213,254 research outputs found
M-quantile regression analysis of temporal gene expression data
No full textIn this paper, we explore the use of M-regression and M-quantile coefficients to detect statistical differences between temporal curves that belong to different experimental conditions. In particular, we consider the application of temporal gene expression data. Here, the aim is to detect genes whose temporal expression is significantly different across a number of biological conditions. We present a new method to approach this problem. Firstly, the temporal profiles of the genes are modelled by a parametric M-quantile regression model. This model is particularly appealing to small-sample gene
expression data, as it is very robust against outliers and it does not make any assumption on the error distribution. Secondly, we further increase the robustness of the method by summarising the M-quantile regression models for a large range of quantile values into an M-quantile coefficient. Finally, we employ a Hotelling T2-test to detect significant differences of the temporal M-quantile profiles across conditions. Simulated data shows the increased robustness of M-quantile regression methods over standard regression methods. We conclude by using the method to detect differentially expressed genes from time-course microarray data on muscular dystrophy
Mechanism of growth reduction of the deceleration-phase Rayleigh-Taylor instability
No full textThe deceleration-phase (dp) ablative Rayleigh-Taylor instability (RTI) of igniting and nonigniting inertial fusion capsules is studied by high-resolution two-dimensional Lagrangian fluid simulations. It is found that growth reduction of the dp-RTI with respect to classical RTI results from the advection of perturbed fluid elements outside a thin unstable fluid layer. Within this layer, at fixed Lagrangian position, perturbations grow approximately classically
Three-dimensional study of radiation symmetrization in some indirectly driven heavy ion ICF targets
No full textSymmetrization of the radiation field inside hohlraum targets for indirectly driven heavy ion beam inertial confinement fusion (ICF) is investigated numerically. The targets considered consist of a casing, enclosing the spherical fuel capsule, and a few cylindrical radiators, schematically representing ion beam irradiated converters. Radiation absorption and re-emission are dealt with as in the paper by Murakami and Meyer-ter-Vehn (Nucl. Fusion 31 (1991) 1333), but with the geometry extended to three dimensions and with the finite size of the radiators taken into account. It is found that, for a practical casing to capsule area ratio (of the order of ten) and practical converter aspect ratios, two converters (allowing for two-side axisymmetric irradiation) cannot provide the uniformity required for ICF. However, with a spherical casing with six converters (placed in couples along the axes of a Cartesian co-ordinate system) it would be possible to illuminate a capsule with a non-uniformity well below 2%, which could satisfy the ICF requirements. The effects of changing the area ratio, the size and position of the converters and the geometry of the hohlraum are also discussed
Studies on Radiation Symmetrization in Heavy-Ion Driven Hohlraum Targets
No full textRadiation symmetrization within spherical, ellipsoidal and cylindrical hohlraum targets for heavy-ion inertial-confinement fusion (ICF) is studied by means of a 3D numerical static model, in which realistic assumptions are made concerning the geometry of the system and, particularly, of the radiation >. Among the systems so far studied, only spherical hohlraums with six converters achieve the illumination symmetry of the fusion capsule considered necessary for ICF applications. A parametric study of cylindrical hohlraums enlightens the effect of several parameter changes, and suggests directions for further studies, aiming at the design of two-converter targets
Proton-beam driven fast ignition of inertially confined fuels: Reduction of the ignition energy by the use of two proton beams with radially shaped profiles
No full textFast ignition of a spherical compressed deuterium-tritium assembly induced by the energy deposition of laser-accelerated proton beams is considered. An efficient way to reduce the ignition energy consists of using a two proton beams scheme [M. Temporal, Phys Plasmas 13, 122704 (2006)]. For a uniformly compressed fuel at 500 g/cm(3) irradiated by proton beams with Maxwellian energy distribution with a temperature of 4 MeV, the ignition energy is 10 kJ using only one proton beam and reduces to a total of 8 kJ with the two-beam scheme. Further reduction of the ignition energy is found by using a first beam with annular radial profile and a second beam with the uniform radial profile. It is found that the first beam causes some additional fuel compression and confinement that decrease the total beam energy required for the ignition to 6 kJ, which is 40% smaller than in the case of a single beam with uniform radial profile. (c) 2008 American Institute of Physics
