Spatial Drift Research Articles

Unstable spiral waves and local Euclidean symmetry in a model of cardiac tissue.

This paper investigates the properties of unstable single-spiral wave solutions arising in the Karma model of two-dimensional cardiac tissue. In particular, we discuss how such solutions can be computed numerically on domains of arbitrary shape and study how their stability, rotational frequency, and spatial drift depend on the size of the domain as well as the position of the spiral core with respect to the boundaries. We also discuss how the breaking of local Euclidean symmetry due to finite size effects as well as the spatial discretization of the model is reflected in the structure and dynamics of spiral waves. This analysis allows identification of a self-sustaining process responsible for maintaining the state of spiral chaos featuring multiple interacting spirals.

Accuracy and efficiency of an infrared based positioning and tracking system for patient set-up and monitoring in image guided radiotherapy

An infrared based positioning and tracking (IPT) system was introduced and its accuracy and efficiency for patient setup and monitoring were tested for daily radiotherapy treatment. The IPT system consists of a pair of floor mounted infrared stereoscopic cameras, passive infrared markers and tools used for acquiring localization information as well as a custom controlled software which can perform the positioning and tracking functions. The evaluation of IPT system characteristics was conducted based on the AAPM 147 task report. Experiments on spatial drift and reproducibility as well as static and dynamic localization accuracy were carried out to test the efficiency of the IPT system.Measurements of known translational (up to 55.0mm) set-up errors in three dimensions have been performed on a calibration phantom. The accuracy of positioning was evaluated on an anthropomorphic phantom with five markers attached to the surface; the precision of the tracking ability was investigated through a sinusoidal motion platform. For the monitoring of the respiration, three volunteers contributed to the breathing testing in real time. The spatial drift of the IPT system was 0.65mm within 60min to be stable. The reproducibility of position variations were between 0.01 and 0.04mm. The standard deviation of static marker localization was 0.26mm. The repositioning accuracy was 0.19mm, 0.29mm, and 0.53mm in the left/right (L/R), superior/inferior (S/I) and anterior/posterior (A/P) directions, respectively. The measured dynamic accuracy was 0.57mm and discrepancies measured for the respiratory motion tracking was better than 1mm. The overall positioning accuracy of the IPT system was within 2mm. In conclusion, the IPT system is an accurate and effective tool for assisting patient positioning in the treatment room. The characteristics of the IPT system can successfully meet the needs for real time external marker tracking and patient positioning as well as respiration monitoring during image guided radiotherapy treatments.

Slice groups for post-compression region of interest encryption in H.264/AVC and its scalable extension

Encrypting regions of interest in H.264/AVC and SVC bit streams after compression is a challenging task due to drift. In this paper, we assess whether the use of slice groups makes this task easier and what its expense in terms of bit rate overhead is. We introduce the concept of all-grey base layers for SVC which simplify the encryption of regions of interest in surveillance camera applications while obeying all standard-imposed base layer restrictions. Furthermore, we show that the use of slice groups is possible with relatively low overhead for medium and high bit rates (below 5% in most of the tested configurations). This applies to H.264/AVC as well as SVC bit streams with two and three spatial layers, including those with the newly introduced all-grey base layers. Although we are able to contain spatial and inter-layer drift with our proposed encryption setup, temporal drift still remains an issue that cannot be solved by sole usage of slice groups.

A Free Matlab Script for Spatial Drift Correction

A Free Matlab Script for Spatial Drift Correction

Controlling phase noise in oscillatory interference models of grid cell firing.

Oscillatory interference models account for the spatial firing properties of grid cells in terms of neuronal oscillators with frequencies modulated by the animal's movement velocity. The phase of such a "velocity-controlled oscillator" (VCO) relative to a baseline (theta-band) oscillation tracks displacement along a preferred direction. Input from multiple VCOs with appropriate preferred directions causes a grid cell's grid-like firing pattern. However, accumulating phase noise causes the firing pattern to drift and become corrupted. Here we show how multiple redundant VCOs can automatically compensate for phase noise. By entraining the baseline frequency to the mean VCO frequency, VCO phases remain consistent, ensuring a coherent grid pattern and reducing its spatial drift. We show how the spatial stability of grid firing depends on the variability in VCO phases, e.g., a phase SD of 3 ms per 125 ms cycle results in stable grids for 1 min. Finally, coupling N VCOs with similar preferred directions as a ring attractor, so that their relative phases remain constant, produces grid cells with consistently offset grids, and reduces VCO phase variability of the order square root of N. The results suggest a viable functional organization of the grid cell network, and highlight the benefit of integrating displacement along multiple redundant directions for the purpose of path integration.

