Inhomogeneous Targets Research Articles

A Novel Hybrid Approach for Computing Electromagnetic Scattering from Objects with Honeycomb Structures

We propose in this paper a novel hybrid numerical modeling method for computing electromagnetic scattering from inhomogeneous targets containing honeycomb structures. In the proposed approach, the whole honeycomb structure is divided into the inner and outer two subregions. Each thin wall of a unit cell in the outer subregion is replaced by a zero-thickness surface, with the aid of a resistive sheet boundary condition (RSBC) to describe the electric and magnetic field discontinuities across the surface. Each unit cell in the inner subregion is homogenized by using the Hashin–Shtrikman and the Mori–Tanaka formulae. The two subregions are further divided into smaller subdomains by introducing the Robin-type transmission condition to couple subregion interfaces, as well as subdomain interfaces. The whole solution region is then discretized and solved using the nonconformal domain decomposition-based hybrid finite element–boundary integral–multilevel fast multipole algorithm (FE-BI-MLFMA). The numerical results demonstrate that the proposed approach exhibits a high accuracy, efficiency, and flexibility. Solutions of scattering by a wing-like object and a practical unmanned aerial vehicle (UAV) model with honeycomb radar-absorbing structures are presented, showing the superior performance of the proposed algorithm.

Расчет компонентного состава поверхностных слоев карбида титана при распылении легкими ионами

On the basis of the previously tested model of sputtering binary layered inhomogeneous targets with light ions, a method for calculating the component composition of the surface layers of titanium carbide under stationary (stoichiometric) sputtering with light ions is proposed. The results of calculations of the component composition of the altered surface layer of titanium carbide are given in comparison with experimental data and the results of computer simulation. The proposed method for calculating the component composition based on the model of sputtering binary layered inhomogeneous targets made it possible to estimate the thickness of the changed surface layer. The proposed calculation method allows developing a technology for creating titanium carbide surfaces with a given ratio of components.

A Feasibility Study on Proton Range Monitoring Using 13N Peak in Inhomogeneous Targets.

Proton irradiations are highly sensitive to spatial variations, mainly due to their high linear energy transfer (LET) and densely ionizing nature. In realistic clinical applications, the targets of ionizing radiation are inhomogeneous in terms of geometry and chemical composition (i.e., organs in the human body). One of the main methods for proton range monitoring is to utilize the production of proton induced positron emitting radionuclides; these could be measured precisely with positron emission tomography (PET) systems. One main positron emitting radionuclide that could be used for proton range monitoring and verification was found to be 13N that produces a peak close to the Bragg peak. In the present work, we have employed the Monte Carlo method and Spectral Analysis (SA) technique to investigate the feasibility of utilizing the 13N peak for proton range monitoring and verification in inhomogeneous targets. Two different phantom types, namely, (1) ordinary slab and (2) MIRD anthropomorphic phantoms, were used. We have found that the generated 13N peak in such highly inhomogeneous targets (ordinary slab and human phantom) is close to the actual Bragg peak, when irradiated by incident proton beam. The feasibility of using the SA technique to estimate the distribution of positron emitter was also investigated. The current findings and the developed tools in the present work would be helpful in proton range monitoring and verification in realistic clinical radiation therapy using proton beams.

Efficient EM Scattering Analysis of Uncertain Inhomogeneous Medium

An efficient solver based on volume-surface integral equation is proposed for the electromagnetic (EM) scattering of inhomogeneous medium targets with uncertain geometrical shapes and dielectric constants. On the one hand, the nonuniform ration B-spline surfaces can be used to constructed the uncertain geometrical shapes with a number of geometrical variables. On the other hand, the uncertain dielectric constant can also be derived as the permittivity variables. Then these random variables can be expanded by the first-order Taylor series, which represent the uncertain geometrical shapes and dielectric constant. At last, the mean and variance of the radar cross section (RCS) are acquired for the uncertain inhomogeneous medium targets. The numerical results can be compared with the traditional Monte Carlo (MC) method, which can indicate the accuracy and effectiveness of the method in this letter.

