Attenuating Medium Research Articles

Modeling of Wave Dispersion Using Continuous Wavelet Transforms

In the estimate of dispersion with the help of wavelet analysis considerable emphasis has been put on the extraction of the group velocity using the modulus of the wavelet transform. In this paper we give an asymptotic expression of the full propagator in wavelet space that comprises the phase velocity as well. This operator establishes a relationship between the observed signals at two different stations during wave propagation in a dispersive and attenuating medium. Numerical and experimental examples are presented to show that the method accurately models seismic wave dispersion and attenuation.

Modelling of nonlinear effects and the response of ultrasound contrast micro bubbles: simulation and experiment

The propagation of diagnostic ultrasonic imaging pulses in tissue and their interaction with contrast micro bubbles is a very complex physical process, which we assumed to be separable into three stages: pulse propagation in tissue, the interaction of the pulse with the contrast bubble, and the propagation of the scattered echo. The model driven approach is used to gain better knowledge of the complex processes involved. A simplified way of field simulation is chosen due to the complexity of the task and the necessity to estimate comparative contributions of each component of the process. Simulations are targeted at myocardial perfusion estimation. A modified method for spatial superposition of attenuated waves enables simulations of low intensity pulse pressure fields from weakly focused transducers in a nonlinear, attenuating, and liquid-like biological medium. These assumptions enable the use of quasi-linear calculations of the acoustic field. The simulations of acoustic bubble response are carried out with the Rayleigh–Plesset equation with the addition of radiation damping. Theoretical simulations with synthesised and experimentally sampled pulses show that the interaction of the excitation pulses with the contrast bubbles is the main cause of nonlinear scattering, and a 2–3 dB increase of second harmonic amplitude depends on nonlinear distortions of the incident pulse.

The effects of ultraviolet background correlations on Lyα forest flux statistics

We examine the possible effects of ultraviolet (UV) background fluctuations on the pixel flux power spectrum and autocorrelation function of the Lyα forest due to a finite number of sources in an attenuating medium. We consider scenarios in which quasi-stellar object (QSO) sources dominate the contribution to the UV background. To estimate their contribution, we use the QSO luminosity functions from the Two-Degree Field and the Sloan Digital Sky Survey. We estimate self-consistent values for the attenuation length at the Lyman edge through the intergalactic medium using particle mesh simulations of the Lya forest, normalized by the measured mean Lya flux. It is necessary to add the contribution from Lyman-limit systems based on their measured statistical properties because the simulations are unable to reproduce the measured abundances of these systems, suggesting that their formation may involve more complex hydrodynamical processes than simple collapse into dark matter haloes. We examine the convergence properties of the flux power spectrum and autocorrelation function under differing assumptions regarding pressure smoothing. On the basis of our tests, we recommend that simulations used to estimate the flux power spectrum at high redshifts (z > 3) be performed in a box of comoving size at least 25 h -1 Mpc on a side and a comoving force mesh as fine as 30-60 h -1 kpc. The requirements become more severe at lower redshifts. Even with these specifications, we find that the flux autocorrelation function does not converge to better than 10 per cent on scales exceeding 3 per cent of the box size without added pressure smoothing. Much better convergence properties are obtained for the relative effects of the UV background fluctuations on the flux power spectrum. The background fluctuations increase the large-scale power and suppress the power at intermediate scales. At z ≤ 4, the effect is only at the few per cent level. By z = 5.5, however, the large-scale power is boosted by more than 10 per cent, growing rapidly at higher redshifts, and suggesting that the flux power spectrum may serve as a useful tool for distinguishing QSO-dominated UV background scenarios from those in which more abundant sources like galaxies dominate.

