Inhomogeneous Region Research Articles

TH‐EF‐BRB‐06: Implementation of a Modulated‐Arc Total Body Irradiation (TBI) Technique Using the RayStation Treatment Planning System

Purpose:To develop a clinical workflow for delivering a modulated‐arc total body irradiation (TBI) with RayStation scripting. This technique uses arc fields with the patient lying at floor level on a padded table and is validated through measurements taken on a custom‐made TBI phantom.Methods:Treatment planning was performed for a retrospective cohort of eight patients with a diverse range of heights and body types. Each was replanned using an open‐field dual arc method, with the patient in supine and prone positions on the floor of the vault. All plans were optimized using Raystation Planning 4.7.2.5 (RaySearch Laboratories, Stockholm, Sweden), with 200 cGy prescribed to the 95% of the body contour − 5mm. This results in an open‐field beam that sweeps craniocaudally across the length of the patient. The technique is validated with measurements at 10 cm intervals in a custom‐milled, 5 cm thick acrylic phantom. A centrally located CC13 ion chamber and a Mobile MOSFET (Best Medical Canada, Ottawa, ON) detector array were used to measure dose. Supine and prone arcs for each patient were consecutively delivered, and the aggregate dose at each point was compared to the planned dose calculated in the phantom.Results:The ion chamber measurements differed from the planned dose by an average of .5%, with a standard deviation of 2.1%. All measured data for the MOSFETS were within 10% of the corresponding planned dose except for two outlying points. The standard deviation of dose differences across the entire cohort was 4.0%. Most significant discrepancies occurred either in inhomogeneous regions with large gradients, or at inferior points where beam angle was steepest.Conclusion:We have confirmed that the planned dose is well matched to our measurements within 10% for this method of planning and delivery. We are currently incorporating this technique into our clinical workflow.This work is supported by RaySearch.

TU‐H‐206‐03: Characterizing B1 Inhomogeneities in DCE MRI

Purpose:Dynamic Contrast Enhanced (DCE) MRI is a valuable technique for measuring perfusion and permeability characteristics of tumors. Exogenous contrast concentrations are calculated based on changes in T1 measured using fast 3D gradient echo (FLASH) sequences. However, the slab selective pulses used in 3D MRI may result in B1 inhomogeneities across the volume of interest that can lead to errors in T1 and thus the estimated gadolinium concentration. We compared three FLASH DCE sequences (GRE, TWIST, and VIBE) to determine their signal homogeneity across slices and the accuracy in calculating T1 using acquisitions with variable flip angles.Methods:The sequences were tested at 3 T on a Siemens mMR (VB20P) using a doped water phantom 3.75 g/L NiSO4 ‐ 6H2O + 5 g/L NaCl (T1 = 104 ms) and a 2% agar, 0.67% NaCl phantom (T1= 1.71 s). 2D EPI B1 maps and inversion recovery T1maps were acquired for ground truth. 3D MRI was acquired at different flip angles to generate a T1 map. Regions of interest were drawn to measure signal inside the phantoms as a function of slice position. The T1 for each slice ROI was fit to the FLASH steady‐state model of magnetization.Results:Based on the data, GRE gave the most uniform signal homogeneity and T1 values in the middle slices of the 3D volume. The 3D VIBE sequence had the largest region of signal inhomogeneity compared to the 3D GRE and TWIST sequences. VIBE's B1 inhomogeneity is inconsistent at low flip angles. However, VIBE resulted in more slices with T1 values similar to the ground truth.Conclusion:The central 1/3 of the slices yielded signals that result in T1 fits consistent with the ground truth. However, the remaining slices required some form of B1 inhomogeneity correction for quantitative DCE analysis.The research was supported in part by NIH NCI Grant R01CA159471.

Improving Intra Prediction in High-Efficiency Video Coding.

