porcine kidney in a MR compatible set-up were carried out to determine the accuracy of blood flow values calculated from arterial spin labeling measurements. In a study of 36 brain tumor patients, spin labeling was compared to non-quantitative contrast-enhanced dynamic susceptibility-weighted perfusion imaging. Relative blood flow values determined with both methods were in good agreement, the linear regression coefficient in the tumor region was R=0.83. Due to the variable individual perfusion state, quantitative blood flow values determined using spin labeling play a minor role in the assessment of tumor grade. Application of spin labeling to angiography of major arteries was investigated. Dynamic spin labeling angiography (DSLA) sequences were implemented and tested on a clinical scanner. This technique allows time-resolved depiction of blood flow in large vessels with very high temporal resolution. As opposed to digital subtraction angiography, the method allows arbitrary projection directions. In a study, 18 patients with one-sided carotid stenoses were examined. In these patients the time differences of blood bolus arrival at both hemispheres were determined. Time differences measured in the carotid siphon show a significant correlation with the degree of stenosis. However, a clear increase is not seen until 80% narrowing of a carotid. Possibilities and limitations of the DSLA method are discussed in comparison to established techniques

Real‐time MR elastography for viscoelasticity quantification in skeletal muscle during dynamic exercises

Purpose: To develop and test real-time MR elastography for viscoelastic parameter quantification in skeletal muscle during dynamic exercises. Methods: In 15 healthy participants, 6 groups of lower-leg muscles (tibialis anterior, tibialis posterior, peroneus, extensor digitorum longus, soleus, gastrocnemius) were investigated by real-time MR elastography using a single-shot, steady-state spiral gradient-echo pulse sequence and stroboscopic undersampling of harmonic vibrations at 40 Hz frequency. One hundred and eighty consecutive maps of shear-wave speed and loss angle (φ) covering 30.6 s of total acquisition time at 5.9-Hz frame rate were reconstructed from 360 wave images encoding 2 in-plane wave components in an interleaved manner. The experiment was carried out twice to investigate 2 exercises-isometric plantar flexion and isometric dorsiflexion-each performed over 10 s between 2 resting periods. Results: Activation of lower-extremity muscles was associated with increasing viscoelastic parameters shear-wave speed and phi, both reflecting properties related to the transverse direction relative to fiber orientation. Major viscoelastic changes were observed in soleus muscle during plantar flexion (shear-wave speed: 20.0% ± 3.6%, φ: 41.3% ± 12.0%) and in the tibialis anterior muscle during dorsiflexion (41.8% ± 10.2%, φ: 27.9% ± 2.8%; all P < .0001). Two of the muscles analyzed were significantly activated by plantar flexion and 4 by dorsiflexion based on shear-wave speed, whereas φ changed significantly in 5 muscles during both exercises. Conclusion: Real-time MR elastography allows mapping of dynamic, nonperiodic viscoelasticity changes in soft tissues such as voluntary muscle with high spatial and temporal resolution. Real-time MR elastography thus opens new horizons for the in vivo study of physiological processes in soft tissues toward functional elastography

Reduction of breathing artifacts in multifrequency magnetic resonance elastography of the abdomen

Purpose: With abdominal magnetic resonance elastography (MRE) often suffering from breathing artifacts, it is recommended to perform MRE during breath-hold. However, breath-hold acquisition prohibits extended multifrequency MRE examinations and yields inconsistent results when patients cannot hold their breath. The purpose of this work was to analyze free-breathing strategies in multifrequency MRE of abdominal organs. Methods: Abdominal MRE with 30, 40, 50, and 60 Hz vibration frequencies and single-shot, multislice, full wave-field acquisition was performed four times in 11 healthy volunteers: once with multiple breath-holds and three times during free breathing with ungated, gated, and navigated slice adjustment. Shear wave speed maps were generated by tomoelastography inversion. Image registration was applied for correction of intrascan misregistration of image slices. Sharpness of features was quantified by the variance of the Laplacian. Results: Total scan times ranged from 120 seconds for ungated free-breathing MRE to 376 seconds for breath-hold examinations. As expected, free-breathing MRE resulted in larger organ displacements (liver, 4.7 ± 1.5 mm; kidneys, 2.4 ± 2.2 mm; spleen, 3.1 ± 2.4 mm; pancreas, 3.4 ± 1.4 mm) than breath-hold MRE (liver, 0.7 ± 0.2 mm; kidneys, 0.4 ± 0.2 mm; spleen, 0.5 ± 0.2 mm; pancreas, 0.7 ± 0.5 mm). Nonetheless, breathing-related displacement did not affect mean shear wave speed, which was consistent across all protocols (liver, 1.43 ± 0.07 m/s; kidneys, 2.35 ± 0.21 m/s; spleen, 2.02 ± 0.15 m/s; pancreas, 1.39 ± 0.15 m/s). Image registration before inversion improved the quality of free-breathing examinations, yielding no differences in image sharpness to uncorrected breath-hold MRE in most organs (P > .05). Conclusion: Overall, multifrequency MRE is robust to breathing when considering whole-organ values. Respiration-related blurring can readily be corrected using image registration. Consequently, ungated free-breathing MRE combined with image registration is recommended for multifrequency MRE of abdominal organs