Acoustically treated structurally reinforced sound absorbing panel

An acoustically treated structurally reinforced sound absorbing panel having particular application for use at the inlet cowl of a jet engine to reduce engine noise leaked into the environment. The sound absorbing panel includes a sound receiving chamber having a honeycomb shaped core manufactured from corrugated metallic ribbons. The sound receiving chamber is sandwiched between a metallic backskin at one side thereof, and a metallic face sheet at the opposite side. The face sheet is perforated so that incoming sound waves which represent the engine noise enter the sound receiving chamber within which they are channeled towards and reflected between the metallic backskin and face sheet. An acoustic attenuation medium having a plurality of short metal fibers that are held together by a sintering process is diffusion bonded to the perforated face sheet so that the reflected sound waves which are channeled through the sound receiving chamber are transmitted through and attenuated by the acoustic attenuation medium.

Method of improved scatterer size estimation and application to parametric imaging using ultrasound.

The frequency dependence of RF signals backscattered from random media (tissues) has been used to describe the microstructure of the media. The frequency dependence of the backscattered RF signal is seen in the power spectrum. Estimates of scatterer properties (average scatterer size) from an interrogated medium are made by minimizing the average squared deviation (MASD) between the measured power spectrum and a theoretical power spectrum over an analysis bandwidth. Estimates of the scatterer properties become increasingly inaccurate as the average signal to noise ratio (SNR) over the analysis bandwidth becomes smaller. Some frequency components in the analysis bandwidth of the measured power spectrum will have smaller SNR than other frequency components. The accuracy of estimates can be improved by weighting the frequency components that have the smallest SNR less than the frequencies with the largest SNR in the MASD. A weighting function is devised that minimizes the noise effects on the estimates of the average scatterer sizes. Simulations and phantom experiments are conducted that show the weighting function gives improved estimates in an attenuating medium. The weighting function is applied to parametric images using scatterer size estimates of a rat that had developed a spontaneous mammary tumor.

Absorption filter correction by spectral shifts of ultrasonic propagation in nonlinear attenuating media

A closed form solution of the general case of frequency shift of a Gaussian pulse in a nonlinear power law attenuating medium of arbitrary exponent including the effect of transient radiation coupling process is presented. Also, this paper describes a simulation of the spectrum pathway in an ultrasonic A-scan. It includes transducer response, frequency dependent absorption in the test medium and radiation coupling transfer function. The measurement and simulation result has been used with the closed form solution to correct the absorption filter (attenuation coefficient). The attenuation coefficient correction for castor oil to fit the simulation result with recorded data is examined.

Scattering by a buried elongated object

The feasibility of combining spherical and spheroidal basis sets in formulating a transition matrix for scattering by elongated objects embedded in an attenuating medium such as an ocean sediment is investigated. While a description completely in terms of spheroidal bases would likely yield optimal stability, spheroidal functions are more difficult to compute and well-tested algorithms are scarce for complex arguments. Therefore, a hybrid approach that expands the surface fields of the scatterer in spheroidal bases and all other exterior fields in spherical functions is tried. This approach mitigates the need for spheroidal functions of a complex argument. The merits of the hybrid approach are assessed by comparison against standard transition matrix formulations. [Work supported by ONR.]

3-D shape recovery of planar and curved surfaces from shading cues in underwater images

Automatic construction of high-resolution topographical maps, or determining the shape of man-made objects, targets, and obstacles for localization, identification, and/or recognition, is an important capability in the application of autonomous or intelligent underwater vehicles. In this paper, we investigate the application of underwater image models for the reconstruction of three-dimensional object shapes from the shading cues in two-dimensional optical images. We first present the results of a sensitivity analysis in support of a simplified model for a Lambertian surface illuminated by a point source in an attenuating medium, proposed in earlier work. We then generalize a closed-form solution for planar scenes, previously proposed for negligible source-camera baseline, to the case where the baseline may be increased to minimize backscatter effects. For curved surfaces, we propose two techniques based on different iterative updating strategies to recursively improve the recovered surface shape. We study the performance of various methods using synthetic data and real images acquired under different turbidity conditions.