An uncertainty principle for star formation – I. Why galactic star formation relations break down below a certain spatial scale

Galactic scaling relations between the (surface densities of) the gas mass and the star formation (SF) rate are known to develop substantial scatter or even change form when considered below a certain spatial scale. We quantify how this behaviour should be expected due to the incomplete statistical sampling of independent star-forming regions. Other included limiting factors are the incomplete sampling of SF tracers from the stellar initial mass function and the spatial drift between gas and stars. We present a simple uncertainty principle for SF, which can be used to predict and interpret the failure of galactic SF relations on small spatial scales. This uncertainty principle explains how the scatter of SF relations depends on the spatial scale and predicts a scale-dependent bias of the gas depletion time-scale when centering an aperture on gas or SF tracer peaks. We show how the scatter and bias are sensitive to the physical size and time-scales involved in the SF process (such as its duration or the molecular cloud lifetime), and illustrate how our formalism provides a powerful tool to constrain these largely unknown quantities. Thanks to its general form, the uncertainty principle can also be applied to other astrophysical systems, e.g. addressing the time-evolution of star-forming cores, protoplanetary discs, or galaxies and their nuclei.

Direct observation of zipper-like wall-to-wall coalescence of double-wall carbon nanotubes

In situ transmission electron microscope (TEM) observation has become a fairly powerful characterization method to uniquely provide the real-time information on the dynamics and kinetics of physical and chemical transformations at the nanometer scale. Here, we report a development of a high-temperature-compatible specimen heating holder equipped with a micro-sized carbon nanotube (CNT) network heater. The CNT network heater can heat samples up to extremely high temperature, as high as >2000°C, while minimizing the spatial drift of the samples induced by the heating. This has directly led to a real-time imaging of a purely thermally-driven nano-welding process between two parallel-aligned double-wall carbon nanotubes (DWCNTs) using low-voltage TEM (80keV); a single larger-diameter DWCNT was formed after the reaction and the reaction rate was determined. This technique developed is basically applicable to the in situ TEM study of various thermal-driven solid-phase dynamical changes/reactions and should provide more significant insight on nanoscience and nanothermodynamics.

Multidimensional SPM applied for nanoscale conductance mapping

Abstract

Stochastic Simulation of Nonstationary Rainfall Fields, Accounting for Seasonality and Atmospheric Circulation Pattern Evolution

A model for generating daily spatial correlated rainfall fields suitable for evaluating the impacts of climate change on water resources is presented. The model, termed Stochastic Rainfall Generating Process, is designed to incorporate two major nonstationarities: changes in the frequencies of different precipitation generating mechanisms (frontal and convective), and spatial nonstationarities caused by interactions of mesoscale atmospheric patterns with topography (orographic effects). These nonstationarities are approximated as discrete sets of the time-stationary Stochastic Rainfall Generating Process, each of which represents the different spatial patterns of rainfall (including its variation with topography) associated with different atmospheric circulation patterns and times of the year (seasons). Each discrete Stochastic Rainfall Generating Process generates daily correlated rainfall fields as the product of two random fields. First, the amount of rainfall is generated by a transformed Gaussian process applying sequential Gaussian simulation. Second, the delimitation of rain and no-rain areas (intermittence process) is defined by a binary random function simulated by sequential indicator simulations. To explore its applicability, the model is tested in the Upper Guadiana Basin in Spain. The result suggests that the model provides accurate reproduction of the major spatiotemporal features of rainfall needed for hydrological modeling and water resource evaluations. The results were significantly improved by incorporating spatial drift related to orographic precipitation into the model.