Intra prediction is an important tool in intra-frame video coding to reduce the spatial redundancy. In current coding standard H.265/high-efficiency video coding (HEVC), a copying-based method based on the boundary (or interpolated boundary) reference pixels is used to predict each pixel in the coding block to remove the spatial redundancy. We find that the conventional copying-based method can be further improved in two cases: 1) the boundary has an inhomogeneous region and 2) the predicted pixel is far away from the boundary that the correlation between the predicted pixel and the reference pixels is relatively weak. This paper performs a theoretical analysis of the optimal weights based on a first-order Gaussian Markov model and the effects when the pixel values deviate from the model and the predicted pixel is far away from the reference pixels. It also proposes a novel intra prediction scheme based on the analysis that smoothing the copying-based prediction can derive a better prediction block. Both the theoretical analysis and the experimental results show the effectiveness of the proposed intra prediction method. An average gain of 2.3% on all intra coding can be achieved with the HEVC reference software.

Computed tomography and magnetic resonance features of intracranial hemangioendothelioma: A study of 7 cases.

The current study aimed to present the neuroradiological and histopathological features of intracranial hemangioendothelioma (HE). The computed tomography (CT; n=3) and magnetic resonance imaging (MRI; n=7) features, and the clinical presentations of 7 patients with pathologically documented HEs were retrospectively analyzed. Lesions were observed in the right side of the skull (the frontal bone in 1 patient and the parietal bone in 1 patient), the tentorium (2 patients), the cerebral falx (1 patient), the right cavernous sinus (1 patient) and the right temporal lobe (1 patient). The tumor was lobulated in 5 cases and round in 2 cases. The majority of tumors appeared isointense or hypointense with multiple scattered hyperintensities on T1-weighted MRI. Moreover, the lesions appeared as inhomogeneous hyperintense regions with multiple enlarged and tortuous blood flow voids on T2-weighted MRI. The lesions also showed marked gadolinium enhancement in a honeycomb pattern. CT scan results showed a isoattenuation region (32-47 HU), with numerous small, round, high-density foci. The 2 cases with skull lesions presented with local bone destruction and discontinuous bone lines of the tabula interna ossis cranii. In 1 case, MR angiography revealed abnormal vessels in the basilar region. A total of 4 cases were epithelial HE, 2 were retiform HE and 1 was kaposiform HE. Histological examination revealed endothelial cell proliferation with vascular lesions and a mucous matrix or dense fibrous mesenchyme. In conclusion, intracranial HE is rare, but should be considered in the differential diagnosis when evaluating intracranial neoplasms. A well-defined lobulated mass and imaging features that include internal heterogeneity, small scattered hemorrhages and thromboses, signal voids of vessels, and marked and delayed enhancement may confirm the diagnosis of HE.

Wave Front Sets of the Buldyrev Head Wave and Whispering Gallery Waves

The diffraction of waves, produced by a point source, on an inhomogeneous half-plane is considered. The square of the wave number inside an inhomogeneous region is assumed to decrease linearly with the distance from the division boundary. Using the results obtained earlier, the Buldyrev head wave and whispering gallery waves are analyzed.

Comparison of selected dose calculation algorithms in radiotherapy treatment planning for tissues with inhomogeneities

Inhomogeneity correction is recommended for accurate dose calculation in radiotherapy treatment planning since human body are highly inhomogeneous with the presence of bones and air cavities. However, each dose calculation algorithm has its own limitations. This study is to assess the accuracy of five algorithms that are currently implemented for treatment planning, including pencil beam convolution (PBC), superposition (SP), anisotropic analytical algorithm (AAA), Monte Carlo (MC) and Acuros XB (AXB). The calculated dose was compared with the measured dose using radiochromic film (Gafchromic EBT2) in inhomogeneous phantoms. In addition, the dosimetric impact of different algorithms on intensity modulated radiotherapy (IMRT) was studied for head and neck region. MC had the best agreement with the measured percentage depth dose (PDD) within the inhomogeneous region. This was followed by AXB, AAA, SP and PBC. For IMRT planning, MC algorithm is recommended for treatment planning in preference to PBC and SP. The MC and AXB algorithms were found to have better accuracy in terms of inhomogeneity correction and should be used for tumour volume within the proximity of inhomogeneous structures.