Spatial resolution performance of a one-dimensional linear amplitude-steered array in an attenuating medium

The amplitude-steered array concept [J. Acoust. Soc. Am. 59, 1040 (1976)] introduced the idea of steering the maximum response of a linear array by amplitude weighting the output signals of the elements. Recently, an evaluation of the linear amplitude-steered array assessed its performance such as lateral and axial resolution in a lossless medium [Frazier, Hughes, and O’Brien, J. Acoust. Soc. Am. 107, 2430 (2000)]. In the present work, the array’s performance is evaluated in an attenuating medium similar to biological tissue (1 dB/cm<th>MHz). Similar to the array’s performance in a lossless medium, the length of the array limits the axial and lateral resolution in the attenuating medium. As the steering direction is decreased, which corresponds to increasing frequency, the array’s performance with attenuation has a worse resolution when compared with its performance without attenuation. For example, using a 10-cm-length array and a 6-deg steering direction (4.6 MHz), the axial resolution (−3-dB point spread function width) with attenuation was 7.9 mm, compared with 6.4 mm without attenuation. When the steering direction was increased to 12 deg (2.3 MHz), the axial resolution with attenuation was 12.8 mm compared with 11.5 mm without attenuation. In the attenuating medium there is increased importance on the steering direction or frequency. [Work supported by NIH CA09067.]

Effects of lipids on n-alkane attenuation in media supporting oil-utilizing microorganisms from the oily Arabian Gulf coasts.

The Arabian Gulf is one of the most extensively oil-polluted areas of the world. The major objectives of this work were to study whether hydrocarbon-utilizing microorganisms indigenous to that area would readily accumulate added lipids, and whether this might affect their hydrocarbon consumption potential. Two prokaryotes, Arthrobacter nicotianae KCC B35 and the unidentified organisms KCC B6, as well as one eukaryote, Candida parapsilosis KCC Y1, were selected for this study. Biomass samples of the test organisms were incubated in an inorganic medium containing various concentrations of cholesterol, stearic acid, triolein or egg-phospholipids. The results revealed that all lipid classes were readily accumulated by the three test organisms. In addition, biomass samples were incubated for 6 h in an inorganic medium containing mixtures of individual lipid classes and either n-octadecane or n-docosane. The cells were removed and the residual alkanes in the medium were quantitatively recovered and analyzed by GLC. The results showed that out of the tested lipid classes only stearic acid exhibited a common stimulatory effect on the consumption of both n-alkanes by all test organisms. Other lipid classes were either inhibitory or had less pronounced effects than stearic acid.

Effects of lipids on n-alkane attenuation in media supporting oil-utilizing microorganisms from the oily Arabian Gulf coasts

The Arabian Gulf is one of the most extensively oil-polluted areas of the world. The major objectives of this work were to study whether hydrocarbon-utilizing microorganisms indigenous to that area would readily accumulate added lipids, and whether this might affect their hydrocarbon consumption potential. Two prokaryotes, Arthrobacter nicotianae KCC B35 and the unidentified organisms KCC B6, as well as one eukaryote, Candida parapsilosis KCC Y1, were selected for this study. Biomass samples of the test organisms were incubated in an inorganic medium containing various concentrations of cholesterol, stearic acid, triolein or egg-phospholipids. The results revealed that all lipid classes were readily accumulated by the three test organisms. In addition, biomass samples were incubated for 6 h in an inorganic medium containing mixtures of individual lipid classes and either n-octadecane or n-docosane. The cells were removed and the residual alkanes in the medium were quantitatively recovered and analyzed by GLC. The results showed that out of the tested lipid classes only stearic acid exhibited a common stimulatory effect on the consumption of both n-alkanes by all test organisms. Other lipid classes were either inhibitory or had less pronounced effects than stearic acid.