Quality assurance program for CatalystTM, a 3D surface imaging system for patient setup verification in radiotherapy

Introduction The aim of this study is to come up with a quality assurance program for CatalystTM (C-Rad, Uppsala, Sweden), a ‘‘non-radiographic” patient setup verification device. Materiel and methods The Catalyst system consists of an optical surface scanning and re-projection device mounted in the ceiling in front of our Varian clinac. The system gives an instant feedback during the patient repositioning and is designed to be integrated in the treatment workflow. The software is interfaced with our clinac console, the motorized couch and the R&V system: ARIA. This report presents the acceptance and the commissioning we performed for this new equipment installed in our center. A quality assurance program has also been established to maintain the safety, accuracy, precision sensitivity and reliability of this patient positioning system in time. Results Acceptance testing provided by the supplier has been made during the installation. These tests are useful to become familiar with equipment operation and its limitations and demonstrate that the equipment is working. The commissioning procedures have been based on the AAPM task group report 147. The main parts of these tests are integration of equipment, measurements of spatial reproducibility and drift and estimation of the localization accuracy. All of this led us to a better understanding of Catalyst and allowed us to establish a complete quality program. The communication and the integration of Catalyst with all our equipment are fully functional. We only detected a potential misusage when we import a treatment plan with multiple isocentres. During the first six months, the daily check operation shows less than 1 mm deviation in all directions, with a mean of 0.6 ± 0.3 mm. And we did not observe any spatial drift due to the system warm-up. Reproducibility of surface acquisition and setup procedure was better than 0.5 mm and 0.5° and 1 mm and 1°, respectively. The system accuracy was better than 1 mm and 1° hen a catalyst image was used as reference. A global worsening was observed using an external surface extracted from CT study. Conclusion This localization and positioning system is expected to become an important component within our radiation therapy process, so a quality assurance of this system is essential. Moreover, this study allows us to refine and to adapt our clinical procedure. The aim of this study is to come up with a quality assurance program for CatalystTM (C-Rad, Uppsala, Sweden), a ‘‘non-radiographic” patient setup verification device. The Catalyst system consists of an optical surface scanning and re-projection device mounted in the ceiling in front of our Varian clinac. The system gives an instant feedback during the patient repositioning and is designed to be integrated in the treatment workflow. The software is interfaced with our clinac console, the motorized couch and the R&V system: ARIA. This report presents the acceptance and the commissioning we performed for this new equipment installed in our center. A quality assurance program has also been established to maintain the safety, accuracy, precision sensitivity and reliability of this patient positioning system in time. Acceptance testing provided by the supplier has been made during the installation. These tests are useful to become familiar with equipment operation and its limitations and demonstrate that the equipment is working. The commissioning procedures have been based on the AAPM task group report 147. The main parts of these tests are integration of equipment, measurements of spatial reproducibility and drift and estimation of the localization accuracy. All of this led us to a better understanding of Catalyst and allowed us to establish a complete quality program. The communication and the integration of Catalyst with all our equipment are fully functional. We only detected a potential misusage when we import a treatment plan with multiple isocentres. During the first six months, the daily check operation shows less than 1 mm deviation in all directions, with a mean of 0.6 ± 0.3 mm. And we did not observe any spatial drift due to the system warm-up. Reproducibility of surface acquisition and setup procedure was better than 0.5 mm and 0.5° and 1 mm and 1°, respectively. The system accuracy was better than 1 mm and 1° hen a catalyst image was used as reference. A global worsening was observed using an external surface extracted from CT study. This localization and positioning system is expected to become an important component within our radiation therapy process, so a quality assurance of this system is essential.

Under-the-barrier dynamics in laser-induced relativistic tunneling.

The tunneling dynamics in relativistic strong-field ionization is investigated with the aim to develop an intuitive picture for the relativistic tunneling regime. We demonstrate that the tunneling picture applies also in the relativistic regime by introducing position dependent energy levels. The quantum dynamics in the classically forbidden region features two time scales, the typical time that characterizes the probability density's decay of the ionizing electron under the barrier (Keldysh time) and the time interval which the electron spends inside the barrier (Eisenbud-Wigner-Smith tunneling time). In the relativistic regime, an electron momentum shift as well as a spatial shift along the laser propagation direction arise during the under-the-barrier motion which are caused by the laser magnetic field induced Lorentz force. The momentum shift is proportional to the Keldysh time, while the wave-packet's spatial drift is proportional to the Eisenbud-Wigner-Smith time. The signature of the momentum shift is shown to be present in the ionization spectrum at the detector and, therefore, observable experimentally. In contrast, the signature of the Eisenbud-Wigner-Smith time delay disappears at far distances for pure quasistatic tunneling dynamics.