Broadband wide-angle absorption enhancement due to mode conversion in cold unmagnetized plasmas with periodic density variations

We study theoretically the mode conversion and the associated resonant absorption of p-polarized electromagnetic waves into longitudinal plasma oscillations in cold, unmagnetized and stratified plasmas with periodic spatial density variations. We consider sinusoidal density configurations for which the frequency band where mode conversion occurs is well included within a transmission band of the one-dimensional plasma photonic crystal. We calculate the mode conversion coefficient, which measures the fraction of the electromagnetic wave energy absorbed into the plasma, and the spatial distribution of the magnetic field intensity for various values of the wave frequency and the incident angle using the invariant imbedding theory of mode conversion. We find that the absorption is greatly enhanced over a wide range of frequency and incident angle due to the interplay between the mode conversion and the photonic band structure. The enhancement occurs because for frequencies within a transmission band, the wave reflection is strongly suppressed and the waves penetrate more deeply into the inhomogeneous region, thereby increasing the possibility for them to reach many resonance points where the dielectric permittivity vanishes.

Origin of the 3.331 eV emission in ZnO nanorods: Comparison of vapour phase transport and pulsed laser deposition grown nanorods

In this work, we report the growth of vertically aligned ZnO nanorods with excellent optical quality by both catalyst free vapour phase transport (VPT) and catalyst free pulsed laser deposition (PLD). We compare the near band edge emission of such deposits, with a focus on the identification of the origin of the 3.331eV emission feature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and low-temperature (13K) photoluminescence (PL) were used to characterise these nanorod deposits. XRD and SEM data reveal that both techniques lead to highly textured ZnO nanorod arrays with uniform c-axis orientation normal to the substrate surface. The VPT-grown nanorods are well separated and show smooth, facetted surfaces whereas the PLD-grown nanorods are more closely packed and display comparatively rougher surfaces. The optical quality of the samples obtained by both growth methods was very good and low-temperature PL spectra were dominated in both cases by a strong I6 bound exciton (BX) emission (3.36eV), and also showed emission from the surface exciton and the free exciton. A comparatively weak visible emission was also observed in samples deposited by both techniques. The main difference between the PLD- and VPT-grown nanorod samples is the presence of the 3.331eV emission in the former, and its complete absence in the latter (as well as in continuous PLD-grown seed layers) which is discussed in light of the differing surface morphologies mentioned above and which provides strong support for our previous assignment of the origin of this defect to structural defects in the inhomogeneous sub-surface region close to the rough nanorod surface.

From Alfvén waves to kinetic Alfvén waves in an inhomogeneous equilibrium structure

AbstractKinetic Alfvén waves are believed to primary form fluctuations in a hydromagnetic turbulence at scales of the order of the ion inertial length. We study a model where an initial Alfvén wave propagates inside an equilibrium structure which is inhomogeneous in the direction perpendicular to the equilibrium magnetic field. In a previous paper this situation has been considered in a particular configuration where the initial wave vector is parallel to the magnetic field and the wave is polarized perpendicular to the inhomogeneity direction. Here we consider other configurations, with a different polarization and possible initial oblique propagation. We employ numerical simulations, using both a Hall‐magnetohydrodynamics and a Hybrid Vlasov‐Maxwell model. Results show that in all the considered cases the time evolution leads to the formation of kinetic Alfvén waves within the inhomogeneity regions, which are identified by a comparison with analytical linear theory results. Then, in this context the formation of kinetic Alfvén waves seems to be a general phenomenon which could be also extended to more complex situations, like turbulence. Kinetic simulations show that kinetic Alfvén waves modify the ion distribution function, generating temperature anisotropy of both parallel and perpendicular to the local magnetic field as well as particle beams aligned along the local magnetic field. These results could be relevant both in the solar corona and in large‐scale structures of the solar wind, where Alfvénic fluctuations are present along with large‐scale inhomogeneities.

Grain size and interfacial interdiffusion influence on the magnetic and dielectric properties of magnetoelectric La0.7Ba0.3MnO3–BaTiO3 composites

We report on specific features of the dielectric and magnetic properties as well as magnetoelectric coupling coefficients (αME) of the La0.7Ba0.3MnO3 (L)-BaTiO3 (B) 2–2 type ceramic composite. The powder of L and B was synthesized by two different methods (solid state reaction and Pechini). Orthorhombic and tetragonal phases were observed for the separated phases L and B of the composite, respectively, for both synthesis methods. The characteristics of grain size and interfacial interdiffusion in the L–B–L composite obtained for different synthesis method were studied, showing that diffusion was a typically physical migration, which can be mainly controlled by the grain size. Anomalies in the observed dielectric behavior are attributed to the internal residual stresses and chemically inhomogeneous regions. The existence of a broad magnetic transition observed in the pure L phase and laminated L–B–L composite was also attributed to its small grain size. A comparison of the maximum transversal (α31ME) and longitudinal (α33ME) ME coupling coefficients, at room temperature, is also shown.