Absolute quantitation of a spherical I-131 activity distribution using a high-resolution rotating collimator: a phantom study

SPECT data were acquired using a high-resolution rotating parallel-hole collimator designed for imaging of medium-energy radionuclides. The authors reconstructed images of I-131 activity distributions for a 2.2 cm-diameter sphere inside a water-filled cylinder, which approximates the activity distribution and attenuation medium encountered in the I-131 monoclonal antibody treatment of brain tumors. Reconstructions were performed using FBP, OSEM and MAP/ICD methods including detector response modeling in the iterative algorithms. The MAP/ICD with an edge-preserving convex prior and OSEM reconstructions provided the more accurate absolute activity distributions with better resolution (FWHM of 0.5 cm). Once the image resolution was accounted for in the determination of volumes of interest, the sphere's activity and activity density were reproduced to within /spl plusmn/5% of their known values. The activity estimation based on MAP/ICD and OSEM reconstructions showed relatively little dependence on the size of the volume of interest compared to FBP images.

Precision Temperature Control of High-Throughput Fluid Flows: Theoretical and Experimental Analysis

A precision method for attenuating temperature variations in a high-throughput control fluid stream is described and analyzed. In contrast to earlier investigations, the present study emphasizes heat transfer analysis of the constituent control device and derives theoretical descriptions of system responses to time-varying fluid temperatures. Experiments demonstrate that the technique provides: (1) frequency-dependent attenuation which is several orders of magnitude greater than that obtained via a perfect mixing volume; (2) attenuation, over two decades of disturbance frequency, that reduces in-flow temperature variations by factors ranging from 10 to ≈104; (3) asymptotic attenuation greater than three orders of magnitude for spectral components having periods less than the device thermal equilibrium time; and (4) attenuation which is fully consistent with theoretical predictions. The model developed provides design criteria for tailoring system performance. In particular, it is shown that for a given control stream flow rate, the magnitude of maximal attenuation can be adjusted by varying the thermal resistance between the flow and attenuating medium, while the range of frequencies maximally attenuated can be adjusted by varying the product of thermal resistance and attenuating medium heat capacity. The analysis and design are general and should prove useful in the design and analysis of other high-throughput precision temperature control systems.

A three-dimensional ray-driven attenuation, scatter andgeometric response correction technique for SPECT in inhomogeneousmedia

The qualitative and quantitative accuracy of SPECT images is degradedby physical factors of attenuation, Compton scatter and spatially varyingcollimator geometric response.This paper presents a 3D ray-tracing technique for modellingattenuation, scatter and geometric response for SPECT imaging in aninhomogeneous attenuating medium. The model is incorporated into athree-dimensional projector-backprojector and used with the maximum-likelihoodexpectation-maximization algorithm for reconstruction of parallel-beam data.A transmission map is used to define the inhomogeneous attenuating andscattering object being imaged. The attenuation map defines the probability ofphoton attenuation between the source and the scattering site, the scatteringangle at the scattering site and the probability of attenuation of thescattered photon between the scattering site and the detector. The probabilityof a photon being scattered through a given angle and being detected in theemission energy window is approximated using a Gaussian function. Theparameters of this Gaussian function are determined using physicalmeasurements of parallel-beam scatter line spread functions from anon-uniformly attenuating phantom. The 3D ray-tracing scatterprojector-backprojector produces the scatter and primary components. Then, a3D ray-tracing projector-backprojector is used to model the geometric response of the collimator.From Monte Carlo and physical phantom experiments, it is shown thatthe best results are obtained by simultaneously correcting attenuation,scatter and geometric response, compared with results obtained with only oneor two of the three corrections. It is also shown that a 3D scatter model ismore accurate than a 2D model.A transmission map is useful for obtaining measurements ofattenuation and scatter in SPECT data, which can be used together witha model of the geometric response of the collimator to obtain corrected imageswith quantitative and diagnostically accurate information.

