278 research outputs found
Quantitative T1 mapping of the normal brain from early infancy to adulthood
Abstract Background Quantitative mapping of MRI relaxation times is expected to uncover pathological processes in the brain more subtly than standard MRI techniques with weighted contrasts. So far, however, most mapping techniques suffer from a long measuring time, low spatial resolution or even sensitivity to magnetic field inhomogeneity. Objective To obtain T1 relaxation times of the normal brain from early infancy to adulthood using a novel technique for fast and accurate T1 mapping at high spatial resolution. Materials and methods We performed whole-brain T1 mapping within less than 3 min in 100 patients between 2 months and 18 years of age with normal brain at a field strength of 3 T. We analyzed T1 relaxation times in several gray-matter nuclei and white matter. Subsequently, we derived regression equations for mean value and confidence interval. Results T1 relaxation times of the pediatric brain rapidly decrease in all regions within the first 3 years of age, followed by a significantly weaker decrease until adulthood. These characteristics are more pronounced in white matter than in deep gray matter. Conclusion Regardless of age, quantitative T1 mapping of the pediatric brain is feasible in clinical practice. Normal age- dependent values should contribute to improved discrimination of subtle intracerebral alterations
Real-time magnetic resonance imaging in pediatric radiology — new approach to movement and moving children
The recent development of highly undersampled radial gradient echo sequences in combination with nonlinear inverse image reconstruction now allows for MRI examinations in real time. Image acquisition times as short as 20 ms yield MRI videos with rates of up to 50 frames per second with spin density, T1- and T2-type contrast. The addition of an initial 180° inversion pulse achieves accurate T1 mapping within only 4 s. These technical advances promise specific advantages for studies of infants and young children by eliminating the need for sedation or anesthesia. Our preliminary data demonstrate new diagnostic opportunities ranging from dynamic studies of speech and swallowing processes and body movements to a rapid volumetric assessment of brain cerebrospinal fluid spaces in only few seconds. Real-time MRI of the heart and blood flow can be performed without electrocardiogram gating and under free breathing. The present findings support the idea that real-time MRI will complement existing methods by providing long-awaited diagnostic options for patients in early childhood. Major advantages are the avoidance of sedation or anesthesia and the yet unexplored potential to gain insights into arbitrary body functions
Outpacing movement — ultrafast volume coverage in neuropediatric magnetic resonance imaging
Background Conventional MRI sequences are often affected in neuropediatric imaging by unavoidable movements. Therefore, children younger than 6 years usually have to be examined under sedation/anesthesia. A new real-time MRI technique with automatic slice advancement allows for motion-robust T2-weighted volume coverage of the whole brain within a few seconds in adults. Objective To evaluate to which extent the new volume coverage method can be used to visualize cerebrospinal fluid and reduce the need for anesthesia in children. Materials and methods We assessed 30 children ages 6 years and younger with suspected or proven hydrocephalus, hygroma or macrocephalus using volume coverage sequences with 20 slices per second in three planes. If necessary, a parent was placed in the bore together with the child for calming and gentle immobilization. We compared visualization of cerebrospinal fluid spaces and course of the shunt catheter in volume coverage sequences vs. fast spin-echo sequences. Results The clinical issue could be sufficiently assessed in all children with use of volume coverage sequences, whereas conventional fast spin-echo sequences performed moderately to poorly. Visualization of the tip of a shunt failed in 16% of volume coverage scans and 27% of turbo spin-echo scans. A subsequent examination under anesthesia was never necessary. None of the examinations had to be stopped prematurely. Conclusion The motion-robust volume coverage sequences with T2-type contrast can be used to avoid sedation of children in the evaluation of cerebrospinal fluid spaces, even in the presence of vigorous motion. For other indications and contrasts, the technique must still be evaluated
Quantum walks of correlated photon pairs in two-dimensional waveguide arrays
We demonstrate quantum walks of correlated photons in a 2D network of
directly laser written waveguides coupled in a 'swiss cross' arrangement. The
correlated detection events show high-visibility quantum interference and
unique composite behaviour: strong correlation and independence of the quantum
walkers, between and within the planes of the cross. Violations of a
classically defined inequality, for photons injected in the same plane and in
orthogonal planes, reveal non-classical behaviour in a non-planar structure.Comment: 5 pages, 5 figure
Oscillations in Quantum Entanglement During Rescattering
We study the time evolution of quantum entanglement between an electron and
its parent ion during the rescattering due to a strong few-cycle laser pulse.
