Evolution of bubble clouds induced by pulsed cavitational ultrasound therapy - histotripsy

Mechanical tissue fractionation can be achieved using successive, high-intensity ultrasound pulses in a process termed histotripsy. Histotripsy has many potential clinical applications where noninvasive tissue removal is desired. The primary mechanism for histotripsy is believed to be cavitation. Using fast-gated imaging, this paper studies the evolution of a cavitating bubble cloud induced by a histotripsy pulse (10 and 14 cycles) at peak negative pressures exceeding 21MPa. Bubble clouds are generated inside a gelatin phantom and at a tissue-water interface, representing two situations encountered clinically. In both environments, the imaging results show that the bubble clouds share the same evolutionary trend. The bubble cloud and individual bubbles in the cloud were generated by the first cycle of the pulse, grew with each cycle during the pulse, and continued to grow and collapsed several hundred microseconds after the pulse. For example, the bubbles started under 10 microm, grew to 50 microm during the pulse, and continued to grow 100 microm after the pulse. The results also suggest that the bubble clouds generated in the two environments differ in growth and collapse duration, void fraction, shape, and size. This study furthers our understanding of the dynamics of bubble clouds induced by histotripsy.     

Spatial variability in acoustic backscatter as an indicator of tissue homogenate production in pulsed cavitational ultrasound therapy

Spatial variability in acoustic backscatter is investigated as a potential feedback metric for assessment of lesion morphology during cavitation-mediated mechanical tissue disruption ("histotripsy"). A 750-kHz annular array was aligned confocally with a 4.5 MHz passive backscatter receiver during ex vivo insonation of porcine myocardium. Various exposure conditions were used to elicit a range of damage morphologies and backscatter characteristics [pulse duration = 14 micros, pulse repetition frequency (PRF) = 0.07-3.1 kHz, average I(SPPA) = 22-44 kW/cm2]. Variability in backscatter spatial localization was quantified by tracking the lag required to achieve peak correlation between sequential RF A-lines received. Mean spatial variability was observed to be significantly higher when damage morphology consisted of mechanically disrupted tissue homogenate versus mechanically intact coagulation necrosis (2.35 +/- 1.59 mm versus 0.067 +/- 0.054 mm, p < 0.025). Statistics from these variability distributions were used as the basis for selecting a threshold variability level to identify the onset of homogenate formation via an abrupt, sustained increase in spatially dynamic backscatter activity. Specific indices indicative of the state of the homogenization process were quantified as a function of acoustic input conditions. The prevalence of backscatter spatial variability was observed to scale with the amount of homogenate produced for various PRFs and acoustic intensities.     

Quantitative blood speed imaging with intravascular ultrasound

Previously, we presented a method of real-time arterial color flow imaging using an intravascular ultrasound (IVUS) imaging system, where real-time RF A-scans were processed with an FIR (finite-impulse response) filter bank to estimate relative blood speed. Although qualitative flow measurements are clinically valuable, realizing the full potential of blood flow imaging requires quantitative flow speed and volume measurements in real time. Unfortunately, the rate of RF echo-to-echo decorrelation is not directly related to scatterer speed in a side-looking IVUS system because the elevational extent of the imaging slice varies with range. Consequently, flow imaging methods using any type of decorrelation processing to estimate blood speed without accounting for spatial variation of the radiation pattern will have estimation errors that prohibit accurate comparison of speed estimates from different depths. The FIR filter bank approach measures the rate of change of the ultrasound signal by estimating the slow-time spectrum of RF echoes. A filter bank of M bandpass filters is applied in parallel to estimate M components of the slow-time DFT (discrete Fourier transform). The relationship between the slow-time spectrum, aperture diffraction pattern, and scatterer speed is derived for a simplified target. Because the ultimate goal of this work is to make quantitative speed measurements, we present a method to map slow time spectral characteristics to a quantitative estimate. Results of the speed estimator are shown for a simulated circumferential catheter array insonifying blood moving uniformly past the array (i.e., plug flow) and blood moving with a parabolic profile (i.e., laminar flow)     

Enhanced pulsed magneto-motive ultrasound imaging using superparamagnetic nanoclusters