Compressional‐wave velocities in attenuating media: A laboratory physical model study

Elastic‐wave velocities are often determined by picking the time of a certain feature of a propagating pulse, such as the first amplitude maximum. However, attenuation and dispersion conspire to change the shape of a propagating wave, making determination of a physically meaningful velocity problematic. As a consequence, the velocities so determined are not necessarily representative of the material’s intrinsic wave phase and group velocities. These phase and group velocities are found experimentally in a highly attenuating medium consisting of glycerol‐saturated, unconsolidated, random packs of glass beads and quartz sand. Our results show that the quality factor Q varies between 2 and 6 over the useful frequency band in these experiments from ∼200 to 600 kHz. The fundamental velocities are compared to more common and simple velocity estimates. In general, the simpler methods estimate the group velocity at the predominant frequency with a 3% discrepancy but are in poor agreement with the corresponding phase velocity. Wave velocities determined from the time at which the pulse is first detected (signal velocity) differ from the predominant group velocity by up to 12%. At best, the onset wave velocity arguably provides a lower bound for the high‐frequency limit of the phase velocity in a material where wave velocity increases with frequency. Each method of time picking, however, is self‐consistent, as indicated by the high quality of linear regressions of observed arrival times versus propagation distance.

Implementation of a Tc-99m and Ce-139 scanning line source for attenuation correction in SPECT using a dual opposing detector scintillation camera.

Image degradation during single photon emission computed tomography (SPECT) due to attenuation and Compton scatter of photons can cause clinical image artifacts and will also result in inaccurate quantitative data. Therefore attenuation correction methods recently received wide interest. Transmission imaging can be performed to obtain the attenuation coefficients of a nonhomogeneous attenuating medium accurately. The aim of this study was firstly to evaluate the imaging characteristics of the scanning line source assembly. The results obtained with Tc-99m and Ce-139 were compared. Secondly the calculated attenuation coefficients were compared with known values from literature, using Tc-99m and Ce-139 as transmission sources. Lastly the method of acquiring simultaneous transmission and emission data was investigated. This study shows that an attenuation coefficient map can be obtained using a scanning line source for transmission imaging with a dual opposing detector camera. The imaging characteristics of Tc-99m and Ce-139 as transmission sources are similar. The resolution obtained with the Ce-139 line source was poorer than that obtained with the Tc-99m line source. A linear relationship was found between CT numbers and attenuation coefficients for transmission images using both Tc-99m and Ce-139 line sources. The attenuation coefficient value for water was underestimated by 1% using the Tc-99m transmission source and underestimated by 10% using Ce-139 as transmission source. This underestimation of attenuation coefficient values was also obtained in the human study. A myocardial perfusion study processed without and with attenuation correction clearly demonstrated the effect of the attenuation correction in the inferior myocardial region. The potential of using a scanning line source as transmission source with a dual opposing detector camera has been demonstrated in this study. The transmission source, Ce-139 was successfully introduced in this investigation for simultaneous acquisition of transmission and emission data.

Methods for measurement of the attenuation coefficient and phase velocity of Optison and their relevance to in vivo efficacy

We report methods and results of measurements of contrast media attenuation coefficient and phase velocity for concentrations ranging from 1.7×105 to 21×106 particles/ml. These concentrations cover the range likely to occur during in vivo application of ultrasound contrast media such as Optison. For the concentrations cited in this study, the attenuation coefficient exhibits a single peak between 1 to 2 MHz, while the phase velocity rises sharply and then levels off, typically near the same frequency at which the attenuation peak occurs. Previous studies from our laboratory have shown that these acoustic properties of ultrasound contrast media can change significantly with changing experimental conditions. For instance, the frequency at which the peak attenuation of Albunex occurs can shift by as much as 1 to 1.5 MHz as the measurement temperature is increased from 22° to 37 °C. Similarly, the attenuation peak width (full width at half maximum) can narrow by more than a factor of 2 (the exact amount depends on concentration). The peak attenuation values can shift by as much as 50% with increasing acoustic pressure. The primary aim of this study is to measure acoustic properties which are as close as possible to those occurring in clinical practice.