Spatial dependencies and spatial drift in public transport seasonal ticket revenue data

When firms' customers are located in spatially dispersed areas, it can be difficult to manage service quality on a geographically small scale because the relative importance of service quality might vary spatially. Moreover, standard approaches discussed so far in the marketing science literature usually neglect spatial effects, such as spatial dependencies (spatial autocorrelation for example) and spatial drift (spatial non-stationarity). We propose a comprehensive approach based on spatial econometric methods that covers both issues. Based on the real company data on seasonal ticket revenue of a local public transport service company, we show that addressing such spatial effects of service data can improve management's ability to implement programs aimed at enhancing seasonal ticket revenue. In particular, the article shows how a spatial revenue response function might be specified.

Dissipative Soliton Excitability Induced by Spatial Inhomogeneities and Drift

We show that excitability is generic in systems displaying dissipative solitons when spatial inhomogeneities and drift are present. Thus, dissipative solitons in systems which do not have oscillatory states, such as the prototypical Swift-Hohenberg equation, display oscillations and type I and II excitability when adding inhomogeneities and drift to the system. This rich dynamical behavior arises from the interplay between the pinning to the inhomogeneity and the pulling of the drift. The scenario presented here provides a general theoretical understanding of oscillatory regimes of dissipative solitons reported in semiconductor microresonators. Our results open also the possibility to observe this phenomenon in a wide variety of physical systems.

A Two-step Bayes method for Spatial Drift in Consumer Classification

In the field of data mining, spatial drift refers to the data used to develop the model consists of only one part of the population, and the differences among the samples or between sample and population are unknown. This paper proposes a two-step Bayes method to improve adaptability for different region samples, which also maintains high model accuracy. The new method first groups region based on similarity, second, sets a model structure without parameters for populations or large samples with good data quality, and then trains parameters using samples in same region group. This method builds a estimation model, proving the method by showing how it can to some extent solve the uncertainty of consumer classification

Radial Domany-Kinzel models with mutation and selection

We study the effect of spatial structure, genetic drift, mutation, and selective pressure on the evolutionary dynamics in a simplified model of asexual organisms colonizing a new territory. Under an appropriate coarse-graining, the evolutionary dynamics is related to the directed percolation processes that arise in voter models, the Domany-Kinzel (DK) model, contact process, and so on. We explore the differences between linear (flat front) expansions and the much less familiar radial (curved front) range expansions. For the radial expansion, we develop a generalized, off-lattice DK model that minimizes otherwise persistent lattice artifacts. With both simulations and analytical techniques, we study the survival probability of advantageous mutants, the spatial correlations between domains of neutral strains, and the dynamics of populations with deleterious mutations. "Inflation" at the frontier leads to striking differences between radial and linear expansions. For a colony with initial radius R(0) expanding at velocity v, significant genetic demixing, caused by local genetic drift, occurs only up to a finite time t(*)=R(0)/v, after which portions of the colony become causally disconnected due to the inflating perimeter of the expanding front. As a result, the effect of a selective advantage is amplified relative to genetic drift, increasing the survival probability of advantageous mutants. Inflation also modifies the underlying directed percolation transition, introducing novel scaling functions and modifications similar to a finite-size effect. Finally, we consider radial range expansions with deflating perimeters, as might arise from colonization initiated along the shores of an island.

Calibration and stability of a SiPM-based simultaneous PET/MR insert

On behalf of the HYPER Image project, a Silicon photomultiplier (SiPM) based preclinical PET insert for a commercial human 3T MRI scanner was built. In this contribution we report on the stability of imaging performance of the PET scanner and MR hardness and compatibility. From data sets that were acquired during the last 7 months we extracted SiPM gain values and their annual drift, the mean energy resolution and the energy resolution drift, spatial resolution and spatial resolution drift, and photo peak position and their annual drift. Further, a point source and a hot rod phantom was imaged fully simultaneously with the MRI scanner and the PET scanner. No interference between either modality was observed.

