Analysis Of Noise Propagation Research Articles

Emulating Low-Dose PCCT Image Pairs with Independent Noise for Self-Supervised Spectral Image Denoising.

Photon counting CT (PCCT) acquires spectral measurements and enables generation of material decomposition (MD) images that provide distinct advantages in various clinical situations. However, noise amplification is observed in MD images, and denoising is typically applied. Clean or high-quality references are rare in clinical scans, often making supervised learning (Noise2Clean) impractical. Noise2Noise is a self-supervised counterpart, using noisy images and corresponding noisy references with zero-mean, independent noise. PCCT counts transmitted photons separately, and raw measurements are assumed to follow a Poisson distribution in each energy bin, providing the possibility to create noise-independent pairs. The approach is to use binomial selection to split the counts into two low-dose scans with independent noise. We prove that the reconstructed spectral images inherit the noise independence from counts domain through noise propagation analysis and also validated it in numerical simulation and experimental phantom scans. The method offers the flexibility to split measurements into desired dose levels while ensuring the reconstructed images share identical underlying features, thereby strengthening the model's robustness for input dose levels and capability of preserving fine details. In both numerical simulation and experimental phantom scans, we demonstrated that Noise2Noise with binomial selection outperforms other common self-supervised learning methods based on different presumptive conditions.

Prediction and analysis of aeroacoustic characteristics of a real-size high-speed train running in open field using compressible Large Eddy Simulation and vortex sound source

With the rapid advancement of technology, high-speed trains have been requested to increases their speeds. Aerodynamic noise generated from the external flow fields around these trains has become a critical factor to be addressed during a design phase. Accurate prediction of flow-induced noise in high-speed trains requires high-resolution sound source generation in the near-field and precise noise propagation analysis in the acoustic field without numerical dissipation. This necessitates the appropriate design of grids, taking into account aerodynamic noise generation and propagation in conjunction with the length scales of the relevant components of a real-size train. To tackle this challenge, the present research employs a three-dimensional compressible Large Eddy Simulation (LES) technique with high-resolution grids to simultaneously calculate the external flow and acoustic fields of a real-size high-speed train consisting of five cars running in an open field. A comprehensive analysis is conducted to evaluate the contributions of major components, namely, a cap, pantograph, bogie, intercoach, and HVAC cover, which are well-known as significant contributors to high-speed train flow-induced noise. The study utilizes the vortex sound source approach to analyze the generation mechanism of flow-induced noise for each component, providing valuable insights that could potentially aid in reducing aerodynamic noise.

Analysis of Cartographic Symbols as Visual Support in Interactive VR Geovisualizations

Abstract. Game engines and head-mounted displays (HMD) can create interactive virtual reality (VR) cartographies. Many VR geovisualizations try to depict places as close to reality as possible. The scaling is 1:1 for most VR geovisualizations. If VR geovisualizations are too close to reality or complex, users can navigate through them better, but the amount of detail makes it more challenging to target information. With the help of cartographic elements, a more targeted focus can again be acquired. The focus and style of VR cartography depend on the topic. The topics selected were "Floods and heavy rainfall events", "Noise propagation analysis", and "Lighting simulation". These are three topic examples in which 3D city models are used. CityGML models and fictitious 3D objects were used. Different rendering techniques and point signatures were implemented for each of the themes. All three topics were implemented in an application as a serious game. With the help of an interactive interface and motion controllers, users can change and navigate all three themes. Solution-oriented objects can be placed on all three topics as an additional interactive element. These options allow users to learn targeted information through various presentation options and implement solutions.

