2,151 research outputs found
A consistent approximation scheme beyond RPA for bosons
In this paper, we develop a consistent extension of RPA for bosonic systems.
In order to illustrate the method, we consider the case of the anharmonic
oscillator. We compare our results with those obtained in mean-field and
standard RPA approaches, with the exact ones and show that they are very close
to the exact ones.Comment: 19 pages, Latex, 1 figure, accepted version in EPJ
Observations and simulation of intense convection embedded in a warm conveyor belt â how ambient vertical wind shear determines the dynamical impact
Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB\u27s large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposhere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyses the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact EXperiment (NAWDEX) field campaign (IOPs 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behaviour of the WCB trajectories reveals that very intense convection with ascent rates of 600âhPa in 30â60âmin occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection â moderate and intense convection â is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: Intense convection produces locally very intense surface precipitation with peak values exceeding 6âmm in 15 minutes and large amounts of graupel of up to 2.8âgâkg in the middle troposphere (compared to 3.9âmm and 1.0âgâkg for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study â in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet â the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications
Transition to Mach reflexion of shock waves in steady and pseudosteady flow with and without relaxation
Experiments were conducted in the free-piston shock tube and shock tunnel with dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon to measure the transition condition in pseudosteady and steady flow. The transition condition in the steady flow, in which the wall was eliminated by symmetry, agrees with the calculated von Neumann condition. In the real gases this calculation assumed thermo-dynamic equilibrium after the reflected shock. In the pseudosteady flow of reflexion from a wedge the measured transition angle lies on the Mach-reflexion side of the calculated detachment condition by an amount which may be explained in terms of the displacement effect of the boundary layer on the wedge surface. A single criterion based on the availability of a length scale at the reflexion point explains the difference between the pseudosteady and steady flow transition condition and predicts a hysteresis effect in the transition angle when the shock angle is varied during steady flow. No significant effects on the transition condition due to finite relaxation length could be detected. However, new experiments in which interesting relaxation effects should be evident are suggested
Parkinson's Disease: Clinical Signs and Symptoms, Neural Mechanisms, Positron Emission Tomography, and Therapeutic Interventions
Parkinson's disease is one of the most
frequent neurodegenerative brain diseases. Its
time course is slow and is characterized by
progressive loss of dopaminergic and other
brainstem neurons resulting in malfunctioning of
the cerebral neuronal systems responsible for
motor functions. The clinical signs are slowness
of movement, muscle rigidity and rest-tremor
amongst other features. The cause of the disease
is unknown, but recently involvement of genetic
factors is being researched. Positron emission
tomography (PET) allows in vivo determination
of striatai dopaminergic activity. This has
increased our insight in the pathophysiology of
the disease and permits direct study of disease
progression at a biochemical level and equally to
monitor whether potential neuroprotective interventions