Technical Note: Detection of gas bubble leakage via correlation of water column multibeam images

Abstract. Hydroacoustic detection of natural gas release from the seafloor has been conducted in the past by using singlebeam echosounders. In contrast, modern multibeam swath mapping systems allow much wider coverage, higher resolution, and offer 3-D spatial correlation. Up to the present, the extremely high data rate hampers water column backscatter investigations and more sophisticated visualization and processing techniques are needed. Here, we present water column backscatter data acquired with a 50 kHz prototype multibeam system over a period of 75 seconds. Display types are of swath-images as well as of a "re-sorted" singlebeam presentation. Thus, individual and/or groups of gas bubbles rising from the 24 m deep seafloor clearly emerge in the acoustic images, making it possible to estimate rise velocities. A sophisticated processing scheme is introduced to identify those rising gas bubbles in the hydroacoustic data. We apply a cross-correlation technique adapted from particle imaging velocimetry (PIV) to the acoustic backscatter images. Temporal and spatial drift patterns of the bubbles are assessed and are shown to match very well to measured and theoretical rise patterns. The application of this processing to our field data gives clear results with respect to unambiguous bubble detection and remote bubble rise velocimetry. The method can identify and exclude the main source of misinterpretations, i.e. fish-mediated echoes. Although image-based cross-correlation techniques are well known in the field of fluid mechanics for high resolution and non-inversive current flow field analysis, we present the first application of this technique as an acoustic bubble detector.

Kernel-based Target Tracking with Spatial Histogram and Template Drift Correction

The traditional mean shift based on kernel-histogram is of invariable kernel bandwidth and unable to represent accurately the color distribution of target and eliminate the accumulative tracking errors. To overcome these limitations, a tracking algorithm with the following improved strategies is proposed. Firstly, a modified second-order spatial histogram including spatial and color information is adopted to describe target model and the candidate, and Bhattacharyya distance is derived to evaluate the similarity between them. Secondly, target model is estimated repeatedly according to the target region parameter resulted from template drift correction which avoids the tracking window drifting problem. Then, an affine transform is established combining corner and edge detection to update the tracking window parameters. Finally, a Kalman filter or linear filter is selected to predict the target motion according to the Kalman residual error. The results of experiments show that the proposed algorithm works well and is robust against similarity distraction, scale and orientation variations and short-term occlusion.

Extending reference scan drift correction to high-magnification high-cone-angle tomography

The reference scan method is a simple yet powerful method for measuring spatial drift of the x-ray spot during a low-cone-angle μ-CT experiment. As long as the drift is smooth, and occurring on a time scale that is long compared to the acquisition time of each projection, this method provides a way to compensate for the drift by applying 2D in-plane translations to the radiographs. Here we show that this compensation may be extended to the regime of high-magnification, high-cone-angle CT experiments where source drift perpendicular to the detector plane can cause significant magnification changes throughout the acquisition.

Second-order selectivity of single units in macaque primary visual cortex (V1) and V2

The way humans perceive texture-defined form suggests that 2nd-order stimuli are encoded by mechanisms that receive rectified input from orientation-selective neurons like those in V1. Direct physiological evidence for 2nd-order neurons in primate visual cortex is elusive. The responses of orientation-selective neurons are often modulated by stimuli falling in the receptive field (RF) surround; a variant 2nd-order model uses inhomogeneities in the RF center and surround to create selectivity for form defined by variations in contrast and texture (Tanaka & Ohzawa, 2009). We tested this model by measuring the responses of single units in V1 and V2 of paralyzed, opiate-anaesthetized macaques. We stimulated the RF center and surround with large, contrast-modulated gratings made from a sinusoidal grating carrier modulated by a second, drifting sinusoidal grating of lower spatial frequency. The carrier was of optimal spatial frequency, orientation, and drift rate; the modulator varied across 8 drift directions and 5 or 6 spatial frequencies. We found cells in both V1 and V2 that were selective for modulator orientation and spatial frequency. We used a 2nd-order difference-of-Gaussians model to capture the spatial arrangement of the contrast-responsive center and surround regions of the RF. Cells unselective for modulator structure had classical circular centers with annular surrounds, while others - with modulator selectivity - showed diverse spatial arrangements of the RF center and surround. In many selective cells, the surround was arranged anisotropically relative to the RF center, and explained the modulator selectivity. In others, selectivity appeared to arise from an elongated RF center, in the absence of an anisotropic, suppressive surround. Selectivity for 2nd-order form may not depend on pure 2nd-order neurons, but might arise instead by selective modulation of neuronal responses to conventional 1st-order stimuli.