Self‐regulation in a stochastic model of chemical self‐replication

AbstractIn this article, we provide an analysis of noise propagation in a stochastic minimal model of chemical self‐replication, where a given species can duplicate itself normally. A feedback from the end product on the source, acting as an inhibitor transcription factor, is considered. Stochasticity involves the intrinsic noise affecting gene expression, which is assumed to happen in bursts. The use of a stochastic approach is a novelty within such a framework. The investigation involves the role of the feedback: how it impacts noise attenuation with respect to different modeling choices of stochastic transcription, and with respect to different strengths of the feedback action. The quantification of noise propagation is measured by means of the so called metabolic noise, that is, the coefficient of variation of the end product. Computations are carried out numerically, according to the stochastic simulation algorithm (SSA) properly adapted for the proposed stochastic hybrid systems, as well as analytically: the latter has been achieved by exploiting the linear approximation of the nonlinear terms involved, since otherwise there are no closed loop solutions for the first‐ and second‐order moments. In such a way, noise propagation may be linked to the model parameters, with the SSA aiming at validating the approximated formulas. Results confirm the noise reduction paradigm with feedback.

Hydrodynamic Analysis of Noise Propagation By the High Skew Marine Propeller Working in Non-Uniform Inflow

Being able to predict ship and marine propulsion noise is an important issue for naval architectures and the international maritime community. The main objective of this paper is the numerical investigation on the noise propagation by the high skew marine propeller working in a non-uniform inflow via RANS solver in the broadband frequency range. The pressure fluctuations were monitored at three points on the propeller blade, then by using the FFT operator we computed the blade passing frequency (BPF) for different propeller loading conditions. Based on these pressure pulses and adopting the Fowcs Williams-Hawking model we calculated noise radiated at the monitoring points. The results showed the BPF and noise level increased by increasing the load on the blades and we also observed that the noise generated at the leading edge was greater than at other points. Furthermore, the study of pressure fluctuations showed the propeller tip has more pressure variations in one revolution than other regions of the propeller surface.

Current Status and Future Challenges for Small Horizontal Axis Wind Turbines

Abstract This paper examines the category of small wind turbines. Numerous definitions are found in the literature. However, this paper's focus is on fixed pitch, small horizontal axis wind turbines, with a direct drive DC generator in the 1–10 kW class. Small wind turbine growth world-wide is analyzed for trends and predicted development. It is necessary to discuss design tools available for design, including computational fluid dynamic models and experimentally testing both open rotors and wind tunnel models. Wind turbines must be optimized for peak performance to include startup/cut-in speeds and other modifications. These wind turbines will rely on new and purposely designed airfoils; however, for low-Reynolds number conditions, computational tools do not accurately predict separation. An analysis of noise generation as well as design techniques for reducing noise is necessary for future designs. Discussions on the technologies particular to small wind turbines should include the topics of aerodynamics and structures/materials. Small wind turbines are contributing to the concept of distributed generation. Urban applications are leading to studies of flow fields in and around buildings. Interest in hybrid systems, which combine wind with other energy generation systems such as solar, internal combustion engines, and diesel engines, is growing. These systems are advantageous for the homeowner, small business, cell phone towers, remote locations, and backup emergency power systems (to include lighting). Finally, the concept of energy storage must be addressed in the context of small wind turbines, especially those turbines used in an isolated application.

Incorporating acoustic objectives into Forest Management Planning when sensitive bird species are relevant.