Spatial Domain Indirect Holography for a Full Phaseless Approach to Biomedical Microwave Imaging

Microwave imaging approaches generally require the knowledge of the complex incident field distribution inside the imaging domain. This task can be accomplished by probing the observation domain, thus resulting in a time-consuming process, or assuming a numerical/simulated model. Alternatively, the incident field can be collected on the measurement domain, and the field distribution can be successfully retrieved on the whole imaging space. In this paper, a full phaseless, two-step strategy is described, where the phase of the incident field is retrieved by exploiting a spatial domain indirect holography technique, applied to the acquisition points only. The retrieved complex incident field can be so used for the reconstruction of the incident field inside the domain of interest. More specifically, a modal expansion technique and a source reconstruction method are adopted and compared to perform the above task. Once performing the characterization of the incident field distribution, the proposed strategy is included in the framework of a phaseless inverse scattering problem, where intensity-only data of the total field are involved. The effectiveness of the combined approach is demonstrated by considering an imaging scenario which involves a series of breast models as inhomogeneous targets.

Calculation of the sputtering coefficients of oxide films from the surface of a homogeneous material by medium-energy helium ions

The paper presents an analytical model for sputtering two-component layered inhomogeneous targets by bombardment with light ions. An analytical formula is obtained that makes it possible to calculate the total and partial sputtering yields of a binary layer of target inhomogeneity by light ions. The obtained formula is used to calculate the sputtering yields of oxide layers from the surface of a homogeneous substrate. The calculation results are in good agreement with the computer simulation data.

Assessment of an Lp‐space multi‐focusing inversion method for subsoil mapping with multi‐frequency data

AbstractA multi‐focusing method for quantitative subsoil inverse scattering (IS), which enhances an inexact‐Newton (IN) technique by means of an iterative zooming strategy, is considered. The approach simultaneously exploits multi‐chromatic scattered field data from a set of multi‐static probes above the air‐soil interface. Numerical experiments have been carried out in order to analyze the performance of the developed technique versus the depth of buried objects, as well as in the presence of inhomogeneous targets. The presented results demonstrate the advantages of the multi‐focusing approach in challenging scenarios as compared to a standard single‐resolution IN method in terms of accuracy and efficiency.

Calculation of sputtering yields of SiO2 layers from the Si surface by helium ions bombardment

An analytical model of the sputtering of two-component layered inhomogeneous targets by light ions bombardment has been developed, an analytical formula has been obtained, which makes it possible to calculate the total and partial sputtering coefficients of a binary layer of target heterogeneity by light ions bombardment. The formula obtained is used to calculate the sputtering coefficients of silicon oxide layers from the surface of a uniform silicon substrate. The calculation results are in good agreement with the data of computer simulation.

Influence of heterogeneous media on Very High Energy Electron (VHEE) dose penetration and a Monte Carlo-based comparison with existing radiotherapy modalities

In this paper we investigate the dosimetric effects target inhomogeneities on VHEE (Very High Energy Electron) beam dose profiles. Dose profile sensitivity to high and low density (0.001-2.2g/cm3) embedded media in water was measured for 156 MeV VHEE beams. To the knowledge of the authors, this is the first systematic experiment on dose penetration in inhomogeneous targets at this high energy electron beams. The results of the dosimetry experiments were compared with simulation results acquired with the TOPAS/GEANT4 Monte Carlo codes and were compared with the sensitivity of photon and proton beams.Dosimetry experiments with VHEE beams at 156 MeV were performed by embedding various density inserts in a cuboid 30×30×10 cm3 water phantom. This setup was then irradiated with a narrow (σ=1.2 mm) Gaussian VHEE beams. Transverse dose profiles were recorded at various depths within the water phantom using radosensitive EBT-XD Gafchromic films. Simulations were performed using the TOPAS user wrap for the GEANT4 Monte Carlo toolkit with the measured beam and target geometry parameters from the CLEAR facility and compared with sensitivity of photon and protons beams to inhomogeneities in water phantoms.Less than 8% dose deviation was shown for all measured dose data across all recorded depths (0–20 cm). Monte Carlo simulations within the whole water phantom volume showed a 15% dose deviation. Simulation studies indicated that using proton and photon beams within a therapeutic energy range in MC simulations of this experimental setup resulted in dose change in the central plane up to 100% and 74% of the dose maximum in the central plane respectively.The dosimetry results of these experiments and TOPAS/GEANT4 simulations compare well. Dose profiles of VHEE beams were found to be relatively insensitive (<15% dose difference across the central plane) to embedded high and low density geometries compared to simulation results with therapeutic proton beams (up to 100% of dose maximum in the plane). Therefore, VHEE is robust to anatomical changes and has the potential to be a reliable mode of radiotherapy for treating tumors in highly inhomogeneous and mobile regions such as the lung.