The Solar Particle Acceleration Radiation and Kinetics (SPARK) Mission Concept

© 2023by the authors. Licensee MDPI, Basel, Switzerland. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/Particle acceleration is a fundamental process arising in many astrophysical objects, including active galactic nuclei, black holes, neutron stars, gamma-ray bursts, accretion disks, solar and stellar coronae, and planetary magnetospheres. Its ubiquity means energetic particles permeate the Universe and influence the conditions for the emergence and continuation of life. In our solar system, the Sun is the most energetic particle accelerator, and its proximity makes it a unique laboratory in which to explore astrophysical particle acceleration. However, despite its importance, the physics underlying solar particle acceleration remain poorly understood. The SPARK mission will reveal new discoveries about particle acceleration through a uniquely powerful and complete combination of γ-ray, X-ray, and EUV imaging and spectroscopy at high spectral, spatial, and temporal resolutions. SPARK’s instruments will provide a step change in observational capability, enabling fundamental breakthroughs in our understanding of solar particle acceleration and the phenomena associated with it, such as the evolution of solar eruptive events. By providing essential diagnostics of the processes that drive the onset and evolution of solar flares and coronal mass ejections, SPARK will elucidate the underlying physics of space weather events that can damage satellites and power grids, disrupt telecommunications and GPS navigation, and endanger astronauts in space. The prediction of such events and the mitigation of their potential impacts are crucial in protecting our terrestrial and space-based infrastructure.Peer reviewe

The Large Imaging Spectrometer for Solar Accelerated Nuclei (LISSAN): A next-generation solar γ-ray spectroscopic imaging instrument concept

Models of particle acceleration in solar eruptive events suggest that roughly equal energy may go into accelerating electrons and ions. However, while previous solar X-ray spectroscopic imagers have transformed our understanding of electron acceleration, only one resolved image of γ-ray emission from solar accelerated ions has ever been produced. This paper outlines a new satellite instrument concept—the large imaging spectrometer for solar accelerated nuclei (LISSAN)—with the capability not only to observe hundreds of events over its lifetime, but also to capture multiple images per event, thereby imaging the dynamics of solar accelerated ions for the first time. LISSAN provides spectroscopic imaging at photon energies of 40 keV–100 MeV on timescales of ≲10 s with greater sensitivity and imaging capability than its predecessors. This is achieved by deploying high-resolution scintillator detectors and indirect Fourier imaging techniques. LISSAN is suitable for inclusion in a multi-instrument platform such as an ESA M-class mission or as a smaller standalone mission. Without the observations that LISSAN can provide, our understanding of solar particle acceleration, and hence the space weather events with which it is often associated, cannot be complete

The Solar Particle Acceleration Radiation and Kinetics (SPARK) mission concept

Particle acceleration is a fundamental process arising in many astrophysical objects, including active galactic nuclei, black holes, neutron stars, gamma-ray bursts, accretion disks, solar and stellar coronae, and planetary magnetospheres. Its ubiquity means energetic particles permeate the Universe and influence the conditions for the emergence and continuation of life. In our solar system, the Sun is the most energetic particle accelerator, and its proximity makes it a unique laboratory in which to explore astrophysical particle acceleration. However, despite its importance, the physics underlying solar particle acceleration remain poorly understood. The SPARK mission will reveal new discoveries about particle acceleration through a uniquely powerful and complete combination of γ-ray, X-ray, and EUV imaging and spectroscopy at high spectral, spatial, and temporal resolutions. SPARK’s instruments will provide a step change in observational capability, enabling fundamental breakthroughs in our understanding of solar particle acceleration and the phenomena associated with it, such as the evolution of solar eruptive events. By providing essential diagnostics of the processes that drive the onset and evolution of solar flares and coronal mass ejections, SPARK will elucidate the underlying physics of space weather events that can damage satellites and power grids, disrupt telecommunications and GPS navigation, and endanger astronauts in space. The prediction of such events and the mitigation of their potential impacts are crucial in protecting our terrestrial and space-based infrastructure

The Solar Particle Acceleration Radiation and Kinetics (SPARK) Mission Concept

Particle acceleration is a fundamental process arising in many astrophysical objects, including active galactic nuclei, black holes, neutron stars, gamma-ray bursts, accretion disks, solar and stellar coronae, and planetary magnetospheres. Its ubiquity means energetic particles permeate the Universe and influence the conditions for the emergence and continuation of life. In our solar system, the Sun is the most energetic particle accelerator, and its proximity makes it a unique laboratory in which to explore astrophysical particle acceleration. However, despite its importance, the physics underlying solar particle acceleration remain poorly understood. The SPARK mission will reveal new discoveries about particle acceleration through a uniquely powerful and complete combination of γ-ray, X-ray, and EUV imaging and spectroscopy at high spectral, spatial, and temporal resolutions. SPARK’s instruments will provide a step change in observational capability, enabling fundamental breakthroughs in our understanding of solar particle acceleration and the phenomena associated with it, such as the evolution of solar eruptive events. By providing essential diagnostics of the processes that drive the onset and evolution of solar flares and coronal mass ejections, SPARK will elucidate the underlying physics of space weather events that can damage satellites and power grids, disrupt telecommunications and GPS navigation, and endanger astronauts in space. The prediction of such events and the mitigation of their potential impacts are crucial in protecting our terrestrial and space-based infrastructure