Inhomogeneous Variation Research Articles

Improved treatment of the pelvis and inguinal nodes using the modified segmental boost technique: dosimetric evaluation

Purpose/Objective: To describe a novel, yet simple modified segmental boost technique (MSBT) for treatment of malignancies requiring irradiation of the pelvis and inguinal nodes. We also compare the dose inhomogeneity of our technique with other traditional methods of treatment for an extended pelvic/winged radiation field. Materials/Methods: A modification of a previously described segmental boost technique (SBT)(1) was developed to decrease “hot spots” that are commonly present at the matchline between the pelvic and inguinal boost fields. At our institution, patients have been simulated and treated in the following manner: The patient is placed supine with the isocenter at midline (Fig 1). Initially, the superior, inferior, and lateral borders of the posterior field are defined by the clinician and resemble a standard pelvic field (PA). The anterior field (AP) is then defined using the same superior/inferior borders and opening the lateral jaws wider to encompass the inguinal/femoral nodes. Two additional anterior inguinal fields, right anterior oblique (RAO) and left anterior oblique (LAO), are added for treating the inguinal nodes. The RAO and LAO are shaped by moving the multileaf collimators (MLC) across the central axis to the lateral edge of the exiting PA field (SBT). The dose inhomogeneity can be significantly reduced by simply angling the gantry for the inguinal fields so that the medial borders match the divergence of the lateral borders of the PA field (determined through field geometry or CT based treatment planning). Film dosimetry using Ready Pack Kodak EDR2 film was performed using our MSBT as well as other traditional techniques including: 1. Segmental boost technique (SBT) 2. Partial transmission block/compensator (PTB) 3. Photon pelvis with electron tags (P-E) Both vertical and horizontal films were exposed in a Virtual WaterTM phantom using beam arrangements specific to each technique. The films were scanned to produce isodose curves and computerized dose distributions were determined for each technique to allow for comparative dosimetry. Results: A comparison of the variation of dose inhomogeneity with depth and location within the treatment volume for SBT and MSBT is shown in Table 1. Data analysis of the techniques showed comparable maximum doses (115-122%), within the target volume for MSBT, PTB, and P-E while the maximum dose for the SBT was 146%. The maximum dose occurred at a depth of 1.5 cm along the matchline between the inguinal fields and the posterior pelvic field. Conclusions: The MSBT significantly improved the dose homogeneity at the matchline in comparison to the SBT. The PTB and P-E techniques have comparable dose homogeneity but are technically more complicated and in our clinical experience, require more time for both simulation and daily set up. Our MSBT proves to be technically simple while optimizing dose homogeneity and allows for maximal utilization of the features of modern linear accelerators. 1. Watson, Barbara; et al. Use of Segmental Boost Fields in the Irradiation of Inguinal Lymphatic Nodes. Medical Dosimetry 24(1): 27-32;1999. Tabled 1Depth (cm)Segmental Boost (SBT)Modified Segmental Boost (MSBT)Average Dose (%)Matchline Dose (%)Dose InhomogeneityAverage Dose (%)Matchline Dose (%)Dose Inhomogeneity1.5 (inguinal field)1181461.241161231.065.0 (inguinal field)1001321.321001121.1210.0 (pelvic field)100N/AN/A100N/AN/A Open table in a new tab

Variations of alpha in space and time

We study inhomogeneous cosmological variations in the fine structure `'constant'' $\ensuremath{\alpha}$ in Friedmann universes. Inhomogeneous motions of the scalar field driving changes in $\ensuremath{\alpha}$ display spatial oscillations that decrease in amplitude with increasing time. The inhomogeneous evolution quickly approaches that found for exact Friedmann universes. We prove a theorem to show that oscillations of \ensuremath{\alpha} in time (or redshift) cannot occur in Friedmann universes in the Bekenstein-Sandvik-Barrow-Magueijo theories considered here.

Thermodynamics of catalytic formation of dimethyl ether from methanol in acidic zeolites.

