Effects of gas pockets on high-intensity focused ultrasound field

The paper describes experimental and numerical studies of the effects of gas pockets on a high-intensity focused ultrasound (HIFU) field. Air bubbles ranging from 0.8 to 2.4 mm in radius were produced in transparent polyacrylamide tissue-mimicking gels. A single-element 3.5-MHz HIFU transducer was used to sonicate the gel phantoms. The changes in the HIFU beam pattern for air bubbles at different positions were visualized by the Schlieren method. Quantitative measurements of pressure at the HIFU focus by a calibrated needle hydrophone showed considerable reduction in the focal pressure with the presence of an air pocket. The presence of a single 1.2-mm-radius air bubble, at a 5 mm axial pre-focal position, reduced the focal intensity by 50% and increased the lateral focal dimension by 50%. For air bubbles at pre-focal position close to the focus, lesion formation was observed not at the theoretical focus, but in front of the air bubble and the air bubble became a barrier for the post-focal ultrasound propagation. The effects of reflection were simulated numerically and were compared with the experiments. The results can be used as guidelines for evaluation of potential safety concerns produced by trapped gas-pockets in various HIFU therapies.     

Angiogenesis imaging by spatiotemporal analysis of ultrasound contrast agent dispersion kinetics

The key role of angiogenesis in cancer growth has motivated extensive research with the goal of noninvasive cancer detection by blood perfusion imaging. However, the results are still limited and the diagnosis of major forms of cancer, such as prostate cancer, are currently based on systematic biopsies. The difficulty in the detection of angiogenesis partly resides in a complex relationship between angiogenesis and perfusion. This may be overcome by analysis of the dispersion kinetics of ultrasound contrast agents. Determined by multipath trajectories through the microvasculature, dispersion permits a better characterization of the microvascular architecture and, therefore, more accurate detection of angiogenesis. In this paper, a novel dispersion analysis method is proposed for prostate cancer localization. An ultrasound contrast agent bolus is injected intravenously. Spatiotemporal analysis of the concentration evolution measured at different pixels in the prostate is used to assess the local dispersion kinetics of the injected agent. In particular, based on simulations of the convective diffusion equation, the similarity between the concentration evolutions at neighbor pixels is the adopted dispersion measure. Six measurements in patients, compared with the histology, provided a receiver operating characteristic curve integral equal to 0.87. This result was superior to that obtained by the previous approaches reported in the literature.     

Identification of ultrasound contrast agent dilution systems for ejection fraction measurements

Left ventricular ejection fraction is an important cardiac-efficiency measure. Standard estimations are based on geometric analysis and modeling; they require time and experienced cardiologists. Alternative methods make use of indicator dilutions, but they are invasive due to the need for catheterization. This study presents a new minimally invasive indicator dilution technique for ejection fraction quantification. It is based on a peripheral injection of an ultrasound contrast agent bolus. Left atrium and left ventricle acoustic intensities are recorded versus time by transthoracic echocardiography. The measured curves are corrected for attenuation distortion and processed by an adaptive Wiener deconvolution algorithm for the estimation of the left ventricle impulse response, which is interpolated by a monocompartment exponential model for the ejection fraction assessment. This technique measures forward ejection fraction, which excludes regurgitant volumes. The feasibility of the method was tested on a group of 20 patients with left ventricular ejection fractions going from 10% to 70%. The results are promising and show a 0.93 correlation coefficient with echographic bi-plane ejection fraction measurements. A more extensive validation as well as an investigation on the method applicability for valve insufficiency and right ventricular ejection fraction quantification will be an object of future study.     

A new formalism for the quantification of tissue perfusion by the destruction-replenishment method in contrast ultrasound imaging

A new formalism is presented for the destruction-replenishment perfusion quantification approach at low mechanical index. On the basis of physical considerations, best-fit methods should be applied using perfusion functions with S-shape characteristics. These functions are first described for the case of a geometry with a single flow velocity, then extended to the case of vascular beds with blood vessels having multiple flow velocity values and directions. The principles guiding the analysis are, on one hand, a linearization of video echo signals to overcome the log-compression of the imaging instrument, and, on the other hand, the spatial distribution of the transmit-receive ultrasound beam in the elevation direction. An in vitro model also is described; it was used to confirm experimentally the validity of the approach using a commercial contrast agent. The approach was implemented in the form of a computer program, taking as input a sequence of contrast-specific images, as well as parameters related to the ultrasound imaging equipment used. The generated output is either flow-parameter values computed in regions-of-interest, or parametric flow-images (e.g., mean velocity, mean transit time, mean flow, flow variance, or skewness). This approach thus establishes a base for extracting information about the morphology of vascular beds in vivo, and could allow absolute quantification provided that appropriate instrument calibration is implemented.     