BackgroundThe potentially negative effects of timber harvesting on biodiversity and habitat conservation leads to the consideration of a wide range of restrictions to forest logging in natural areas. In particular, high noise levels produced by forest machinery present a challenge to developing sustainable forest management plans. The Cinereous vulture (Aegypius monachus), the largest bird of prey whose nests are located in mature trees, is considered to be appropriate as an indicator species for environment-friendly forest planning. In this work, we evaluated spatially differences in sound propagation between stands. We hypothesized that differences due to the influence of orography in mountainous forests would allow the relaxation of spatial and temporary restrictions to timber logging, without causing any great disturbance at nesting sites of sensitive species.MethodsOur study was conducted in a Scots pine (Pinus sylvestris) forest of Spain, where an important colony of the Western European population of Cinereous vulture is located. We built 62 noise maps to characterize noise pollution due to tree logging at planning level. We modeled two different scenarios, in order to characterize; (i) the effect of a chainsaw operator during a complete cycle for felling a tree (Scenario 1), and (ii) the effect of the peak level produced by the breaking noise emitted by the trunk of the tree and its impact on the ground (Scenario 2). A strategy of three logical steps was designed; (i) landscape-scale analysis of noise propagation in stands, (ii) hierarchical cluster analysis of stands, (iii) assessment of the potentially significant influence of noise management in timber harvesting.ResultsThe minimum distance (DIST) from chainsaw operator sites to the 40 dB(A) contour lines was the only variable that had a significant influence on the clustering results. On the other hand, mean values of a newly proposed metric called average radius or radial distance (ARD) oscillated between 174 m in cluster #1 (Scenario 1) and 407 m in cluster #2 (Scenario 2).DiscussionOur results highlight the convenience of considering noise modeling tools at a forest planning level in order to address the compatibility of forest logging and the necessity of protecting nature. We found that spatial propagation of noise made by chainsaws at felling sites does not differ between stands even in a mountainous terrain, contrary to what we initially hypothesized. However, temporary logging restrictions could be excluded in about 36% of the current conditioned management areas according to ARD results in Scenario 2 (400 m). This proposal could be based on a sound pressure level (in decibels) criteria instead of conventional buffer protection distance criteria. In addition, it is suggested that the current size of restricted management areas could be generally extended from a 100 m radius to a 200 m one around the Cinereous vulture nest sites.

Analysis of Noise Generation by Turbulent Jets from Consideration of Their Near Acoustic Field

The paper studies the generation of acoustic waves near the boundaries of swirling and nonswirling turbulent jets outside a jet flow. Nonstationary motion of the medium is analyzed. In the case of swirling jets, the occurrence and propagation of perturbations were studied experimentally, while in the case of free turbulent jets, both experimentally and numerically on the basis of LES technology. The results of the study show that, near the jet boundaries, there is a region in which the phase difference between pressure and velocity pulsations at fixed frequencies is 90° or more; i.е., there are no perturbations propagating outward. The phases of velocity and pressure pulsations coincide starting from some significant distance from the jet boundaries. The region in which the phase difference varies from 90° to 0° lies outside the jet flow and is presumably the sound generation region. It is proposed that this region be identified with the near acoustic field of jets.

Computation of exact g-factor maps in 3D GRAPPA reconstructions.

To characterize the noise distributions in 3D-MRI accelerated acquisitions reconstructed with GRAPPA using an exact noise propagation analysis that operates directly in k-space. We exploit the extensive symmetries and separability in the reconstruction steps to account for the correlation between all the acquired k-space samples. Monte Carlo simulations and multi-repetition phantom experiments were conducted to test both the accuracy and feasibility of the proposed method; a high-resolution in-vivo experiment was performed to assess the applicability of our method to clinical scenarios. Our theoretical derivation shows that the direct k-space analysis renders an exact noise characterization under the assumptions of stationarity and uncorrelation in the original k-space. Simulations and phantom experiments provide empirical support to the theoretical proof. Finally, the high-resolution in-vivo experiment demonstrates the ability of the proposed method to assess the impact of the sub-sampling pattern on the overall noise behavior. By operating directly in the k-space, the proposed method is able to provide an exact characterization of noise for any Cartesian pattern sub-sampled along the two phase-encoding directions. Exploitation of the symmetries and separability into independent blocks through the image reconstruction procedure allows us to overcome the computational challenges related to the very large size of the covariance matrices involved.

Analysis of noise generation and electric conduction at grain boundaries in CVD-grown MoS2 field effect transistors

Grain boundaries in a chemical vapour deposition (CVD)-grown monolayer of MoS2 induce significant effects on the electrical and low frequency noise characteristics of the MoS2. Here, we investigated the electrical properties and noise characteristics of MoS2 field effect transistors (FETs) made with CVD-grown monolayer MoS2. The electrical and noise characteristics of MoS2 FETs were analysed and compared for the MoS2 channel layers with and without grain boundaries. The grain boundary in the CVD-grown MoS2 FETs can be the dominant noise source, and dependence of the extracted Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS2 were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS2 devices.