Imaging memory in temporal lobe epilepsy: predicting the effects of temporal lobe resection
No full textFunctional magnetic resonance imaging can demonstrate the functional anatomy of cognitive processes. In patients with refractory temporal lobe epilepsy, evaluation of preoperative verbal and visual memory function is important as anterior temporal lobe resections may result in material specific memory impairment, typically verbal memory decline following left and visual memory decline after right anterior temporal lobe resection. This study aimed to investigate reorganization of memory functions in temporal lobe epilepsy and to determine whether preoperative memory functional magnetic resonance imaging may predict memory changes following anterior temporal lobe resection. We studied 72 patients with unilateral medial temporal lobe epilepsy (41 left) and 20 healthy controls. A functional magnetic resonance imaging memory encoding paradigm for pictures, words and faces was used testing verbal and visual memory in a single scanning session on a 3T magnetic resonance imaging scanner. Fifty-four patients subsequently underwent left (29) or right (25) anterior temporal lobe resection. Verbal and design learning were assessed before and 4 months after surgery. Event-related functional magnetic resonance imaging analysis revealed that in left temporal lobe epilepsy, greater left hippocampal activation for word encoding correlated with better verbal memory. In right temporal lobe epilepsy, greater right hippocampal activation for face encoding correlated with better visual memory. In left temporal lobe epilepsy, greater left than right anterior hippocampal activation on word encoding correlated with greater verbal memory decline after left anterior temporal lobe resection, while greater left than right posterior hippocampal activation correlated with better postoperative verbal memory outcome. In right temporal lobe epilepsy, greater right than left anterior hippocampal functional magnetic resonance imaging activation on face encoding predicted greater visual memory decline after right anterior temporal lobe resection, while greater right than left posterior hippocampal activation correlated with better visual memory outcome. Stepwise linear regression identified asymmetry of activation for encoding words and faces in the ipsilateral anterior medial temporal lobe as strongest predictors for postoperative verbal and visual memory decline. Activation asymmetry, language lateralization and performance on preoperative neuropsychological tests predicted clinically significant verbal memory decline in all patients who underwent left anterior temporal lobe resection, but were less able to predict visual memory decline after right anterior temporal lobe resection. Preoperative memory functional magnetic resonance imaging was the strongest predictor of verbal and visual memory decline following anterior temporal lobe resection. Preoperatively, verbal and visual memory function utilized the damaged, ipsilateral hippocampus and also the contralateral hippocampus. Memory function in the ipsilateral posterior hippocampus may contribute to better preservation of memory after surgery
Identification and bisection of temporal durations and tone frequencies: common models for temporal and nontemporal stimuli
No full textTwo experiments examined identification and bisection of tones varying in temporal duration (Experiment 1) or frequency (Experiment 2). Absolute identification of both durations and frequencies was influenced by prior stimuli and by stimulus distribution. Stimulus distribution influenced bisection for both stimulus types consistently, with more positively skewed distributions producing lower bisection points. The effect of distribution was greater when the ratio of the largest to smallest stimulus magnitude was greater. A simple mathematical model, temporal range frequency theory, was applied. It is concluded that (a) similar principles describe identification of temporal durations and other stimulus dimensions and (b) temporal bisection point shifts can be understood in terms of psychophysical principles independently developed in nontemporal domains, such as A. Parducci's (1965) range frequency theory
Energetics and Symmetry of Hohlraum Targets Driven by Ion Beam Pulses with Simple Time Shape