Recently, pulsed magneto-motive ultrasound (pMMUS) imaging augmented with ultra-small magnetic nanoparticles has been introduced as a tool capable of imaging events at molecular and cellular levels. The sensitivity of a pMMUS system depends on several parameters, including the size, geometry and magnetic properties of the nanoparticles. Under the same magnetic field, larger magnetic nanostructures experience a stronger magnetic force and produce larger displacement, thus improving the sensitivity and signal-to-noise ratio (SNR) of pMMUS imaging. Unfortunately, large magnetic iron-oxide nanoparticles are typically ferromagnetic and thus are very difficult to stabilize against colloidal aggregation. In the current study we demonstrate improvement of pMMUS image quality by using large size superparamagnetic nanoclusters characterized by strong magnetization per particle. Water-soluble magnetic nanoclusters of two sizes (15 and 55 nm average size) were synthesized from 3 nm iron precursors in the presence of citrate capping ligand. The size distribution of synthesized nanoclusters and individual nanoparticles was characterized using dynamic light scattering (DLS) analysis and transmission electron microscopy (TEM). Tissue mimicking phantoms containing single nanoparticles and two sizes of nanoclusters were imaged using a custom-built pMMUS imaging system. While the magnetic properties of citrate-coated nanoclusters are identical to those of superparamagnetic nanoparticles, the magneto-motive signal detected from nanoclusters is larger, i.e. the same magnetic field produced larger magnetically induced displacement. Therefore, our study demonstrates that clusters of superparamagnetic nanoparticles result in pMMUS images with higher contrast and SNR.     

Flow imaging with pulsed Doppler ultrasound and flow phantoms

The use of a multigate profiling system with steady laminar flow in plastic tubes revealed spectral artifacts not previously described. In particular, a double or split profile was often observed. In this paper, these artifacts are related to the dual mode ultrasound propagation in the plastic tube. The propagation speeds and, therefore, refraction angles and propagation paths are different for the longitudinal and the shear wave. The power transmission can be extraordinarily sensitive to small variations in the angle of incidence, and this may combine with the existence of a range of angles of incidence within any focused ultrasound beam to produce spectral distortions. The plastic tube is thus shown equivalent to a selective filter, which diminishes some frequency components in the Doppler spectrum relative to others. The spectral artifacts are explained in terms of the relative power transmitted by each mode, and the degree of beam defocusing experienced by each. Spectral distortions persist even when the beam-to-flow orientation is well away from the critical angle. The results of this study show that it is feasible to understand the acoustic transmission behavior of a flow phantom, based on a knowledge of the material properties, and to demonstrate the usefulness of doing so.     

High frequency ultrasound imaging with optical arrays

Dynamically focused and steered high frequency ultrasound imaging systems require arrays with fine element spacing, wide bandwidths, and large apertures. However, these characteristics are difficult to achieve at frequencies greater than 30 MHz using conventional array construction methods. Optical schemes offer a solution. Focused laser beams incident on a suitable surface can generate and detect acoustic radiation. Precisely controlling the position and size of the beams defines points of transmission and detection, making it possible for pulse-echo image formation by synthetic aperture methods. An optical detection array was built, relying on a conventional piezoelectric transducer as an ultrasound source. The detection system, with near optimal resolution over a wide depth of field, demonstrates the potential for high frequency array implementation using optical techniques. A possible application is in pathology, where 2-D or 3-D fine resolution pulse-echo imaging can be performed in situ without the need for biopsies.     

Exposimetry of unfocused pulsed ultrasound

Therapeutic applications of unfocused pulsed ultrasound have recently been identified and require specialized exposimetry. Spatial distributions of pressure and several measures of intensity are presented for an unfocused pulsed ultrasound field produced by a PZT-4 transducer with diameter 3.8 cm and center frequency 2 MHz. Distributions of pressure I_tp and I_m were adequately uniform over a 4-cm² area in a transverse plane at an axial distance of 7 cm, but not so for I_ta and I_pa , which both exhibited off-axis asymmetric maxima     

High frequency ultrasound imaging using an active optical detector

Optical detection of ultrasound has numerous advantages over traditional piezoelectric methods. These systems offer noncontact inspection, rapid scanning capabilities, fine spatial sampling, and large bandwidths. In addition, difficulties associated with conventional ultrasound imaging systems such as cross-talk between elements, electrical connections, and electromechanical resonances are greatly reduced or even eliminated. Because of this, high frequency phased arrays for ultrasound detection can be emulated by accurately positioning and focusing optical beams on a suitable surface, which defines array elements. However, optical systems have lower sensitivity than their piezoelectric counterparts, limiting their widespread use in ultrasound imaging. Active optical detection offers a solution. An active ultrasound detector consisting of a neodymium-doped glass waveguide laser with an optical demodulation system, was built demonstrating enhanced sensitivity while preserving the benefits of traditional passive optical detection.     