Attenuation and scatter correction of list-mode data driven iterative and analytic image reconstruction algorithms for rotating 3D PET systems

Image reconstruction directly from list-mode data requires different data correction techniques to standard projection-data based reconstruction, particularly in the case of iterative reconstruction. Attenuation and scatter correction techniques have been developed for two list-mode data driven reconstruction algorithms (FAIR-B (iterative) and Atrax (analytic/iterative)) recently proposed by the authors, and the results compared with two fast projection-data based algorithms (FORE+OSEM (iterative) and FORE+FBP (analytic)). List-mode data driven algorithms require event-by-event correction schemes, or alternatively image space procedures, as no direct operations to the completely sampled projection data set can be practically carried out. The methods developed in this work allow correction of list-mode data driven EM-ML type algorithms, such as FAIR, as well as analytic list-mode algorithms. The correction schemes have been applied to simulated data from various activity and attenuating medium distributions for a rotating 3D PET system. Both list-mode algorithms show some improvements in noise-contrast behaviour compared to the projection-data based methods.

A new method for photon transport in Monte Carlo simulation

Monte Carlo methods are used to evaluate data methods such as scatter and attenuation compensation in single photon emission CT (SPECT), treatment planning in radiation therapy, and in many industrial applications. In Monte Carlo simulation, photon transport requires calculating the distance from the location of the emitted photon to the nearest boundary of each uniform attenuating medium along its path of travel, and comparing this distance with the length of its path generated at emission. Here, the authors propose a new method that omits the calculation of the location of the exit point of the photon from each voxel and of the distance between the exit point and the original position. The method only checks the medium of each voxel along the photon's path. If the medium differs from that in the voxel from which the photon was emitted, the authors calculate the location of the entry point in the voxel, and the length of the path is compared with the mean free path length generated by a random number. Simulations using the MCAT phantom show that the ratios of the calculation time were 1.0 for the voxel-based method, and 0.51 for the proposed method with a 256/spl times/256/spl times/256 matrix image, thereby confirming the effectiveness of the algorithm.

Implementation and quantitative evaluation of analytical methods for attenuation correction in SPECT: a phantom study

Three differing exact methods of inverting the two-dimensional (2D) exponential Radon transform were implemented and evaluated quantitatively with a phantom study. The phantom had the shape of a pie-chart divided into six cavities, each 480 ml in volume and 10 cm in height, that were symmetrically positioned in a cylinder that was 20 cm in diameter and 10 cm in height. This phantom tests for linearity between true activity concentration and measured activity concentration, and it is denoted as a linearity phantom in the present study. Each cavity contained a different concentration of a homogeneous solution of 99mTc (74, 148, 222, 296, 370 and 444 kBq ml-1). Data acquisition was performed with two energy windows: a 20% photopeak energy window set symmetrically over the 140 keV of 99mTc and a secondary 5% energy window set over the 122 keV peak. We optimized a triple-energy window scatter correction method for a gamma camera-collimator system to obtain accurate scatter-corrected projections. A circular ROI 3 cm in diameter was identified over each cavity region, and count density (counts per pixel) was calculated. This value was converted to activity concentration (kBq ml-1) using a cross-calibration coefficient between SPECT counts and the gamma well counter. The relation between true activity (x) and measured activity concentration (y) was fitted to a line using the least-squares method. Regression lines were y = 0.63+1.0255x (R2 = 0.9987), y = -2.62+1.0278x (R2 = 0.9995), and y = 0.092+1.0241x (R2 = 0.9989) for the Bellini, Inouye and Metz-Pan methods respectively. In another phantom study using two different types of phantoms, contrast of a cold region in the two was 96% and 101% for all three methods. Combined optimized scatter correction and analytical attenuation correction methods achieve good accuracy in quantification of activity distribution with a uniform attenuating medium.