We present a theoretical study of the formation of the first intermediate, dimethyl ether, in the methanol to gasoline conversion within the framework of an ab initio molecular dynamics approach. The study is performed under conditions that closely resemble the reaction conditions in the zeolite catalyst including the full topology of the framework. The use of the method of thermodynamic integration allows us to extract the free-energy profile along the reaction coordinate. We find that the entropic contribution qualitatively alters the free-energy profile relative to the total energy profile. Different transition states are found from the internal and free energy profiles. The entropy contribution varies significantly along the reaction coordinate and is responsible for stabilizing the products and for lowering the energy barrier. The hugely inhomogeneous variation of the entropy can be understood in terms of elementary processes that take place during the chemical reaction. Our simulations provide new insights into the complex nature of this chemical reaction.

Analysis of anisotropy of cosmic rays with the energy of about 1017 eV by Yakutsk EAS array data

A harmonic analysis of the directions of arrival of cosmic-ray particles with an energy of about 1017 eV in the vicinity of the registration threshold of the Yakutsk extensive air showers (EAS) array is given. A method for determining the contribution of inhomogeneous observation conditions and seasonal variations of the frequency of extensive air showers to the observed anisotropy is suggested. Taking into account these factors results in a considerable decrease of the amplitude characterizing the degree of anisotropy of cosmic-ray primaries. The amplitude of the first harmonic with respect to the right ascension is (0.45 ± 0.55)%, which shows that no probably significant anisotropy of the primary radiation is observed at 1017 eV.

Spectroscopic ellipsometry of TiO 2 layers prepared by ion-assisted electron-beam evaporation

Single layer TiO 2 films deposited on BK7 glass substrates by the ion-assisted electron-beam evaporation technique have been characterized by phase modulated spectroscopic ellipsometry. The ellipsometry spectra were recorded in the wavelength range of 300–1000 nm. The measured spectra were then fitted with theoretically simulated curves generated assuming different model sample structures. Analyses were carried out separately in three different wavelength regimes, the transparent regime with no dispersion of refractive index (650–1000 nm), the transparent regime with dispersion of refractive index (400–650 nm) and the absorbing regime (300–450 nm). Modeling has been attempted with both homogeneous and inhomogeneous sample structures. For the inhomogeneous structure, both linear and non-linear variation of refractive index along the depth of the sample were used. Refractive indices of the samples were determined separately from the best-fit ellipsometric data in the above three wavelength regimes. Finally, variation of refractive index with the variation in ion beam currents has been studied.

The characteristics of variation in inhomogeneity of temporal-spatial distribution of short to imminent precursors before strong earthquakes

C v is used in this paper to describe the variation in inhomogeneity of spatial and temporal distribution of precursors. The inhomogeneous variation in the spatial and temporal distribution of the anomalies in water radon and electromagnetic waves before M s≈6 earthquakes in northern North China is analyzed in detail. Results show that before a moderate to strong earthquake, the distribution of anomalies in water radon and electromagnetic waves changes inhomogeneously, i.e., C v increases significantly. The inhomogeneity in spatial distribution of short-to-imminent precursors increases before a strong earthquake, which may relates to the enhancement of crustal strain field. The research will not only help us to know more about the process of seismogeny and to improve practical earthquake prediction, but also blaze a new way to do earthquake prediction with present precursor data.

Compensation of nonlinear thermal bias drift of Resonant Rate Sensor using fuzzy logic

In this paper, our attention is focused on the compensation of the nonlinear thermal bias (zero-rate-output) drift of RRS (Resonant Rate Sensor), which originates from a number of sources, including manufacturing tolerances, material inhomogeneity and inevitable mechanical characteristic variation of the cylinder with temperature. Motivated by the capability of fuzzy logic in managing nonlinearity, the nonlinearity of bias was represented by Takagi–Sugeno (TS) fuzzy model over the entire range of the operating temperature. Then, the fuzzy model was directly used for compensation of nonlinear bias drift by subtracting the estimated output from the raw data of RRS. By doing this, we can guarantee robust (against temperature variations) sensor performance throughout the entire operating temperature range.

