22,838 research outputs found
Locoregional hyperthermia of deep-seated tumours applied with capacitive and radiative systems. A simulation study
Background: Locoregional hyperthermia is applied to deep-seated tumours in the pelvic region. Two very different heating techniques are often applied: capacitive and radiative heating. In this paper, numerical simulations are applied to compare the performance of both techniques in heating of deep-seated tumours. Methods: Phantom simulations were performed for small (30 × 20 × 50 cm 3 ) and large (45 × 30 × 50 cm 3 ), homogeneous fatless and inhomogeneous fat-muscle, tissue-equivalent phantoms with a central or eccentric target region. Radiative heating was simulated with the 70 MHz AMC-4 system and capacitive heating was simulated at 13.56 MHz. Simulations were performed for small fatless, small (i.e. fat layer typically 3 cm) patients with cervix, prostate, bladder and rectum cancer. Temperature distributions were simulated using constant hyperthermic-level perfusion values with tissue constraints of 44 °C and compared for both heating techniques. Results: For the small homogeneous phantom, similar target heating was predicted with radiative and capacitive heating. For the large homogeneous phantom, most effective target heating was predicted with capacitive heating. For inhomogeneous phantoms, hot spots in the fat layer limit adequate capacitive heating, and simulated target temperatures with radiative heating were 2–4 °C higher. Patient simulations predicted therapeutic target temperatures with capacitive heating for fatless patients, but radiative heating was more robust for all tumour sites and patient sizes, yielding target temperatures 1–3 °C higher than those predicted for capacitive heating. Conclusion: Generally, radiative locoregional heating yields more favourable simulated temperature distributions for deep-seated pelvic tumours, compared with capacitive heating. Therapeutic temperatures are predicted for capacitive heating in patients with (almost) no fat
Modelling light scattering by absorbing smooth and slightly rough facetted particles
A method for approximating light scattering properties of strongly absorbing facetted particles which are large compared to the wavelength is presented. It consists in adding the approximated external diffraction and reflection far fields and is demonstrated for a smooth hexagonal prism. This computationally fast method is extended towards prisms with slightly rough surfaces by introducing a surface scaling factor in order to account for edge effects on subfacets forming the rough surface. These effects become more pronounced with decreasing subfacet dimension to wavelength ratio. Azimuthally resolved light scattering patterns, phase functions and degree of linear polarisation obtained by this method and by the Discrete Dipole Approximation are compared for hexagonal prisms with smooth and slightly rough surfaces, respectively.Peer reviewedSubmitted Versio
Production of TeV gamma-radiation in the vicinity of the supermassive black hole in the giant radiogalaxy M87
Although the giant radiogalaxy M 87 harbors many distinct regions of
broad-band nonthermal emission, the recently reported fast variability of TeV
gamma rays from M 87 on a timescale of days strongly constrains the range of
speculations concerning the possible sites and scenarios of particle
acceleration responsible for the observed TeV emission. A natural production
site of this radiation is the immediate vicinity of the central supermassive
mass black hole (BH). Because of the low bolometric luminosity, the nucleus of
M 87 is effectively transparent for gamma rays up to energy of 10 TeV, which
makes this source an ideal laboratory for study of particle acceleration
processes close to the BH event horizon. We critically analyse different
possible radiation mechanisms in this region, and argue that the observed very
high-energy gamma ray emission can be explained by the inverse Compton emission
of ultrarelativistic electron-positron pairs produced through the development
of an electromagnetic cascade in the BH magnetosphere. We demonstrate, through
detailed numerical calculations of acceleration and radiation of electrons in
the magnetospheric vacuum gap, that this ``pulsar magnetosphere like'' scenario
can satisfactorily explain the main properties of TeV gamma-ray emission of M
87.Comment: 11 pages, ApJ, in prin
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