Based on a simple one-dimensional model, we compute the Neumann entropy during
the process for several values of the carrier-envelope phase. The local maxima
of the oscillations in the Neumann entropy coincide with the zero crossings of
the electric field of the laser pulse. We employ the Wigner function to
qualitatively explain the quantum dynamics of rescattering in the phase space.Comment: 2 page
Imaging the Renner-Teller effect using laser-induced electron diffraction
Structural information on electronically excited neutral molecules can be
indirectly retrieved, largely through pump-probe and rotational spectroscopy
measurements with the aid of calculations. Here, we demonstrate the direct
structural retrieval of neutral carbonyl disulfide (CS) in the BB
excited electronic state using laser-induced electron diffraction (LIED). We
unambiguously identify the ultrafast symmetric stretching and bending of the
field-dressed neutral CS molecule with combined picometer and attosecond
resolution using intrapulse pump-probe excitation and measurement. We invoke
the Renner-Teller effect to populate the BB excited state in neutral
CS, leading to bending and stretching of the molecule. Our results
demonstrate the sensitivity of LIED in retrieving the geometric structure of
CS, which is known to appear as a two-center scatterer
Molecular isomerization and fragmentation of polyatomic molecules controlled by inner-valence recollision-ionization
Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets
Spielen, Bewegen, Erkunden - Praktiken der Raumaneignung von Grundschulkindern in der DrauĂźenschule
"In den Sozial- und Erziehungswissenschaften ist seit den 1990er-Jahren zunehmend eine neue Aufmerksamkeit für den Raum (spatial turn) zu beobachten, die sich auch auf die Ordnung kindlicher Lern- und Lebensräume richtet. Einen prominenten Stellenwert nimmt insbesondere die Betrachtung des Schulraumes ein. Mit dem Diskurs um die sozialräumliche Öffnung von Schule geraten dabei auch außerschulische Lernorte in den Fokus. Weithin vernachlässigt werden bislang jedoch zwei Perspektiven: 1) Die Sicht der Kinder auf den Unterricht an außerschulischen Lernorten und 2) die Analyse kindlicher Praktiken der Aneignung des (Draußen-)Schulraumes. Dem widmet sich der vorliegende Beitrag, indem er, ausgehend von raum- und aneignungstheoretischen Ansätzen sowie auf der empirischen Basis von Gruppengesprächen mit Grundschulkindern, deren Raumaneignungspraktiken in der Draußenschule - dem regelmäßigen Unterricht an außerschulischen Lernorten - diskutiert." (Autorenreferat)"The observable new attention for space in social and educational sciences since the 1990s (spatial turn) focuses more and more on the spatial constitution of children's learning and living spaces. Of relative importance is especially the classroom as a learning environment. But the discourse about the sociospatial opening of school also brings extracurricular educational facilities into focus. So far, two perspectives are still widely neglected: 1) The children’s point of view concerning lessons at extracurricular learning places, and 2) the analysis of the children’s practices of appropriation of the (outdoor) school space. The present article uses approaches of spatial and activity theory as well as group discussions with primary school children as empirical basis. It is about to shed light on this topic by discussing the children's practices of appropriating space in the German Outdoor School (Draußenschule) - the regular instruction at extracurricular learning places." (author's abstract
Ultrafast electron diffraction imaging of bond breaking in di-ionized acetylene
Visualizing chemical reactions as they occur requires atomic spatial and femtosecond temporal resolution. Here, we report imaging of the molecular structure of acetylene (C2H2) 9 femtoseconds after ionization. Using mid-infrared laser–induced electron diffraction (LIED), we obtained snapshots as a proton departs the [C2H2]2+ ion. By introducing an additional laser field, we also demonstrate control over the ultrafast dissociation process and resolve different bond dynamics for molecules oriented parallel versus perpendicular to the LIED field. These measurements are in excellent agreement with a quantum chemical description of field-dressed molecular dynamicsPostprint (author's final draft
Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction
Ubiquitous to most molecular scattering methods is the challenge to retrieve
bond distance and angle from the scattering signals since this requires
convergence of pattern matching algorithms or fitting methods. This problem is
typically exacerbated when imaging larger molecules or for dynamic systems with
little a priori knowledge. Here, we employ laser-induced electron diffraction
(LIED) which is a powerful means to determine the precise atomic configuration
of an isolated gas-phase molecule with picometre spatial and attosecond
temporal precision. We introduce a simple molecular retrieval method, which is
based only on the identification of critical points in the oscillating
molecular interference scattering signal that is extracted directly from the
laboratory-frame photoelectron spectrum. The method is compared with a
Fourier-based retrieval method, and we show that both methods correctly
retrieve the asymmetrically stretched and bent field-dressed configuration of
the asymmetric top molecule carbonyl sulfide (OCS), which is confirmed by our
quantum-classical calculations
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