Dosimetric impact of dental metallic crown on intensity-modulated radiotherapy and volumetric-modulated arc therapy for head and neck cancer.

Metal dental restoration materials cause dose enhancement upstream and dose disturbance downstream of the high‐density inhomogeneous regions in which these materials are used. In this study, we evaluated the impact of a dental metallic crown (DMC) on intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for head and neck cancer. Additionally, the possibility of sparing the oral mucosa from dose enhancement using an individual intraoral mouthpiece was evaluated. An experimental oral phantom was designed to verify the dosimetric impact of a DMC. We evaluated the effect on single beam, parallel opposing beam, arc beam, IMRT, and VMAT treatment plans. To evaluate the utility of a 3‐mm‐thick intraoral mouthpiece, the doses across the mouthpiece were measured. For single beam irradiation, the measured doses at the entrance and exit planes of the DMC were 51% higher and 21% lower than the calculated dose by the treatment planning system, respectively. The maximum dose enhancements were 22% and 46% for parallel opposing beams and the 90° arc rotation beam, respectively. For IMRT and VMAT, the measured doses adjacent to the DMC were 12.2%±6.3% (mean±1.96 SD) and 12.7%±2.5% higher than the calculated doses, respectively. With regard to the performance of the intraoral mouthpiece for the IMRT and VMAT cases, the disagreement between measured and calculated doses at the outermost surface of the mouthpieces were −2.0%, and 2.0%, respectively. Dose enhancements caused by DMC‐mediated radiation scattering occurred during IMRT and VMAT. Because it is difficult to accurately estimate the dose perturbations, careful consideration is necessary when planning head and neck cancer treatments in patients with DMCs. To spare the oral mucosa from dose enhancement, the use of an individual intraoral mouthpiece should be considered.PACS numbers: 87.55.km, 87.55.N‐, 87.55.Qr

Oscillatory frequencies in spatiotemporal system with local inhomogeneity

Target waves usually emit concentric circular waves, whereas spiral waves rotate around a central core (topological defect) region, the two forms of waves are closely related due to the similarity of their spatial structures. Spiral waves can be generated spontaneously in a homogeneous system, while target waves usually cannot be self-sustained in the same system. Therefore, spiral waves can be found in diverse natural systems, and target waves can be produced from the spirals with special boundary configurations or central pacemakers. The pacemaker of target wave is an oscillatory source or medium inhomogeneity. To model the inhomogeneity in some realistic situations, we introduce local parameter shifts and simulate the transition from spiral waves to target waves. In this research, the evolution of the spiral waves in the complex Ginzburg-Landau equation is investigated by numerical simulations, and the multi-spiral patterns can be transformed into stable target waves with local inhomogeneous parameter shifts in a two-dimensional (2D) spatiotemporal system. The detailed study shows that the initial multi-spiral waves can be influenced by introducing inhomogeneity in the local area of the system space, and the oscillatory frequency of the system plays an important role in changing the pattern. A successful transition from inwardly propagating spirals to target waves can be observed when the oscillatory frequencies of non-controlled and local inhomogeneous region, which have equal values, are both less than the inherent frequency of system. When we inspect the relationship between oscillatory frequencies and the characteristics of the inhomogeneous region, an intriguing V-shaped line is found in parameter-frequency diagram, and the V-shaped area presents three features. Firstly, the left and right sides of the V-shaped area are symmetrical. Secondly, the propagating directions of target waves from the left and right sides are opposite. An inwardly propagating target wave is formed on the left side of the V-shaped area, and an outwardly propagating target wave stably exists on the right side of the line. Thirdly, as local inhomogeneous parameter 2 increases, the V-shaped area moves towards the local inhomogeneous parameter 2 and decreases simultaneously, and the width of the V-shaped area remains approximately the same. To our knowledge, this V-shaped line is a novel observation, hence the changes of the system frequencies are thought to be provoking. This work presents the numerical experiments and theoretical analyses for the stable conditions of target waves, and therefore provides the ideas in the applications of signal propagation and mode competition.