Perfectly Flat-Top and Equiripple Flat-Top Cosine Windows

This paper describes a new method for designing flat-top cosine time windows. New perfectly flat-top windows, i.e., having monotonically decaying main lobe, and new equiripple flat-top windows, i.e., with some minor ripples of equal amplitude in the main lobe, are proposed. Perfect flatness (monotonic main lobe) is obtained by zeroing frequency derivatives of the window’s spectrum at the origin. Perfect flatness is a new quality in flat-top windows, as all known flat-top windows have some ripples in the main lobe. New equiripple flat-top windows are designed based on the conditions imposed on the window’s frequency response. The new flat-top windows are compared with the existing ones through the figures of merit, amplitude estimation example, and noise propagation analysis, showing superior performance of the former. The new windows are an excellent choice for popular digital waveform (spectrum) analyzers, as sine-waves amplitudes can be accurately estimated directly from the discrete Fourier transform spectrum without any further postprocessing.

Frequency comb offset dynamics of SESAM modelocked thin disk lasers.

We present a detailed study of the carrier-envelope offset (CEO) frequency dynamics of SESAM modelocked thin disk lasers (TDLs) pumped by kW-class highly transverse multimode pump diodes with a typical M(2) value of 200-300, and give guidelines for future frequency stabilization of multi-100-W oscillators. We demonstrate CEO frequency detection with > 30 dB signal-to-noise ratio with a resolution bandwidth of 100 kHz from a SESAM modelocked Yb:YAG TDL delivering 140 W average output power with 748-fs pulses at 7-MHz pulse repetition rate. We compare with a low-power CEO frequency stabilized Yb:CALGO TDL delivering 2.1 W with 77-fs pulses at 65 MHz. For both lasers, we perform a complete noise characterization, measure the relevant transfer functions (TFs) and compare them to theoretical models. The measured TFs are used to determine the propagation of the pump noise step-by-step through the system components. From the noise propagation analysis, we identify the relative intensity noise (RIN) of the pump diode as the main contribution to the CEO frequency noise. The resulting noise levels are not excessive and do not prevent CEO frequency stabilization. More importantly, the laser cavity dynamics are shown to play an essential role in the CEO frequency dynamics. The cavity TFs of the two lasers are very different which explains why at this point a tight CEO frequency lock can be obtained with the Yb:CALGO TDL but not with the Yb:YAG TDL. For CEO stabilization laser cavities should exhibit high damping of the relaxation oscillations by nonlinear intra-cavity elements, for example by operating a SESAM in the roll-over regime. Therefore the optimum SESAM operation point is a trade-off between enough damping and avoiding multiple pulsing instabilities. Additional cavity components could be considered for supplementary damping independent of the SESAM operation point.

Study of Noise Propagation for Small Vessels

AbstractThe paper presents the results of the noise propagation analysis in ship structures tested in a number of AHTS (Anchor Handling Tug Supply) vessels. Statistical Energy Analysis (SEA) based on numerical model developed specially for the purpose of this numerical investigation were conducted. This numerical model enabled the analysis of both the structural elements and the acoustic spaces. For the detailed studies 47 points fixed at various ship locations were selected. Prediction results with use of the numerical model were compared with the experimental results carried out in six identical AHTS vessels. Experimental studies were performed in accordance with the requirements of the International Maritime Organization (IMO) Resolution A.468 (XII). As a result one presented a comparison of the model analysis and experimental tests results.

