20 research outputs found
Sound radiation of 3 MHz driven gas bubbles
The sound radiation of 3 MHz acoustically driven air bubbles in liquid is
analysed with respect to possible applications in second harmonic ultrasound
diagnostics devices, which have recently come into clinical use. In the forcing
pressure amplitude P_a = 1-10 atm and ambient radius R_0 = 0.5-5 \mu m
parameter domain a narrow regime around the resonance radius R_0 \sim 1-1.5 \mu
m and relatively modest P_a \sim 2-2.5 atm is identified in which optimal sound
yield in the second harmonic is achieved while maintaining spherical stability
of the bubble. For smaller P_a and larger R_0 hardly any sound is radiated; for
larger P_a bubbles become unstable towards non-spherical shape oscillations of
their surface. The computation of these instabilities is essential for the
evaluation of the optimal parameter regime. A region of slightly smaller R_0
and P_a \sim 1-3 atm is best suited to achieve large ratios of the second
harmonic to the fundamental intensity. Spherical stability is guaranteed in the
suggested regimes for liquids with an enhanced viscosity compared to water,
such as blood.Comment: 19 pages, 10 low resolution ps-figures; higher resolution figures can
be retrieved from http://staff-www.uni-marburg.de/~hilgenfe/hires.htm
Analysis of Rayleigh-Plesset dynamics for sonoluminescing bubbles
Recent work on single bubble sonoluminescence (SBSL) has shown that many
features of this phenomenon, especially the dependence of SBSL intensity and
stability on experimental parameters, can be explained within a hydrodynamic
approach. More specifically, many important properties can already be derived
from an analysis of bubble wall dynamics. This dynamics is conveniently
described by the Rayleigh-Plesset (RP) equation. In this work we derive
analytical approximations for RP dynamics and subsequent analytical laws for
parameter dependences. These results include (i) an expression for the onset
threshold of SL, (ii) an analytical explanation of the transition from
diffusively unstable to stable equilibria for the bubble ambient radius
(unstable and stable sonoluminescence), and (iii) a detailed understanding of
the resonance structure of the RP equation. It is found that the threshold for
SL emission is shifted to larger bubble radii and larger driving pressures if
surface tension is enlarged, whereas even a considerable change in liquid
viscosity leaves this threshold virtually unaltered. As an enhanced viscosity
stabilizes the bubbles against surface oscillations, we conclude that the ideal
liquid for violently collapsing, surface stable SL bubbles should have small
surface tension and large viscosity, although too large viscosity (>40 times
the viscosity of water) will again preclude collapses.Comment: 41 pages, 21 eps and ps figures; LaTeX stylefiles replaced because
the PostScript file produced at the archive had misplaced and misscaled
figure
Sonoluminescence light emission
Single bubble sonoluminescence is not an exotic phenomenon but can quantitatively be accounted for by applying a few well-known, simple concepts: the Rayleigh¿Plesset dynamics of the bubble's radius, polytropic uniform heating of the gas inside the bubble during collapse, the dissociation of molecular gases, and thermal radiation of the remaining hot noble gas, where its finite opacity (transparency for its own radiation) is essential. A system of equations based on these ingredients correctly describes the widths, shapes, intensities, and spectra of the emitted light pulses, all as a function of the experimentally adjustable parameters, namely, driving pressure, driving frequency, water temperature, and the concentration and type of the dissolved gas. The theory predicts that the pulse width of strongly forced xenon bubbles should show a wavelength dependence, in contrast to argon bubble
Pediatric craniospinal irradiation with a short partial-arc VMAT technique for medulloblastoma tumors in dosimetric comparison
Background!#!This study aimed to contrast four different irradiation methods for pediatric medulloblastoma tumors in a dosimetric comparison regarding planning target volume (PTV) coverage and sparing of organs at risk (OARs).!##!Methods!#!In sum 24 treatment plans for 6 pediatric patients were realized. Besides the clinical standard of a 3D-conformal radiotherapy (3D-CRT) treatment plan taken as a reference, volumetric modulated arc therapy (VMAT) treatment plans ('VMAT_AVD' vs. 'noAVD' vs. 'FullArc') were optimized and calculated for each patient. For the thoracic and abdominal region, the short partial-arc VMAT_AVD technique uses an arc setup with reduced arc-length by 100°, using posterior and lateral beam entries. The noAVD uses a half 180° (posterior to lateral directions) and the FullArc uses a full 360° arc setup arrangement. The prescription dose was set to 35.2 Gy.!##!Results!#!We identified a more conformal dose coverage for PTVs and a better sparing of OARs with used VMAT methods. For VMAT_AVD mean dose reductions in organs at risk can be realized, from 16 to 6.6 Gy, from 27.1 to 8.7 Gy and from 8.0 to 1.9 Gy for the heart, the thyroid and the gonads respectively, compared to the 3D-CRT treatment method. In addition we have found out a superiority of VMAT_AVD compared to the noAVD and FullArc trials with lower exposure to low-dose radiation to the lungs and breasts.!##!Conclusions!#!With the short partial-arc VMAT_AVD technique, dose exposures to radiosensitive OARS like the heart, the thyroid or the gonads can be reduced and therefore, maybe the occurrence of late sequelae is less likely. Furthermore the PTV conformity is increased. The advantages of the VMAT_AVD have to be weighed against the potentially risks induced by an increased low dose exposure compared to the 3D-CRT method
Hyperresponsiveness of mice defi cient in plasma-secreted sphingomyelinase reveals its pivotal role in early phase of host response
Plasma secretion of acid sphingomyelinase is a hallmark of cellular stress response resulting in the formation of membrane embedded ceramide-enriched lipid rafts and the reorganization of receptor complexes. Consistently, decompartmentalization of ceramide formation from inert sphingomyelin has been associated with signaling events and regulation of the cellular phenotype. Herein, we addressed the question of whether the secretion of acid sphingomyelinase is involved in host response during sepsis. We found an exaggerated clinical course in mice genetically deficient in acid sphingomyelinase characterized by an increased bacterial burden, an increased phagocytotic activity, and a more pronounced cytokine storm. Moreover, on a functional level, leukocyte-endothelial interaction was found diminished in sphingomyelinase-deficient animals corresponding to a distinct leukocytes’ phenotype with respect to rolling and sticking as well as expression of cellular surface proteins. We conclude that hydrolysis of membrane-embedded sphingomyelin, triggered by circulating sphingomyelinase, plays a pivotal role in the first line of defense against invading microorganisms. This function might be essential during the early phase of infection leading to an adaptive response of remote cells and tissues