Frequency dependence of tissue attenuation measured by acoustic microscopy

Broadband scanning acoustic microscopy (SAM) has been used to investigate the mechanical properties of sections of tissue with a resolution of around 8 microns. The work reported here extends these results by reporting the frequency dependence of the attenuation coefficient from 100-500 MHz. A discussion of the theory of the measurements is presented. The scanning laser acoustic microscope (SLAM) is used to characterize similar tissue sections at 100 MHz. The data obtained with the two forms of acoustic microscopy are compared with results from the literature.     

Acoustic microscope lens modeling and its application in determining biological cell properties from single- and multi-layered cell models

The acoustic microscopy technique provides some extraordinary advantages for determining mechanical properties of living cells. It is relatively fast, of excellent spatial resolution, and of minimal invasiveness. Sound velocity is a measure of the cell stiffness. Attenuation of cytoplasm is a measure of supramolecular interactions. These parameters are of crucial interest for studying cell motility and volume regulations and to establish the functional role of the various elements of the cytoskeleton. Using a scanning acoustic microscope, longitudinal wave speed, attenuation and thickness profile of a biological cell were measured earlier by Kundu et al. [Biophys. J. 78, 2270-2279 (2000)]. In that study it was assumed that the cell properties did not change through the cell thickness but could vary in the lateral direction. In that effort the acoustic-microscope-generated signal was modeled as a plane wave striking the cell at normal incidence. Such assumptions ignored the effect of cell inhomogenity and the surface skimming Rayleigh waves. In this paper a rigorous lens model, based on the DPSM (distributed point source method), is adopted. For the first time in the literature the cell is modeled here as a multi-layered material and the effect of some external drug stimuli on a living cell is studied.     

A guided wave technique for the characterization of highly attenuative viscoelastic materials

The measurement of the acoustic properties of highly attenuative materials such as bitumen is very difficult. One possibility is to use measurements of the extent to which filling a cylindrical waveguide with the material affects the dispersion relationship of the cylinder. Torsional modes have been excited using piezoelectric transducers placed at one end of the cylinder, while the phase velocity and attenuation spectra have been measured by means of laser scanning. At each frequency, under the hypothesis of linear viscoelasticity, the phase velocity and attenuation of the fundamental torsional mode have been calculated as a function of the bulk shear velocity and the bulk shear attenuation of the inner core at that frequency. The resulting phase velocity and guided wave attenuation contour plots have been employed for deriving the unknown shear properties from the measured velocity and attenuation of the guided wave. The monochromaticity of the approach has not required a particular frequency dependence of the material properties to be assumed. Results for bitumen are given.     

Ultrasonic attenuation and velocity properties in rat liver as a function of fat concentration: a study at 100 MHz using a scanning laser acoustic microscope

This study examines the extent to which ultrasonic attenuation coefficients and velocity properties change between normal and fatty rat liver. The view of this problem is toward the application in clinical medicine in the future. Fatty livers were produced in rats by feeding them alcohol diets in liquid form. The animals were sacrificed and the fat concentration of the liver specimens determined. The fat concentration varied from 2.5% to 16.8% wet weight. The ultrasonic attenuation coefficient and velocity properties in 28 specimens were measured at 100 MHz with the scanning laser acoustic microscope (SLAM). Regression analysis was applied to the liver's ultrasonic propagation properties as a function of fat concentration. The results show that the attenuation coefficient increases at a rate of 1.08 dB/mm/% fat and the velocity decreases at a rate of 2.3 m/s/% fat as the fat concentration increases.     

The elastic microstructure of various tissues

Previous work has indicated that a modified Quate-Lemons scanning acoustic microscope (SAM) is capable of measuring the acoustic propagation properties of sections of biological tissue. The lens is excited by an impulse, rather than a tone burst, and the undemodulated returning signal from the tissue is recorded. The variations in received signal with time are used to deduce the sound speed, attenuation, impedance, and section thickness. In this article, the technique is applied to various types of tissue, and the variations in acoustic propagation properties are computed. Conventional tone burst SAM images at 425 MHz are compared with the time resolved data in order to elucidate the contrast mechanisms. The effects of varying the frequency and position of the focal plane on the tone burst images are interpreted in the light of the broadband results.     