are indeed effective. Thus far no drug
has emerged but promising substances are
currently being studied
Untersuchung neuroprotektiver Effekte von RadikalfÀngern in Tiermodellen des Morbus Parkinson
Im Fluorogold/6-OHDA-Ratten-Modell (FG/6-OHDA) und
dem MPTP-Maus-Modell des idiopathischen Parkinsonsyndroms (IPS)
sollten in der vorgelegten Studie die RadikalfÀnger
SalicylsÀure (SS), alpha-Phenyl-N-Tert-Butyl Nitron (PBN) und
TroloxÂź Methyl Ether (Trolox) auf ihre neuroprotektive Wirkung
untersucht werden. Ziel der Arbeit war es die potenziell
protektiven Effekte der Substanzen auf den striatalen
Dopamingehalt, die Dopaminumsatzrate, die Anzahl nigraler
Tyrosinhydroxylase-immunreaktiver (TH-ir) Neurone und im
FG/6-OHDA-Modell auf die Anzahl nigraler Fluorogold-positiver
(FG-p) Neurone sowie im MPTP-Modell auf die lokomotorische
AktivitÀt der MÀuse zu erfassen. Anhand der genannten Parameter
konnte nach unilateral durchgefĂŒhrter intrastriataler
In-jektion von 6-OHDA eine signifikante ipsilaterale LĂ€sion
nigraler dopaminerger Neurone mit konsekutiv signifikant
verÀnderten neurochemischen Parametern erhoben werden. SS
zeigte bis auf einen tendenziell positiven Effekt auf die
Verminderung FG-p Neurone und den striatalen Dopamingehalt
keinen schĂŒtzenden Einfluss auf die durch 6-OHDA
hervorgerufenen VerÀnderungen. Bei mit 10mg/kg KG niedrig
dosierter Langzeitbehandlung (2 Wochen pre und 4 Wochen post
laesionem; i.p.) im Vergleich zu Studien im MPTP-Modell der
Maus mit 5 - 10-facher Dosis der SS, welche einen signifikant
protektiven Effekt beschrieben (Aubin et al. 1998), wurde auch
fĂŒr das 6-OHDA-Modell eine neuroprotektive Wirkung in diesem
Dosierungsbereich postuliert, zumal die LĂ€sionseffekte so
ausgeprÀgt waren, dass es möglich er-scheint, dass sich die
gesehenen tendenziellen Effekte bei unter diesen
Voraussetzungen zu geringer Fallzahl nicht signifikant
abzeichneten. Dies bleibt durch weitere Studien zu beweisen.
Hingewiesen wurde auf die zu erwartenden gastrointestinalen
Nebenwirkungen bei chronischer Therapie mit SalicylsÀure im
Hochdosisbereich. PBN wurde im FG/6-OHDA-Modell niedrig- und
hochdosiert 2 Wochen vor und 4 Wochen nach LĂ€sion verabreicht.
Auf sÀmtliche bestimmte Parameter konnte mit beiden Dosierungen
kein signifikant neuroprotektiver Effekt erhoben werden. Eine
durch 150 mg PBN signifikant verminderte Anzahl nigraler TH-ir
Neurone, welche sich tendenziell in der Anzahl FG-p Neurone,
allerdings nicht auf neurochemischer Ebene widerspiegelte, lieĂ
sogar einen dosisabhÀngigen neurotoxischen Effekt in höherer
Dosierung vermuten. Entscheidend scheint die lange
Behandlungsdauer im Vergleich zu anderen tierexperimentellen
Studien im selben Dosierungsbereich, da hier bisher kein
toxischer Effekt auf behavioraler Ebene beschrieben wurde.
Morphologische Parameter wurden in diesen Arbeiten nicht
erhoben. Dies spricht gegen eine neuroprotektive Therapie beim
Menschen, da diese am ehesten in Form einer Langzeitbehandlung
durchgefĂŒhrt wĂŒrde. Eine Stimulation der Dopaminsynthese durch
PBN in den residualen nigrostriatalen Neuronen wurde bei
fehlender gleichzeitiger Verminderung des striatalen
Dopamingehaltes diskutiert. Die einmalige subcutane Applikation
von 30 mg/kg KG MPTP fĂŒhrte ebenfalls zu einem signifikanten
RĂŒckgang nigraler dopaminerger TH-ir Neurone, welcher sich
wiederum signifikant auf die bestimmten neurochemischen und
behavioralen Parameter auswirkte. Das fettlösliche Vitamin E
Derivat TroloxÂź wurde in hoher und niedriger Dosierung im
MPTP-Maus-Modell mittels einmaliger Gabe s.c. unmittelbar vor
Applikation des Toxins getestet. Bis auf eine signifikante
Senkung der postlÀsionell erhöhten Umsatzrate des Dopamins
unter Behandlung mit 300 mg/kg KG TroloxÂź zeigte sich kein
signifikant positiver Effekt auf sÀmtliche Parameter.