Acoustic Analysis of Condenser Fan of Split Air Conditioner Using Numerical and Experimental Method

The present work aims to investigate the accurate method of performing computational fluid dynamics (CFD) — Acoustic analysis for axial flow fans in split air conditioner system. A comprehensive simulation procedure is developed to predict flow-induced noise in a system. The three-dimensional domain using k–ε turbulence model and Ffowcs Williams and Hawkings (FW-H) acoustic model is considered to predict noise generated by the fan blade surface. The acoustic and flow performances of the fan are predicted simultaneously using a computational aero-acoustic technique (combining steady flow and noise propagation analysis). The different cases are simulated by varying the blade angle, blade depth, blade width and serrations at trailing edge of fan blade. An impact of each of these parameter on A-weighted sound pressure level (SPL) and mass flow rate at outlet is determined. The numeric value of obtained A-weighted SPL by CFD simulation is found to be in close agreement with the experimental result within 5.4%. Finally, above mentioned parameters are varied in simulation and optimized design is proposed based on A-weighted SPL and cubic feet per minute (CFM). All simulations are carried out in commercially available CFD solver; ANSYS FLUENT 13.

Virtual resistive network and conductivity reconstruction with Faradayʼs law

A network-based conductivity reconstruction method is introduced using the third Maxwell equation, or Faradayʼs law, for a static case. The usual choice in electrical impedance tomography is the divergence-free equation for the electrical current density. However, if the electrical current density is given, the curl-free equation for the electrical field gives a direct relation between the current and the conductivity and this relation is used in this paper. Mimetic discretization is applied to the equation, which gives the virtual resistive network system. Properties of the numerical schemes introduced are investigated and their advantages over other conductivity reconstruction methods are discussed. Numerically simulated results, with an analysis of noise propagation, are presented.

Accelerating sequences in the presence of metal by exploiting the spatial distribution of off‐resonance

To demonstrate feasibility of exploiting the spatial distribution of off-resonance surrounding metallic implants for accelerating multispectral imaging techniques. Multispectral imaging (MSI) techniques perform time-consuming independent three-dimensional acquisitions with varying radio frequency offsets to address the extreme off-resonance from metallic implants. Each off-resonance bin provides a unique spatial sensitivity that is analogous to the sensitivity of a receiver coil and, therefore, provides a unique opportunity for acceleration. Fully sampled MSI was performed to demonstrate retrospective acceleration. A uniform sampling pattern across off-resonance bins was compared with several adaptive sampling strategies using a total hip replacement phantom. Monte Carlo simulations were performed to compare noise propagation of two of these strategies. With a total knee replacement phantom, positive and negative off-resonance bins were strategically sampled with respect to the B0 field to minimize aliasing. Reconstructions were performed with a parallel imaging framework to demonstrate retrospective acceleration. An adaptive sampling scheme dramatically improved reconstruction quality, which was supported by the noise propagation analysis. Independent acceleration of negative and positive off-resonance bins demonstrated reduced overlapping of aliased signal to improve the reconstruction. This work presents the feasibility of acceleration in the presence of metal by exploiting the spatial sensitivities of off-resonance bins.

A general framework and review of scatter correction methods in x‐ray cone‐beam computerized tomography. Part 1: Scatter compensation approaches

Since scattered radiation in cone-beam volume CT implies severe degradation of CT images by quantification errors, artifacts, and noise increase, scatter suppression is one of the main issues related to image quality in CBCT imaging. The aim of this review is to structurize the variety of scatter suppression methods, to analyze the common structure, and to develop a general framework for scatter correction procedures. In general, scatter suppression combines hardware techniques of scatter rejection and software methods of scatter correction. The authors emphasize that scatter correction procedures consist of the main components scatter estimation (by measurement or mathematical modeling) and scatter compensation (deterministic or statistical methods). The framework comprises most scatter correction approaches and its validity also goes beyond transmission CT. Before the advent of cone-beam CT, a lot of papers on scatter correction approaches in x-ray radiography, mammography, emission tomography, and in Megavolt CT had been published. The opportunity to avail from research in those other fields of medical imaging has not yet been sufficiently exploited. Therefore additional references are included when ever it seems pertinent. Scatter estimation and scatter compensation are typically intertwined in iterative procedures. It makes sense to recognize iterative approaches in the light of the concept of self-consistency. The importance of incorporating scatter compensation approaches into a statistical framework for noise minimization has to be underscored. Signal and noise propagation analysis is presented. A main result is the preservation of differential-signal-to-noise-ratio (dSNR) in CT projection data by ideal scatter correction. The objective of scatter compensation methods is the restoration of quantitative accuracy and a balance between low-contrast restoration and noise reduction. In a synopsis section, the different deterministic and statistical methods are discussed with respect to their properties and applications. The current paper is focused on scatter compensation algorithms. The multitude of scatter estimation models will be dealt with in a separate paper.