Acoustic properties of aortic aneurysm obtained with scanning acoustic microscopy

In aortic aneurysm tissues, macrophages and their secretion of matrix metalloproteinases (MMPs) are playing important role for tissue degeneration. Some studies have shown that weakening of the mechanical properties of the degenerated tissues may progress the expansion of the aneurysm. However, actual measurement of the mechanical properties has not been investigated at microscopic level. The objective of the present study is to assess the mechanical properties of aortic aneurysm tissues by measuring acoustic properties by scanning acoustic microscopy (SAM). Twenty-one cases of aortic aneurysm including renal and common iliac aneurysm tissues were surgically excised. Each tissue was fixed by 4% formaldehyde and the specimens were treated as (1) picrosirius red staining for normal and polarized light microscopy, (2) CD68 staining for macrophage detection, and (3) no staining for acoustic microscopy. A specially developed SAM system operating in the frequency range of 100-200 MHz, was employed in the measurement. Images of amplitude and phase are obtained in a field of 2x2 mm. The intima was mainly consisted of degenerated collagen without polarization of picrosirius red staining. Macrophages stained by CD68 were observed near the degenerated collagen fibers. The sound speed was 1567 m/s in the intima, 1576 m/s in the media, 1640 m/s in the adventitia, respectively. Infiltration of macrophages showed higher values of attenuation and sound speed than the surrounding tissues. The sound speed of the intima was significantly lower than our previous measurement of atherosclerotic aorta without aneurismal change. As the tissue elasticity is closely correlated with the sound speed, the elasticity of the intima was considered to be lower in aneurysm tissues. This mechanical weakness may contribute to the expansion of the diameter of the aneurysm. Acoustic microscopy provided important data for assessing tissue mechanical properties of abdominal aneurysm.     

重塑砂质含气沉积物的声响应特性与模型预测分析

海洋沉积物中气泡的存在对沉积物的声学特性有显著影响.为实现在实验室内对不同初始应力条件下含气沉积物的声学特性测量,研制了一套可用于土工三轴仪的双探针声学测量系统,基于CT扫描试验获得重塑含气砂样品中气泡群的尺寸分布,确定其共振频率范围.试验结果表明:细砂沉积物声速随气体含量的增加呈指数型降低,衰减系数随气体含量的增加呈...     

Non-contact acoustic method for the simultaneous measurement of thickness and acoustic properties of biological tissues

The frequency dependence of the magnitude and phase shift of reflected waves from a thin tissue specimen has been found to yield characteristics patterns as a function of the specimen thickness. By analysis of those characteristic patterns, the specimen thickness can be determined so that the attenuation constant and the sound speed can be obtained by a non-contact procedure. The method is demonstrated in the frequency range 100 MHz to 200 MHz, using a scanning acoustic microscope and approximately 10 μm thick myocardial tissue samples of human origin, one paraffin embedded.     

阻抗复合消声器声学性能有限元预测方法及分析

为计算和分析具有复杂结构的阻抗复合式消声器的宽频消声性能,建立了一种高效声学有限元方法,给出了不同边界条件下的边界积分处理细节,得到有限元全局系数矩阵表达式,设计出计算程序框架以实现这些算法,其求解规模和计算速度与商业软件相比有优势。为计算阻抗复合式消声器的传递损失,通过阻抗管测量和数据拟合得到了吸声材料声学特性的经验公式。计算和测量了两通穿孔阻抗复合式消声器的传递损失,二者良好的吻合验证了声学有限元方法和计算程序的正确性。研究表明,插管长度影响消声器在中高频段的消声特性,右侧隔板上穿孔会消除共振峰,中高频消声性能随着出口管穿孔率的增加而提升。      

Acoustic characteristics of composite materials as acoustic window at oblique incidence of sound waves

This paper described a method for estimating the acoustic characteristics of composite materials at oblique incidence of sound waves. Composite materials are used as acoustic windows of SONAR to protect the internal sensors and electronic parts from water. In this study the composite material of glass reinforced plastic and polyurethane was used as the specimen. As the acoustic characteristics the velocities and attenuation coefficients of sound waves through the composite material were measured in the high frequency range. The insertion loss was also measured as a function of incident angle at 200 and 76 kHz, respectively. The attenuation coefficients in the low frequency range were estimated by interpolating the measured attenuation in the high frequency range with power-law form fitting. A four-medium layer model was proposed to estimate the insertion loss of composite materials at oblique incidence of sound waves in the low frequency range. The four-medium layer model well described the experimentally measured insertion loss at the high frequency range. It suggests that the insertion loss of the composite materials can be well estimated as a function of incident angle in the low frequency range.     