Two-phase fluid flow in geometric packing.

We investigate how a plug of obstacles inside a two-dimensional channel affects the drainage of high viscous fluid (oil) when the channel is invaded by a less viscous fluid (water). The plug consists of an Apollonian packing with, at most, 17 circles of different sizes, which is intended to model an inhomogeneous porous region. The work aims to quantify the amount of retained oil in the region where the flow is influenced by the packing. The investigation, carried out with the help of the computational fluid dynamics package ANSYS-FLUENT, is based on the integration of the complete set of equations of motion. The study considers the effect of both the injection speed and the number and size of obstacles, which directly affects the porosity of the system. The results indicate a complex dependence in the fraction of retained oil on the velocity and geometric parameters. The regions where the oil remains trapped is very sensitive to the number of circles and their size, which influence in different ways the porosity of the system. Nevertheless, at low values of Reynolds and capillary numbers Re<4 and n(c)≃10(-5), the overall expected result that the volume fraction of oil retained decreases with increasing porosity is recovered. A direct relationship between the injection speed and the fraction of oil is also obtained.

Adaptive method for the detection of infrared small target

Background suppression is an important problem in infrared small target detection. The two-dimensional least mean square (TDLMS) filter is a widely used method, but its performance will decline when targets are embedded in a complex cluttered background. To fill the gap, variable step-size TDLMS, the neighborhood analysis technique, and the edge-directional TDLMS filter are developed but still cannot achieve a satisfying performance. Here, an adaptive method for background suppression is proposed. According to different characteristics of the pixels in homogeneous/target regions and inhomogeneous regions, two basic filters are first designed. Then a fuzzy edge estimation factor is introduced to combine them into a uniform framework, in which the two basic filters can be switched automatically to fit different kinds of pixels. Finally, a new mechanism to update and propagate the coefficients of the prediction window is constructed. It makes sure that the adaptive method works smoothly and reveals a potential to be implemented in parallel. The experimental results demonstrate that the proposed method achieves significant improvement in background suppression and detection performance.

Simulation of therapeutic ultrasound treatments using the hybrid angular spectrum technique

Numerical simulations play an important role in therapeutic ultrasound treatments. Simulations can help with transducer design, retroactively analyze temperature patterns for insight into treatment effectiveness, and ultimately be used for patient treatment planning. Our group has developed a rapid 3D simulation tool for ultrasound beam propagation named the hybrid angular spectrum (HAS) method. HAS is an extension of the traditional angular spectrum approach that uses fast Fourier transforms to alternate between the spatial frequency domain and the space domain as the beam propagates through inhomogeneous tissue regions. In this presentation, we briefly cover the physical and algorithmic principles underlying the HAS technique, then give examples of its use in two promising high-intensity focused ultrasound (HIFU) applications. First, we employ it to retrospectively predict the heating efficiency of transcranial treatments of 14 patients undergoing treatment for essential tremor using a large phased-arra...

On solving 3D elasticity problems for inhomogeneous region with cracks, pores and inclusions

Studying stress concentration and stress intensity factors (SIFs), being of importance for multiple scientific and engineering applications, we focus on developing an efficient means for accurate, robust and stable evaluation of these quantities. 3D elasticity problems for a piece-wise homogeneous region (finite or infinite), containing cracks, pores and inclusions, are considered. The combination of singular and hypersingular boundary integral equations, specially tailored for such problems, is used as start point. The boundary is represented by four-vertex, in general curvilinear, boundary elements. An element is transformed into a plane trapezoid by a transformation with Jacobian expanded into a power series in local coordinates. In particular cases, the trapezoidal element may be a parallelepiped, rectangle, square or triangle. Higher order approximation of the density, accounting for its asymptotic behavior near a crack edge, is employed to increase the accuracy of SIF evaluation. We show that using power-type asymptotics results in recurrent equations. They reduce evaluation of influence coefficients to arithmetic operations with relatively small number of starting integrals over the element. In the case of zero exponent, an element is ordinary (non-singular). Then, the number of starting integrals is at most 3 and we present analytical formulae for their evaluation. In the case of an arbitrary exponent, the number of starting integrals is at most 8 and they are evaluated numerically. In the case of square-root asymptotics, important for fracture mechanics, hydraulic fracturing and mining applications, the number of starting integrals is 5, and we show that they can be evaluated by using the highly efficient algorithms suggested. Then the approach developed provides drastic (an order) accuracy increase for SIF evaluation under practically insignificant growth of the computation cost as compared with using ordinary elements only. The remarkable feature of the method developed is that being of higher order accuracy it evaluates the influence coefficients almost analytically in many practically important cases involving ordinary elements and singular edge elements. Known benchmark solutions serve us to check the accuracy and stability of the method and to make conclusions on a reasonable form and on the sizes of edge elements. The examples demonstrate the potential of the method for solving problems under consideration.