Scattering coefficients of inhomogeneous objects and their application in target classification in wave imaging

The aim of this paper is to provide and numerically test in the presence of measurement noise a procedure for target classification in wave imaging based on comparing frequency-dependent distribution descriptors with precomputed ones in a dictionary of learned distributions. Distribution descriptors for inhomogeneous objects are obtained from the scattering coefficients. First, we extract the scattering coefficients of the (inhomogeneous) target from the perturbation of the reflected waves. Then, for a collection of inhomogeneous targets, we build a frequency-dependent dictionary of distribution descriptors and use a matching algorithm in order to identify a target from the dictionary up to some translation, rotation and scaling.

Microwave Imaging of 3D Dielectric Structures by Means of a Newton-CG Method in lp Spaces

An increasing number of practical applications of three-dimensional microwave imaging require accurate and efficient inversion techniques. In this context, a full-wave 3D inverse-scattering method, aimed at characterizing dielectric targets, is described in this paper. In particular, the inversion approach has a Newton-based structure, in which the internal linear solver is a conjugate gradient-like algorithm in lp spaces. The presented results, which include the inversion of both numerical and experimental scattered-field data obtained in the presence of homogeneous and inhomogeneous targets, validate the reconstruction capabilities of the proposed technique.

Electro-sensing of inhomogeneous targets

This paper addresses the electro-sensing problem for weakly electric fish in the case of inhomogeneous targets. It aims at providing a shape descriptor-based classification for inhomogeneous targets from measurements of the potentials on the skin of the fish. The approach is based on new invariants for the contracted generalized polarization tensors associated with inhomogeneous objects. The numerical simulations show that by comparing these invariants with those in a dictionary of precomputed homogeneous and inhomogeneous targets, one can successfully classify the inhomogeneous target.

Inverse Profiling of Inhomogeneous Subsurface Targets With Arbitrary Cross Sections Using Covariance Matrix Adaptation Evolution Strategy

The problem of subsurface inverse profiling of a 2-D inhomogeneous buried dielectric target is addressed in this letter. An iterative optimization technique is proposed that utilizes Covariance Matrix Adaption Evolutionary Strategy (CMA-ES) as its inverse solver and Method of Moments, using Conjugate Gradient-fast Fourier transform, as the forward solver. The numerical results indicate that CMA-ES, as its first reported implementation in buried target reconstruction, can successfully be applied to this challenging reconstruction problem. Also, comparison with Evolutionary Programming and Particle Swarm Optimization indicates that CMA-ES can significantly outperform the other two-optimization techniques in the inhomogeneous subsurface imaging. In addition, examples of various scenarios involving noisy data, lossy targets and multiple targets further demonstrate that CMA-ES can be considered as a robust, simple, and efficient optimization tool in the reconstruction of complex buried targets.

Discrete dipole approximation for black carbon-containing aerosols in arbitrary mixing state: A hybrid discretization scheme

An accurate and efficient simulation of light scattering by an atmospheric black carbon (BC)-containing aerosol—a fractal-like cluster of hundreds of carbon monomers that is internally mixed with other aerosol compounds such as sulfates, organics, and water—remains challenging owing to the enormous diversities of such aerosols׳ size, shape, and mixing state. Although the discrete dipole approximation (DDA) is theoretically an exact numerical method that is applicable to arbitrary non-spherical inhomogeneous targets, in practice, it suffers from severe granularity-induced error and degradation of computational efficiency for such extremely complex targets. To solve this drawback, we propose herein a hybrid DDA method designed for arbitrary BC-containing aerosols: the monomer-dipole assumption is applied to a cluster of carbon monomers, whereas the efficient cubic-lattice discretization is applied to the remaining particle volume consisting of other materials. The hybrid DDA is free from the error induced by the surface granularity of carbon monomers that occurs in conventional cubic-lattice DDA. In the hybrid DDA, we successfully mitigate the artifact of neglecting the higher-order multipoles in the monomer-dipole assumption by incorporating the magnetic dipole in addition to the electric dipole into our DDA formulations. Our numerical experiments show that the hybrid DDA method is an efficient light-scattering solver for BC-containing aerosols in arbitrary mixing states. The hybrid DDA could be also useful for a cluster of metallic nanospheres associated with other dielectric materials.