High-frequency ultrasound characterization of microporous biointegrable polymers in cornea using acoustic parameters

The current work deals with the use of high-frequency quantitative backscatter acoustic microscopy for the evaluation of the biointegration of microporous polymer implants used as support of artificial cornea. A three-dimensional (3-D) 80 MHz ultrasound microscope (25 microns axial resolution at focus) was used for the imaging and characterization of the progressive biointegration of polymers implanted in rabbit cornea. In-vitro and in-vivo studies were performed. Quantitative assessment of the structural changes in the biomaterial was performed using the spectral analysis of the radio frequency signal and the estimation of acoustic parameters in the 10-65 MHz frequency bandwidth. Correlation of in-vitro ultrasound data with histologic findings has shown that attenuation and backscatter coefficients are sensitive to the changes in the polymer pore content with time. Our results obtained in vivo demonstrated that 3-D 80 MHz echography coupled with quantitative characterization provide a unique tool for the non-invasive and objective follow-up of the implant biointegration and should contribute to clinical management of artificial cornea.     

Ultrasonic speed microscopy for imaging of coronary artery

We have been developing a scanning acoustic microscope (SAM) system for medicine and biology featuring quantitative measurement of ultrasonic speed and attenuation of soft tissues. In the present study, we will propose a new concept ultrasonic speed microscopy that can measure the thickness and ultrasonic speed using fast Fourier transform of a single pulsed wave instead of continuous waves used in conventional SAM systems. Six coronary arteries were frozen and sectioned approximately 10 microm in thickness. They were mounted on glass slides without cover slips. The scanning time of a frame with 300 x 300 pixels was 121 s and two-dimensional distribution of ultrasonic speed was obtained. The ultrasonic speed was 1720 m/s in the thickened intima with collagen fiber, 1520 m/s in lipid deposition underlying fibrous cap and 1830 m/s in calcified lesion in the intima. These basic measurements will help understanding echogenecity in intravascular ultrasound (IVUS) images. Imaging of coronary artery with the ultrasonic speed microscopy provides important information for study of IVUS coronary imaging.     

非侵入式超声波浆料浓度测量技术研究*

设计了一种基于非侵入式超声波透射衰减法的浆料浓度测量系统,根据超声传播衰减原理,建立超声衰减值与浆料浓度之间的关系。实验中采用生物显微镜和激光粒度仪对颗粒标称粒径进行验证,采用中心频率为200 kHz的超声波换能器,利用一发一收模式对超声波在有机玻璃管内的浆料进行非侵入式测量并分析透射波信号,对体积百分浓度小于25%、不同粒径的石英砂浆料进行测量,通过拟合方法获得浆料温度、体积百分浓度与声衰减对应的关系,并据此构造浓度求解方程,通过现场实时在线测量并与取样结果进行对比验证方程的准确性,结果显示,本方法可有效测量浆料浓度。     

Mechanical characterization of sintered piezo-electric ceramic material using scanning acoustic microscope

Lead Zirconate Titanate (PZT) is a piezo-electric ceramic material that needs to be characterized for its potential use in microelectronics. Energy dispersive X-ray analysis (EDX) is conducted to determine the chemical composition of the PZT ceramics. The scanning electron microscope (SEM) is performed to study the surface morphology, grain structure and grain boundaries. The SEM image helps us to understand the surface wave propagation and scattering phenomena by the PZT and the reason for its anisotropy and inhomogeneity due to the grain structure. In this paper scanning acoustic microscopy at 100 MHz excitation frequency is conducted for determining mechanical properties of PZT. Earlier works reported only the longitudinal wave speed in PZT while in this paper longitudinal, shear and surface acoustic wave speeds of sintered PZT are measured from its acoustic material signature (AMS) curves, also known as V(z) curves. AMS or V(z) curve is the variation of the output voltage as a function of the distance between the acoustic lens focal point and the reflecting surface. The average velocities of longitudinal, shear and surface acoustic waves in a PZT specimen are determined from its V(z) curve generated at 100 MHz excitation frequency and found to be over 5000 m/s, over 3000 m/s and between 2500 and 3000 m/s, respectively. From these velocities all elastic constants of the specimen are obtained.     

Absorption and scatter of encapsulated gas filled microspheres: theoretical considerations and some measurements.