Effects of damage thresholds on stresses in composite laminates under transverse impact

Studies on stresses and damage in fiber reinforced polymeric matrix composite laminates subjected to transverse impact are conducted by a 3D finite element analysis. The stress analysis is carried out by developing a constitutive equation including damage variables, therefore, effects of damage and damage thresholds on the stresses in the laminates can be investigated. Effects of damage threshold of matrix materials on stresses suggest suitable matrix materials for composite laminates, which could improve damage tolerance of the composite laminates, and resistance of the composite laminates to impact could be improved significantly by increasing the damage threshold.     

超声造影剂Optison的次谐波激励及阈值研究

1.引言: 超声造影剂大多是含包膜微气泡的液体,如Albunex,Optison,Levovist等.由于包膜微气泡可有效地增强散射回波强度,从而提高超声图像的对比度,因而超声造影剂在微血管血流显示、心脏灌注成像等得以广泛应用.近年来,超声造影剂的非线性特性受到广泛关注,包膜微气泡在超声作用下可产生非线性振动,激发二次谐波、次谐波和超谐波等.基于二次谐波及次谐波的超声非线性成像有重要的应用前景.     

Evaluation of ultrasound tissue damage based on changes in image echogenicity in canine kidney

Sufficiently high intensity ultrasound can create hyperechoic regions in an ultrasound image due to local bubble generation. We explore the link between the temporal extent of these hyperechoic regions and tissue damage caused by ultrasound therapy. The decay rate of increased echogenicity from the focal zone in insonated live exteriorized canine kidney was quantified and correlated to the spatial extent of tissue damage. The decay half-time, t(half), defined as the time for echogenicity enhancement to decay by a factor of 2, was observed in all cases to be greater than 41 s in spatial zones in which extensive histological damage was observed. In cases in which the measured thalf was less than 11 s, the damage was limited to minor hemorrhage, or it was not detected. These t(half) discrimination boundaries of 41 and 11 s were not statistically different for cases in which contrast agent was used to enhance therapeutic efficiency. This was true even though contrast agent infusion significantly reduced the therapy pulse duration threshold for damage production.     

An in vitro study of the correlation between bubble distribution, acoustic emission, and cell damage by contrast ultrasound

The objective of this study was to investigate the influences of total exposure duration and pulse-to-pulse bubble distribution on contrast-mediated cell damage. Murine macrophage cells were grown as monolayers on thin polyester sheets. Contrast agent microbubbles were attached to these cells by incubation. Focused ultrasound exposures (P/sub r/= 2 MPa) were implemented at a frequency of 2.25 MHz with 46 cycle pulses and pulse repetition frequencies (PRF) of 1 kHz, 500 Hz, 100 Hz, and 10 Hz in a degassed water bath at 10 or 100 pulses. A 1 MHz receive transducer measured the scattered signal. The frequency spectrum was normalized to a control spectrum from linear scatterers. Photomicrographs were captured before, during, and after exposure at a frame rate of 2000 fps and a pixel resolution of 960 times 720. Results clearly show that cell death is increased, up to 60%, by increasing total exposure duration from 0 ms to 100 ms. There was an increasing difference in cell damage between a 10-pulse exposure and a 100-pulse exposure with increasing PRF. The greatest change in damage occurred at 1000 Hz PRF with a 53% increase between 10-pulse and 100-pulse exposures. For each pulse from 0 to 10, an overlay of the 2 mum bubble count with corresponding emission shows consistent behavior in its pulse-to-pulse changes, indicating a correlation between acoustic emission, bubble distribution, and cell damage.     

Effects of phase aberration on tissue heat generation andtemperature elevation using therapeutic ultrasound

The effects of inhomogeneous propagation media on tissue heating patterns and steady-state temperature distributions are investigated analytically for continuous-wave therapeutic ultrasound using a random-phase screen model. Formulas for the statistical moments of the on-axis heat distribution, total heat deposited in a target volume, and axial temperature elevation are derived. Based on the statistical moments, we propose figures of merit to quantitatively assess therapeutic performance for a range of experimental parameters. The analysis relates statistical properties of the aberrator to those of heat generation in tissue. The mean on-axis heat distribution is substantially reduced in the focal zone, while its variance is increased at other axial positions. The amount of heat delivered to a target volume depends on the location of the target and the size of the volume. Compared to the results for homogeneous media, the greatest reduction in temperature increase occurs in the focal zone. Distortions in the temperature elevation patterns are greatest when the perfusion in tissue is large     

Effects of gage length, loading rates, and damage on the strength of PPTA fibers

Axial tension and transverse compression experiments on single fibers were performed to investigate the mechanical behavior of three high-performance fibers (Kevlar^®, Kevlar^® 129, and Twaron^®) with diameters in the order of 9-12 μm. The single fibers were manufactured from 1998 through 2008. A miniaturized tensile Kolsky bar was used to determine the tensile response of PPTA single fibers at a high strain rate. Gage length and strain rate were found to have minimum effects on the tensile strength of PPTA single fibers. Manufacturing time over a decade was found to have negligible effects on the tensile strength of the fibers. Initial transverse compression on the fibers reduces their ultimate tensile strengths. A high resolution scanning electron microscope (SEM) was also used to examine the fracture modes of transversely deformed fibers. Different types of fracture morphology were observed.     