Detection of mixed-range proton pencil beams with a prompt gamma slit camera

With increasing availability of proton and particle therapy centers for tumor treatment, the need for in vivo range verification methods comes more into the focus. Imaging of prompt gamma rays emitted during the treatment is one of the possibilities currently under investigation. A knife-edge shaped slit camera was recently proposed for this task and measurements proved the feasibility of range deviation detection in homogeneous and inhomogeneous targets. In the present paper, we concentrate on laterally inhomogeneous materials, which lead to range mixing situations when crossed by one pencil beam: different sections of the beam have different ranges. We chose exemplative cases from clinical irradiation and assembled idealized tissue equivalent targets. One-dimensional emission profiles were obtained by measuring the prompt gamma emission with the slit camera. It could be shown that the resulting range deviations can be detected by evaluation of the measured data with a previously developed range deviation detection algorithm. The retrieved value, however, strongly depends on the target composition, and is not necessarily in direct relation to the ranges of both parts of the beam. By combining the range deviation detection with an analysis of the slope of the distal edge of the measured prompt gamma profile, the origin of the detected range deviation, i.e. the mixed range of the beam, is also identified. It could be demonstrated that range mixed prompt gamma profiles exhibit less steep distal slopes than profiles from beams traversing laterally homogeneous material. For future application of the slit camera to patient irradiation with double scattered proton beams, situations similar to the range mixing cases are present and results could possibly apply.

Measurement of prompt gamma profiles in inhomogeneous targets with a knife-edge slit camera during proton irradiation

Proton and ion beam therapies become increasingly relevant in radiation therapy. To fully exploit the potential of this irradiation technique and to achieve maximum target volume conformality, the verification of particle ranges is highly desirable. Many research activities focus on the measurement of the spatial distributions of prompt gamma rays emitted during irradiation. A passively collimating knife-edge slit camera is a promising option to perform such measurements. In former publications, the feasibility of accurate detection of proton range shifts in homogeneous targets could be shown with such a camera. We present slit camera measurements of prompt gamma depth profiles in inhomogeneous targets. From real treatment plans and their underlying CTs, representative beam paths are selected and assembled as one-dimensional inhomogeneous targets built from tissue equivalent materials. These phantoms have been irradiated with monoenergetic proton pencil beams. The accuracy of range deviation estimation as well as the detectability of range shifts is investigated in different scenarios. In most cases, range deviations can be detected within less than 2 mm. In close vicinity to low-density regions, range detection is challenging. In particular, a minimum beam penetration depth of 7 mm beyond a cavity is required for reliable detection of a cavity filling with the present setup. Dedicated data post-processing methods may be capable of overcoming this limitation.

SPARSE: Seed Point Auto-Generation for Random Walks Segmentation Enhancement in medical inhomogeneous targets delineation of morphological MR and CT images.

In medical image processing, robust segmentation of inhomogeneous targets is a challenging problem. Because of the complexity and diversity in medical images, the commonly used semiautomatic segmentation algorithms usually fail in the segmentation of inhomogeneous objects. In this study, we propose a novel algorithm imbedded with a seed point autogeneration for random walks segmentation enhancement, namely SPARSE, for better segmentation of inhomogeneous objects. With a few user‐labeled points, SPARSE is able to generate extended seed points by estimating the probability of each voxel with respect to the labels. The random walks algorithm is then applied upon the extended seed points to achieve improved segmentation result. SPARSE is implemented under the compute unified device architecture (CUDA) programming environment on graphic processing unit (GPU) hardware platform. Quantitative evaluations are performed using clinical homogeneous and inhomogeneous cases. It is found that the SPARSE can greatly decrease the sensitiveness to initial seed points in terms of location and quantity, as well as the freedom of selecting parameters in edge weighting function. The evaluation results of SPARSE also demonstrate substantial improvements in accuracy and robustness to inhomogeneous target segmentation over the original random walks algorithm.PACS number: 87.57.nm