Albunex is an ultrasound contrast agent for use in echocardiology and other areas. It is capable of passing the lung circulation after intravenous injection. A theoretical model is developed for some acoustic properties, particularly the scatter and absorption, of this contrast agent, considering the individual microspheres as air bubbles surrounded by a thin shell. The attenuation, the sum of absorption and scatter, of this contrast medium is measured with five transducers to cover the frequency range from 700 kHz to 8.5 MHz. It is concluded that the model correlates well with these acoustic measurements. When Albunex is used intravenously the backscatter enhancement in the left ventricle is caused mainly by the microspheres with diameters between 5 and 8 microns.     

Subsurface defect of amorphous carbon film imaged by near field acoustic microscopy

Amorphous carbon films were examined by low frequency scanning-probe acoustic microscopy (LF-SPAM). Local elastic properties as well as topography were imaged in the acoustic mode. Two kinds of subsurface defects were revealed by the LF-SPAM method. The influence of the subsurface defects on the elastic properties was also discussed. The ability to image subsurface defects was dependent on the scan area and the scan speed. Our results showed that the low frequency scanning-probe acoustic microscopy is a useful method for imaging subsurface defects with high resolution. PACS 68.37.Ps; 68.37.Uv; 61.43.Dq; 68.35.Gy     

Assessment of bone structure and acoustic impedance in C3H and BL6 mice using high resolution scanning acoustic microscopy

Two hundred-MHz time-resolved scanning acoustic microscopy was applied for the investigation of acoustic and structural bone properties of mice from two inbred strains. Transverse sections of femur taken from 5 C57BL/6J@Ico and 5 C3H/HeJ@Ico mice were explored. Both strains had the same bone diameter, but the C3H/HeJ@Ico mice had greater cortical thickness, smaller cancellous diameter, and greater acoustic impedance values than C57BL/6J@Ico mice. The strong differences in the measured acoustic impedances among the two inbred strains indicate that the impedance is a good parameter to detect genetic variations of the skeletal phenotype in small animal models.     

Frequency scanning ultrasonic pulse echo reflectometer

The frequency scanning ultrasonic pulse echo reflectometer (FSUPER) is a device which can be used to measure the ultrasonic velocity, attenuation coefficient and specific acoustic impedance of liquid samples as a function of frequency (0.3–6 MHz).     

Reverberation and frequency attenuation in forests--implications for acoustic communication in animals

Rates of reverberative decay and frequency attenuation are measured within two Australian forests. In particular, their dependence on the distance between a source and receiver, and the relative heights of both, is examined. Distance is always the most influential of these factors. The structurally denser of the forests exhibits much slower reverberative decay, although the frequency dependence of reverberation is qualitatively similar in the two forests. There exists a central range of frequencies between 1 and 3 kHz within which reverberation varies relatively little with distance. Attenuation is much greater within the structurally denser forest, and in both forests it generally increases with increasing frequency and distance, although patterns of variation differ between the two forests. Increasing the source height generally reduces reverberation, while increasing the receiver height generally reduces attenuation. These findings have considerable implications for acoustic communication between inhabitants of these forests, particularly for the perching behaviors of birds. Furthermore, this work indicates the ease with which the general acoustic properties of forests can be measured and compared.     

Assessment of precipitates of aged Ti-6Al-4V alloy by ultrasonic attenuation

Abstract

Ti-6Al-4V alloy with different microstructures was investigated by means of ultrasonic attenuation measurements. Widmanstätten and equiaxed microstructures were obtaining by heat treating a Ti-6Al-4V alloy. These two microstructures were over-aged at 545 °C at different ageing times. In order to find out the factors affecting the variation in the ultrasonic attenuation, the heat-treated samples were examined by optical microscopy and scanning electron microscopy. Based on the theory of ultrasonic attenuation in a solid media, the mechanisms of ultrasonic attenuation in the Ti-6Al-4V alloy with different microstructures were analysed. It was found that in both cases with Widmanstätten and equiaxed microstructures, the ultrasonic attenuation increased with frequency. After ageing, the ultrasonic attenuation was mainly attributed to the scattering loss which included the stochastic and the Rayleigh scattering due to the precipitation of Ti₃Al particles homogeneously distributed in the α phase. Data analysis presented in the study showed that ultrasonic attenuation yields more accurate area fractions of precipitates predictions when a polynomial fit is performed.