Identification of signal bias in the variable flip angle method by linear display of the algebraic ernst equation

A novel linear parameterization for the variable flip angle method for longitudinal relaxation time T1 quantification from spoiled steady state MRI is derived from the half angle tangent transform, τ, of the flip angle. Plotting the signal S at coordinates x = Sτ and y = S/τ, respectively, establishes a line that renders signal amplitude and relaxation term separately as y-intercept and slope. This representation allows for estimation of the respective parameter from the experimental data. A comprehensive analysis of noise propagation is performed. Numerical results for efficient optimization of longitudinal relaxation time and proton density mapping experiments are derived. Appropriate scaling allows for a linear presentation of data that are acquired at different short pulse repetition times, TR << T1 thus increasing flexibility in the data acquisition by removing the limitation of a single pulse repetition time. Signal bias, like due to slice-selective excitation or imperfect spoiling, can be readily identified by systematic deviations from the linear plot. The method is illustrated and validated by 3T experiments on phantoms and human brain. Magn Reson Med, 2011. © 2011 Wiley-Liss, Inc.

Apparent diffusion model assessment in extraction processes by means of a Luenberger observer

In this work a clear methodology is proposed that allows the estimation of diffusion–concentration functional relationship in extraction processes, which are modelled by Fick’s second law. A criterion is proposed for deciding when the diffusivity should be considered a constant value (comparable to those obtained by the standard regression technique) and when a function depending on concentration. The methodology is based in a Luenberger observer. The diffusion–concentration relationship is refined by an optimization procedure making use of a numerical solution of the diffusion equation. The robustness of the methodology is assessed by a noise propagation analysis. The methodology is illustrated for oil extraction from tung seeds with experimental data taken from the literature.

Study of noise propagation and the effects of insufficient numbers of projection angles and detector samplings for iterative reconstruction using planar-integral data

A rotating slat collimator can be used to acquire planar-integral data. It achieves higher geometric efficiency than a parallel-hole collimator by accepting more photons, but the planar-integral data contain less tomographic information that may result in larger noise amplification in the reconstruction. Lodge evaluated the rotating slat system and the parallel-hole system based on noise behavior for an FBP reconstruction. Here, we evaluate the noise propagation properties of the two collimation systems for iterative reconstruction. We extend Huesman's noise propagation analysis of the line-integral system to the planar-integral case, and show that approximately 2.0(D/dp) SPECT angles, 2.5(D/dp) self-spinning angles at each detector position, and a 0.5dp detector sampling interval are required in order for the planar-integral data to be efficiently utilized. Here, D is the diameter of the object and dp is the linear dimension of the voxels that subdivide the object. The noise propagation behaviors of the two systems are then compared based on a least-square reconstruction using the ratio of the SNR in the image reconstructed using a planar-integral system to that reconstructed using a line-integral system. The ratio is found to be proportional to the square root of F/D, where F is a geometric efficiency factor. This result has been verified by computer simulations. It confirms that for an iterative reconstruction, the noise tradeoff of the two systems is not only dependent on the increase of the geometric efficiency afforded by the planar projection method, but also dependent on the size of the object. The planar-integral system works better for small objects, while the line-integral system performs better for large ones. This result is consistent with Lodge's results based on the FBP method.