Manufacture and Testing of a Functionality Graded Material

A novel technique is presented for the fabrication and fracture testing of large-scale polymeric based Functionality Graded Materials (FGMs). The technique generates a continuously inhomogeneous property variation by taking advantage of the susceptibility of a polyethylene carbon monoxide copolymer (ECO) to ultraviolet irradiation. The resulting FGMs exhibit a varying Young’s modulus, usually in a linear fashion, from approximately 160 MPa to 250 MPa over 150 mm wide specimens. The fracture behaviour of the FGM is experimentally investigated through the use of single edge notch fracture tests on both homogeneously irradiated and functionally graded ECO. Two approaches are used to evaluate fracture parameters: The first, a hybrid numerical-experimental method, is based on far field measurements only. The second uses digital image correlation to obtain near tip measurements. The energy release rates of uniformly irradiated ECO and of several FGMs are measured and compared. It was seen that the FGM showed a built-in fracture resistance behavior implying that it requires increased driving force to sustain crack growth.

A novel technique for the fabrication of laboratory scale model functionally graded materials

In this work, the authors describe the design, fabrication and testing of model functionally graded materials (FGMs). The inhomogeneous property variations were generated by altering material properties through selective ultraviolet (UV) irradiation. Poly(ethylene co-carbon monoxide) (ECO) was chosen to make the FGMs because of its rapid degradation under UV light. Irradiated ECO becomes stiffer, stronger and more brittle with increasing irradiation time. Through a series of tension tests, the authors characterized the mechanical behavior of the specific ECO used as a function of UV exposure time. Furthermore, by controlling exposure time, specimens with continuously and discretely varying mechanical properties were produced. The resulting graded materials exhibited a Young's modulus that varied from about 160 MPa to 250 MPa and a strain to failure that varied from about 900 percent to 10 percent over the width in a 150 mm wide specimen. Microhardness measurements were used to determine the differences between discretely and continuously varying mechanical properties.

Superconducting Inhomogeneity and Local Field Variation in YBa2Cu3O7-δInvestigated through ESR Observations

Inhomogeneous ESR broadening due to local field variation was studied in the superconducting YBa 2 Cu 3 O 7-δ . The change in the ESR lineshape was observed for a paramagnetic “spin probe” applied on one surface of the pellet sample. The ESR line just below T c exhibited tendency of asymmetric broadening. However, upon further cooling, the lineshape evolved into double peak structure. The obtained ESR profiles were analyzed with the assumption that respective peaks correspond to two different spin sites with the aid of computer lineshape calculations. The linewidths for both sites increased linearly with temperature below T c . On the other hand, the resonance fields varied with temperature very differently for each site, inferring difference in superconductivity for different spin positions. Mechanisms for the observed ESR variations were discussed along with some crossover points such as superconducting transition or irreversibility temperatures. Electron microscopic observations were also carried out for examining possible origins of the superconducting inhomogeneity.

Estimation of Markov random field prior parameters using Markov chain Monte Carlo maximum likelihood

Recent developments in statistics now allow maximum likelihood estimators for the parameters of Markov random fields (MRFs) to be constructed. We detail the theory required, and present an algorithm that is easily implemented and practical in terms of computation time. We demonstrate this algorithm on three MRF models--the standard Potts model, an inhomogeneous variation of the Potts model, and a long-range interaction model, better adapted to modeling real-world images. We estimate the parameters from a synthetic and a real image, and then resynthesize the models to demonstrate which features of the image have been captured by the model. Segmentations are computed based on the estimated parameters and conclusions drawn.