Bemerkens-werterweise kamen allerdings die Daten fĂŒr sĂ€mtliche
Parameter der geschÀdigten Tiere in der Reihenfolge Trolox 300
> Trolox 100 > NaCl zu liegen, was eine dosisabhÀngige
Wirkung vermuten und die DurchfĂŒhrung weiterer Studien mit
höher dosiertem Trolox erfolgversprechend erscheinen lÀsst,
zumal unter den angewandten Dosierungen keine unerwĂŒnschten
Wirkungen bei den Tieren zu beobachten waren. DarĂŒber hinaus
stellten sich auch in diesem Versuch die LĂ€sionseffekte so
ausgeprÀgt dar, dass sich die gesehenen tendenziellen Effekte
bei gröĂerer Fallzahl möglicherweise signifikant abzeichnen
wĂŒrden. Zusammenfassend lĂ€sst sich sagen, dass ein
neuroprotektiver Effekt von Trolox und SalicylsÀure in höherer
als in der hier verwendeten Dosierung oder bei gröĂerer
Fallzahl möglich erscheint. Des weiteren erscheint die
Kombination eines wasser-lös-lichen mit einem fettlöslichen
Antioxidanz mit differierendem Wirkort und möglicher
Addition/Potenzierung der neuroprotektiven Effekte
erfolgversprechend. PBN erwies sich im 6-OHDA-Modell der Ratte
als nicht neuroprotektiv. Es zeigte in höherer Dosierung sogar
neurotoxische Effekte, was insbesondere fĂŒr
Langzeitbehandlungen, wie sie beim Menschen im Falle einer
chronisch neurodegenerativen Erkrankung Anwendung finden wĂŒrde,
Relevanz hat und in Folge-Studien nÀher spezifiziert werden
sollte
Mobile intraoperative CT-assisted frameless stereotactic biopsies achieved single-millimeter trajectory accuracy for deep-seated brain lesions in a sample of 7 patients
BACKGROUND
Brain biopsies are crucial diagnostic interventions, providing valuable information for treatment and prognosis, but largely depend on a high accuracy and precision. We hypothesized that through the combination of neuronavigation-based frameless stereotaxy and MRI-guided trajectory planning with intraoperative CT examination using a mobile unit, one can achieve a seamlessly integrated approach yielding optimal target accuracy.
METHODS
We analyzed a total of 7 stereotactic biopsy trajectories for a variety of deep-seated locations and different patient positions. After rigid head fixation, an intraoperative pre-procedural scan using a mobile CT unit was performed for automatic image fusion with the planning MRI images and a peri-procedural scan with the biopsy cannula in situ for verification of the definite target position. We then evaluated the radial trajectory error.
RESULTS
Intraoperative scanning, surgery, computerized merging of MRI and CT images as well as trajectory planning were feasible without difficulties and safe in all cases. We achieved a radial trajectory deviation of 0.97â±â0.39 mm at a trajectory length of 60â±â12.3 mm (meanâ±âstandard deviation). Repositioning of the biopsy cannula due to inaccurate targeting was not required.
CONCLUSION
Intraoperative verification using a mobile CT unit in combination with frameless neuronavigation-guided stereotaxy and pre-operative MRI-based trajectory planning was feasible, safe and highly accurate. The setting enabled single-millimeter accuracy for deep-seated brain lesions and direct detection of intraoperative complications, did not depend on a dedicated operating room and was seamlessly integrated into common stereotactic procedures
Optical coherence tomography in neuromyelitis optica spectrum disorders: potential advantages for individualized monitoring of progression and therapy
Neuromyelitis optica spectrum disorders (NMOSD) are mostly relapsing inflammatory disorders of the central nervous system (CNS). Optic neuritis (ON) is the first NMOSD-related clinical event in 55% of the patients, which causes damage to the optic nerve and leads to visual impairment. Retinal optical coherence tomography (OCT) has emerged as a promising method for diagnosis of NMOSD and potential individual monitoring of disease course and severity. OCT not only detects damage to the afferent visual system caused by ON but potentially also NMOSD-specific intraretinal pathology, i.e. astrocytopathy. This article summarizes retinal involvement in NMOSD and reviews OCT methods that could be used now and in the future, for differential diagnosis, for monitoring of disease course, and in clinical trials
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