Effects of changes in roller compactor parameters on granulations produced by compaction

A Chilsonator (Model L-83, The Fitzpatrick Company, Elmhurst, IL) was studied to gain a better understanding of how compactor parameters influence product characteristics. The materials chosen for this study were Avicel PH 101, hydrous lactose and an acetaminophen blend. The blend consisted of 20% Avicel PH 101, 20% hydrous lactose and 60% acetaminophen crystals. The compactor parameters studied were the roll speed, horizontal feed speed and vertical feed speed. Various combinations of high, medium and low levels of these compactor parameters were tested. The system was modeled using a quadratic regression model. Changes in particle size distribution and recompressibility of the roller compacted granulations were used to monitor the effects of the changes in compactor parameter levels. The models differed for each of the materials evaluated. The response surface plots produced using the models helped in predicting the compactor parameter levels which would produce granulations with the necessary physical characteristics for successful tablet production.     

Influence of ion energy on damage induced by Au-ion implantation in silicon carbide single crystals

This article reports on the influence of the ion energy on the damage induced by Au-ion implantation in silicon carbide single crystals. 6H-SiC samples were implanted with Au ions at room temperature at two different energies: 4 and 20 MeV. Both Rutherford Backscattering spectrometry in channelling geometry (RBS/C) and Raman spectroscopy were used to probe the ion implantation-induced damage. Results show that the accumulated damage increases with the fluence up to the amorphization state. RBS/C data indicate that 4-MeV implantation induces more damage than 20-MeV implantation at a given fluence. This effect is attributed to nuclear collisions since the amount of damage is identical at 4 or 20 MeV when the fluence is rescaled in dpa. Surprisingly, Raman data detect more damage for 20-MeV implantation than for 4-MeV implantation at low fluence (below 10¹³ cm⁻²) where point defects are likely formed.     

Investigation of the effects of microbubble shell disruption on population scattering and implications for modeling contrast agent behavior

In a previous study, quantitative measurement of nonlinear scattering revealed some quantitative discrepancies with a model for ultrasound scattering by a population ensemble of microbubbles. This study is designed to investigate the effect of the shell on single-pulse scattering by a population of contrast microbubbles. Nonlinear scattering was measured shortly (15 /spl mu/s) after exposure to a previous ultrasound pulse with variable intensity. The short time delay eliminated the influence of gas diffusion and focused the study on the consequence of ultrasound exposure. The results suggest that single-pulse scattering is dependent on the disruption properties of the bubble shell and confirm the significance of the shell properties.     

Effects of complexing agent on CdS thin films prepared by chemical bath deposition

CdS films have been prepared by chemical bath deposition (CBD) without stirring using weak and strong complexing agents, i.e., ammonia and ethylenediaminetetraacetic acid (EDTA). The optical, structural, and morphological properties of chemical bath deposited CdS films have been investigated. When the complexing agent is ammonia, five peaks in the X-ray diffraction (XRD) patterns from the CdS film, respectively, correspond to the interplanar spacing of 3.5498, 3.3429, 3.1449, 2.0574, and 1.7487 Å, which are definitely ascribed to hexagonal structure; unfortunately, this hexagonal CdS film is with poor morphology and its optical property in the visible region is not desirable for the solar cells. While, when the complexing agent is EDTA, three diffraction peaks in the XRD patterns from the CdS film, respectively, correspond to the interplanar spacing of 3.1164, 2.6716, and 1.8507 Å, indicating that the film is of a cubic structure. Furthermore, the CdS film has good morphology and its optical property in the visible region is compliant to the requirements of solar cells.     

Effect of oxide films produced by treatment in water vapor on the friction and surface damage of cast irons

The phase composition and microstructure of the rubbing surfaces, the degree of wear, and the friction coefficients of cast irons were studied on unoxidized specimens and on specimens oxidized in water vapor. It was established that the nature of wear of these cast irons tested under the same conditions differs.     

Effects of additives Si on casting structure of ceramic liner produced by a centrifugal-thermit process

In order to control the quality of ceramic layer produced by a centrifugal—thermit process, effects of the amount of additives Si on phase constituents and casting structure of the layer are studied. The results show that silicon is mainly distributed over the matrix phase of the inner part of the layer, and the matrix belongs to a spinel“solid solution” having many Si⁴⁺ ions replacing Al³⁺. Because of its low melting point, the fluidity of liquid ceramic and thus the surface quality are improved. However, the effect of silicon is not always positive, since the dissolution of Si⁴⁺ ions enlarges the temperature range of freezing, thus it increases the tendency of “constitutional supercooling” and facilitates the development of equiaxed zone. In addition, with increasing silicon content the viscosity of liquid ceramic will increase as well. All these factors make the liquid feeding more difficult and lead to poor surface quality and high porosity of the layer. The changes in casting structure with various Si contents are discussed from the above point of view.