Carrier Recombination Dynamics of AlxGa1−xN Epilayers Grown by MOCVD

ABSTRACTWe present a comprehensive study of the optical characteristics of AlxGa1−xN epilayers by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. All AlxGa1−xN epilayers were grown by metalorganic chemical vapor deposition and the Al mole fraction (x) was varied from 0 to 0.6. We observed that (i) the full width at half maximum of the PL emission, (ii) the energy difference between the PL emission peak energy and the PLE absorption edge, and (iii) the effective lifetime increase with increasing x. These facts indicate that degree of band-gap fluctuation due to a spatially inhomogeneous Al alloy content distribution increases with increasing x. We observed anomalous temperature-induced emission shift behavior for AlxGa1−xN epilayers, specifically, an S-shaped (decrease-increase-decrease) temperature dependence of the peak energy with increasing temperature. This anomalous temperature-dependent emission behavior was enhanced as the Al mole fraction was increased. Since the band-gap fluctuation in AlxGa1−xN epilayers due to inhomogeneous spatial variations of the Al content increases with increasing Al content, we believe that band-gap fluctuation causes the PL peak energy to deviate from the typical temperature dependence of the energy gap shrinkage. Therefore, the anomalous temperature-induced emission shift can be attributed to energy tail states due to alloy potential inhomogeneities in the AlxGa1−xN epilayers with large Al content.

Assessing the factors affecting SI engine cycle-to-cycle variations at idle

The extent of the cycle-to-cycle variations of charge composition and their contributions to the variations in Gross Indicated Mean Effective Pressure (GIMEP) in a spark ignition (SI) engine at idle are assessed. The methodology is to use a set of charge perturbation experiments to determine the sensitivity of the engine output parameters to the charge composition variations. Then through this sensitivity matrix, the observed engine output variations at idle are inverted to obtain the charge variations. At idle, the air, residual and fuel mass variations are 5.5, 4.4 and 0.64% respectively. The residual gas fluctuation contributes to ∼1/3 (in terms of the square of the coefficient of variations) of the GIMEP variations. The air and fuel fluctuations contribute much less: 7.5 and 4.6%, respectively. The remaining ∼54% is attributed to the variations in flowfield inhomogeneity.

Transfer of nonsorbing solutes to a streambed with bed forms: Laboratory experiments

Experiments were performed to determine the mechanisms and rates of solute exchange between a flowing stream and porous streambed. Mass balance, flow visualisation, and depth profiles of concentration were used to investigate the exchange of an inert tracer in a laboratory flume. The measured mass transfer was compared to the exchange predicted by models of exchange related to bed forms, which are presented in a companion paper. For the initial stages with stationary bed forms or slowly moving bed forms the net exchange was predicted satisfactorily and was dominated by bed form‐induced interstitial flows (pumping). In the initial stages with rapidly moving bed forms the exchange was dominated by scour/deposition as bed forms propagated (turnover). At later times the models of bed form‐related exchange significantly underpredicted the measured exchange. The additional exchange, and the exchange to a flat bed, may be related to bed inhomogeneity or irregular variations in pressure at the bed surface which are not related to bed forms. Such effects are likely to be more pronounced in the field situation.

Phase diagram of domain walls in the cubic superstructures of the fcc lattice

The thermodynamical behavior of non-conservative [100] domain walls is investigated in the L12 and L10 binary alloys, close to their respective first order–disorder transition lines. Calculations are made using an inhomogeneous cluster variation method in the tetrahedron approximation, within the framework of antiferromagnetic Ising Hamiltonian formulation. Critical phenomena are discussed close to the L12–disorder and L10–disorder transitions, and in the vicinity of the triple point. Different kinds of wetting are exhibited in relation to the relative orientations of the outer variants. The following features are examined: (i) the translational domain wall in the L12 phase is found to be wetted by the disordered phase on the coexistence line; (ii) the orientational domain wall between two L10 variants undergoes a first-order dewetting transition close to the triple point; and (iii) the translational domain wall in the L10 phase undergoes a prewetting transition followed by a wetting process up to the transition line.

Alternate k-space sampling in EPI: compensation for T2* and adjustable T2 weighting.