No full textThe energetics of hohlraum targets for inertial fusion are studied by means of one-dimensional radiation hydrodynamics simulations, assuming that a pulse of thermal X-rays with a simple time shape is fed into the cavity. A fusion yield Efus = 160–250 MJ is released by a capsule with fuel mass mDT = 3.3 mg, driven by a two-step pulse. The required input energy is Ex ≈ 3.4 MJ for a hohlraum area ratio a = 9 and 6.6 MJ for a = 20, corresponding to gains of Gx = Efus/Ex = 50–73 and 25–35 respectively. Higher gains are obtained by three-step pulses. Targets with mDT = 0.4 mg require better-shaped pulses, with at least three steps. Driven by Ex = 0.85−1.7 MJ, they release Efus = 8–10 MJ. Symmetry aspects of axially symmetric hohlraums driven by heavy ion beams are studied by a viewfactor code, employing wall motion and re-emissivities provided by the one-dimensional hydro-simulations. The dependence of the capsule irradiation asymmetry on the hohlraum aspect ratio, area ratio and fill density is analyzed. Reductions of wall motion and converter expansion, and the use of shields appear necessary to allow for the use of a moderate area ratio a ≈ 10–15
Defining Meyer's loop-temporal lobe resections, visual field deficits and diffusion tensor tractography
No full textAnterior temporal lobe resection is often complicated by superior quadrantic visual field deficits (VFDs). In some cases this can be severe enough to prohibit driving, even if a patient is free of seizures. These deficits are caused by damage to Meyer's loop of the optic radiation, which shows considerable heterogeneity in its anterior extent. This structure cannot be distinguished using clinical magnetic resonance imaging sequences. Diffusion tensor tractography is an advanced magnetic resonance imaging technique that enables the parcellation of white matter. Using seed voxels antero-lateral to the lateral geniculate nucleus, we applied this technique to 20 control subjects, and 21 postoperative patients. All patients had visual fields assessed with Goldmann perimetry at least three months after surgery. We measured the distance from the tip of Meyer's loop to the temporal pole and horn in all subjects. In addition, we measured the size of temporal lobe resection using postoperative T1-weighted images, and quantified VFDs. Nine patients suffered VFDs ranging from 22% to 87% of the contralateral superior quadrant. In patients, the range of distance from the tip of Meyer's loop to the temporal pole was 24–43 mm (mean 34 mm), and the range of distance from the tip of Meyer's loop to the temporal horn was –15 to +9 mm (mean 0 mm). In controls the range of distance from the tip of Meyer's loop to the temporal pole was 24–47 mm (mean 35 mm), and the range of distance from the tip of Meyer's loop to the temporal horn was –11 to +9 mm (mean 0 mm). Both quantitative and qualitative results were in accord with recent dissections of cadaveric brains, and analysis of postoperative VFDs and resection volumes. By applying a linear regression analysis we showed that both distance from the tip of Meyer's loop to the temporal pole and the size of resection were significant predictors of the postoperative VFDs. We conclude that there is considerable variation in the anterior extent of Meyer's loop. In view of this, diffusion tensor tractography of the optic radiation is a potentially useful method to assess an individual patient's risk of postoperative VFDs following anterior temporal lobe resection
Numerical study of fast ignition of ablatively imploded DT fusion capsules by ultra-intense proton beams
No full textCompression and ignition of deuterium–tritium fuel under conditions relevant to the scheme of fast ignition by laser generated proton beams [Roth et al., Phys. Rev. Lett. 86, 436 (2001)] are studied by numerical simulation. Compression of a fuel containing spherical capsule driven by a pulse of thermal radiation is studied by a one-dimensional radiation hydrodynamics code. Irradiation of the compressed fuel by an intense proton beam, generated by a target at distance d from the capsule center, and subsequent ignition and burn are simulated by a two-dimensional code. A robust capsule, absorbing 635 kJ of 210 eV (peak) thermal x rays, with fusion yield of almost 500 MJ, has been designed, which could allow for target gain of 200. On the other hand, for a reasonable proton spectrum the required proton beam energy Eig, exceeds 25 kJ (for d = 4 mm), even neglecting beam losses in the hohlraum and assuming that the beam can be focused on a spot with radius of 10 μm. The effects of proton range lengthening due to the increasing plasma temperature and of beam temporal spread caused by velocity dispersion are discussed. Ways to reduce Eig to about 10 kJ are discussed and analyzed by simulations
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