Near-Field Tomography and Holography of Low-Contrast Subsurface Objects

We develop a new method for electromagnetic computer tomography of distributed subsurface inhomogeneities and holography (i.e., retrieval of the form) of internally homogeneous subsurface objects. The method is based on solving the inverse problem of near-field scattering with allowance for the data of multifrequency measurements of the two-dimensional distribution of the scatteredfield complex amplitudes along the medium surface over the region of inhomogeneities.

Comparison of pencil-beam, collapsed-cone and Monte-Carlo algorithms in radiotherapy treatment planning for 6-MV photons

Treatment planning system calculations in inhomogeneous regions may present significant inaccuracies due to loss of electronic equilibrium. In this study, three different dose calculation algorithms, pencil beam (PB), collapsed cone (CC), and Monte-Carlo (MC), provided by our planning system were compared to assess their impact on the three-dimensional planning of lung and breast cases.

Deformation Behavior of the Zr53.5Cu26.5Ni5Al12Ag3 Bulk Metallic Glass Over a Wide Range of Strain Rate and Temperatures

The stress–strain relations for the Zr53.5Cu26.5Ni5Al12Ag3 bulk metallic glass (BMG) over a broad range of temperatures (room temperature to its supercooled liquid region) and strain rates (10−4 to 10−1 s−1) were established in uniaxial compression using a thermal-mechanical simulation system. The superplastic flow was seen above its glass transition temperature (Tg = 694 K) and strain rates of up to 10−1 s−1 from the variation of stress–strain curves. A deformation map of strain rate vs temperature of Zr53.5Cu26.5Ni5Al12Ag3 was obtained, which was mainly composed of homogeneous and inhomogeneous deformation regions, the former featuring either Newtonian or non-Newtonian flow while the latter characterizing linear elastic behavior followed by shear localization, respectively. A phenomenological constitutive equation used to describe a master curve of viscosity with respect to the strain rate was obtained by fitting the experimental results, which determines the viscosity of the present alloy at the temperature near and above Tg. The results show the Zr53.5Cu26.5Ni5Al12Ag3 BMG is the subject suitable for net shape forming process at the supercooled liquid region.

Origin of room temperature weak-ferromagnetism in antiferromagnetic Pb(Fe2/3W1/3)O3 ceramic

We report the origin of room temperature weak ferromagnetic behavior of polycrystalline Pb(Fe2/3W1/3)O3 (PFW) powder. The structure and magnetic properties of the ceramic powder prepared by a Columbite method were characterized by X-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements. Rietveld analysis of diffraction data confirm the formation of single phase PFW, without traces of any parasitic pyrochlore phase. PFW was found to crystallize in the cubic structure at room temperature. The Rietveld refinement of neutron diffraction data measured at room temperature confirmed the G-type antiferromagnetic structure of PFW in our sample. However, along with the antiferromagnetic (AFM) ordering of the Fe spins, we have observed the existence of weak ferromagnetism at room temperature through: (i) a clear opening of hysteresis (M–H) loop, (ii) bifurcation of the field cooled and zero-field cooled susceptibility; supported by Mössbauer spectroscopy results. The P–E loop measurements showed a non-linear slim hysteresis loop at room temperature due to the electronic conduction through the local inhomogeneities in the PFW crystallites and the inter-particle regions. By corroborating all the magnetic measurements, especially the spin glass nature of the sample, with the conduction behavior of the sample, we report here that the observed ferromagnetism originates at these local inhomogeneous regions in the sample, where the Fe-spins are not perfectly aligned antiferromagnetically due to the compositional disordering.