132: Monitoring of carbon ion beams using secondary ions: investigations in inhomogeneous targets

Purpose: Radiation therapy with ion beams provides highly conformal dose distributions. These are sensitive to possible variations of the beam settings and changes of the tissue in the beam path, which can deteriorate the dose distribution in the patient. To monitor the ion beam, it was suggested in [Amaldi et al. NIM A 617, 2010] to exploit the information carried by prompt secondary ions produced within the patient during irradiation. Our initial experimental investigations of monitoring the beam position, width and energy in a homogeneous phantom have been published in [Gwosch et al. PMB 58, 2013]. In this contribution we address the performance of the method in phantoms with inhomogeneities. Materials and Methods: The experiments were performed at the Heidelberg Ion-Beam Therapy Center using beam energies and widths typical for therapy. Secondary charged particles emerging from phantoms during irradiation were registered by the pixelated semiconductor detector Timepix [Llopart et al. NIM A 581, 2007], which was developed by the Medipix Collaboration at CERN. Its multi-layered version (3D voxel detector) [Soukup et al. JINST 6, 2011] was used to determine the direction of single particles. Using an anthropomorphic head phantom containing real bones, it was tested whether the distributions of secondary ions around the phantom are sensitive to variations of the beam parameters. In a plastic slab phantom it was studied how changes of its structure affect the secondary ion distribution. Experiments and Results: The measured particle directions were projected to the vertical beam plane. An example of such a projection is shown in figure 1 (inset). Changes of this projection with the beam parameters and phantom structure were analysed. This new imaging modality [Jakubek IEEE, 2011] was found to be capable of detecting lateral beam shifts, changes of the beam energy (affecting the range) and beam width (see figure 1) of a few millimeters in the head phantom. In a homogeneous plastic phantom, missing slabs of 1cm thickness were found to influence the distribution of secondary ions significantly. Conclusion: The presented experimental investigations demonstrate that measurements of charged particle tracks around phantoms irradiated with carbon ion beams enable to detect missing slices of 1cm thickness in an otherwise homogeneous phantom. In the head-phantom it was shown that the method provides an attractive source of information on the actual beam delivery, despite the inherent tissue inhomogeneities.

Interaction of the radiation of the high-power ytterbium-fiber laser with inhomogeneous dielectric targets

The action of the radiation of the ytterbium-fiber laser (λ = 1.07 μm) on the Nd3+Y2O3 target with nonuniform transparency in the course of the nanopowder production is studied. It is demonstrated that the laser irradiation leads to an extremely rough surface with the stalagmite roughness due to a relatively large melting depth. The resulting powder consists of two fractions. The first fraction (99% of the total mass of the powder) consists of nanoparticles with a mean size of 29 nm (BET data). The second fraction consists of micro- and submicroparticles that represent circular drops condensed from the melt and shapeless debris of the target. The peaks on the diameter distribution of the drops at 2, 8, and 80 μm are determined by different effects. The laser heating of the inhomogeneous target with the nonlinear refractive index is numerically analyzed. It is demonstrated that the melting of the target is initiated at a mean laser power of 700 W, a power density of 5.6 × 105 W/cm2, and an irradiation time of 150 μs.

Photoelectron emission for layers of finite thickness

Basic tools that describe the energy and angular distributions of photoelectrons emitted by inhomogeneous targets are presented. These are the functions of reflection and transmission, the function of photoelectron emission, the equation for photoelectron flow, and the equation for the transmission function, which describes the dynamics of the angular and energy spectra of photoelectrons when they move in the tar-get. The determining influence of the effects of multiple elastic scattering on the angular distribution of photoelectrons is shown. It is demonstrated in which way and by how much the effect of brightness body rotation, well known in optics, influences the X-ray photoelectron spectroscopy signal. Along with analytical solutions, the results of simulation of the X-ray photoelectron spectroscopy signals are presented.