In this work, preliminary results are described for a modification of the MBEST sampling scheme such that image resolution can be increased while preserving image contrast. In this new approach, a single spin-echo is used in sampling k-space. The basic idea relies on acquiring a conventional EPI image from the center of k-space and applying a pi pulse to permit the acquisition of the two outer edges of k-space. Using this new approach, it is possible to obtain an enhancement in EPI image resolution, while reducing the extent of T2* weighting. As a result, the resulting images possess reduced T2* contrast and suffer less signal loss from T2* effects such as spatial variations in susceptibility and field inhomogeneity.

A method to distinguish between chemical shift and susceptibility effects in NMR microscopy and its application to insect larvae.

We propose a simple method of distinguishing Zeeman broadening arising from susceptibility inhomogeneity and chemical shift variation, applicable to NMR microscopy. The method is based on the use of a specially built probe-head in which orthogonal sample alignment is possible using the same radiofrequency (RF) coil. This allows the investigation of alignment effects in image distortion and relies on the fact that the isotropic chemical shift is invariant under reorientation, whereas the susceptibility-related local field will depend strongly on relative orientation of bounding surfaces with the external polarizing field. We apply this approach to the study of a simple phantom, and an insect larva (Spodoptera litura Fabricius), demonstrating in the latter case that susceptibility variations are sufficiently small to allow chemical shift imaging on a scale greater than 1 ppm.

The AAPM/RSNA physics tutorial for residents. Contrast mechanisms in spin-echo MR imaging.

The majority of sequences used in routine clinical magnetic resonance imaging rely on the concepts involving the spin echo. Spin-echo sequences require long acquisition times (1-10 minutes), but compared with faster gradient-recalled echo methods, spin-echo methods are relatively immune to signal loss and distortions from field inhomogeneity and tissue-induced susceptibility variations. Through modifications of intersequence repetition time (TR), echo formation interval (echo time [TE]), and various gradient moments, image contrast can be altered to emphasize tissue relaxation times T1, T2, or proton density. The TR and TE values control the amount of T1 weighting and T2 weighting, respectively. At long TR intervals (approximately 10 x tissue T1 values) and minimum TE values, the difference in signal intensity arising from relaxation vanishes, and contrast arises solely from the differences in proton density between the two tissues. Images formed with short TR intervals and long TE values exhibit very low signal-to-noise ratio and negligible contrast and should be avoided. Recently, fast spin-echo sequences have partially overcome the limitation of long acquisition times, with up to 16-fold reduction, by acquiring multiple lines in k space with multiecho sequences.

Finite element models of thoracic conductive anatomy: sensitivity to changes in inhomogeneity and anisotropy.

A moderately detailed 3-D finite element model of the conductive anatomy of a canine thorax was used to examine the sensitivity of the results obtained during simulated transthoracic defibrillation to variations in skeletal muscle anisotropy and differing degrees of model inhomogeneity. Our results suggest that the myocardial current density distribution is not particularly sensitive to the method used to model skeletal muscle anisotropy. However, anisotropy variations caused defibrillation parameters such as paddle to paddle impedance and threshold current to change by as much as 50%. We found a greater sensitivity in the myocardial current density and the defibrillation parameters to variations in model inhomogeneity. The changes observed in both depended substantially on paddle placement. This sensitivity to paddle placement highlights the difficulty in predicting how a reduction in anatomical detail will affect the myocardial current density distribution. In general, we found the defibrillation parameters to be more sensitive than the myocardial current density distribution to the variations in anatomical detail we examined.

Compressive mechanical properties of the human anulus fibrosus and their relationship to biochemical composition.

To enhance understanding of the biomechanical role of the intervertebral disc, the compressive properties and biochemical composition of nondegenerate samples of anulus fibrosus were determined as a function of radial position, region, and level. Because of the large swelling propensity of this tissue, a method was developed to test excised specimens while maintaining their in situ geometry and hydration. Using an analysis based on linear biphasic theory, the compressive modulus, hydraulic permeability, and isometric swelling pressure of the anulus fibrosus were determined and correlated with the tissue composition. The findings indicate that the anulus fibrosus is inhomogeneous, with regional and radial variations in both material properties and biochemical composition. The results of this study suggest that both structural and compositional factors may determine the mechanical behavior.