一种基于气泡群的低频主动声呐目标模拟器设计

为了在水声测试过程中提供尺寸小、成本低、应用灵活、低频和高目标强度的目标模拟器,文中根据水中气泡群具有强散射这一特性,提出了一种面向主动声呐试验的气泡群模拟器设计方法。基于等效介质理论 (EffectiveMedium Theory, EMT)建立了气泡群声散射模型。首先将气泡群划分成立方体网格,然后应用图像处理方法获取立方体网格内气泡群的尺寸分布函数,接着使用 EMT 计算了每个立方体网格中的声反射系数和声波入射到立方体网格中产生的衰减,最后利用声波叠加原理计算了模拟器的反向散射目标强度。不同参数下的数值分析结果表明,气泡群尺寸分布函数呈伽马分布时,目标强度较高;目标强度随着尺寸分布区间减小和孔隙率增大而变大;共振频率随孔隙率增大而降低。结合气泡群孔隙率分布模型,文中给出了一组在低频时具有较高目标强度的小尺寸模拟器设计参数,可供相关设计者参考。     

High speed data collection system for magnetic field imaging by CTtechnique

A method of high-speed data collection used in imaging the magnetic-field distribution leaking from magnetic devices along any plane is proposed. The method uses the computed tomography (CT) technique. A high-speed data collection system developed for demonstrating the method is described. An image of the magnetic field is reconstructed from projection data provided by the voltage induced in the sensors of a line conductor and a rectangular coil moving in the plane of observation. Rotational scanning of the sensors is used for collecting the projections so that data-acquisition time can be reduced. To demonstrate the performance of the system, the reconstructed images of the magnetic-field distributions made by some permanent magnets are shown     

Optimal sound speed estimation using modified nonlinear anisotropic diffusion to improve spatial resolution in ultrasound imaging

In ultrasound exams of obese patients and the breast, the spatial and contrast resolutions of ultrasound images are severely deteriorated when a constant sound speed corresponding to soft tissue is used in receive dynamic beamformation. This degradation is due to the defocusing of the ultrasound beam because of the disparity in sound speed between soft tissue and fatty layers. To minimize the degradation, this paper proposes a new method of estimating an optimal sound speed that can be used to achieve the best beamforming performance in a region of interest (ROI). The proposed method employs a new focusing quality factor (FQF) as an indicator of how well the focusing is conducted with a given sound speed. The FQF is closely associated with the degree of edge conspicuity, which can be obtained using the proposed modified nonlinear anisotropic diffusion (MNAD) technique. To calculate FQF, ultrasound images are formed with different sound speeds ranging from 1400 to 1600 m/s and, subsequently, the ROI is chosen. In the ROI, the degrees of edge conspicuity (i.e., FQF) are calculated. The sound speed can be considered an optimal one for the ROI if it is used to construct the image that provides the maximum FQF. The performances of the proposed method were evaluated through simulation and in vitro experiments with a tissue-mimicking phantom. The performance was also compared with that of the conventional image-based method employed in a commercial ultrasound imaging system. The experimental results demonstrated that the proposed method is capable of estimating an optimal sound speed with an error of 10 m/s regardless of whether strong targets are included in the ROI or not. On the other hand, the conventional image-based method generated an estimation error of 60 m/s maximally in the case in which there were no strong targets in ROI. This indicates that the proposed method is a useful tool to improve ultrasound image quality for clinical applications, especially for ultrasound exams of obese patients and the breast.     

Experimental and theoretical spectral reflection properties of ice clouds generated in a laboratory chamber

In a preliminary experimental program, the measured bidirectional reflection properties between 1.0 and 3.5 mum from a grating spectrometer with a resolution of approximately 0.1 mum for ice crystal clouds generated in a cold chamber are compared with theoretical results computed from a line-by-line equivalent solar radiative transfer model. The theoretical calculations are based on the measured habits, concentrations, and sizes of the ice particles from replicas of the ice crystals that show a mean maximum size of approximately 7 mum. The experimental design was first tested with transmission measurements in a pure water-vapor environment that compare closely with theoretical expectations. Within the uncertainties and in consideration of the assumptions necessitated by the preliminary nature of this program, there is a close comparison between the experimental and theoretical results.     

安全壳内水池气泡破碎后夹带液滴的粒径分布

针对安全壳内气溶胶再夹带问题,设计液池表面气泡破碎特性研究可视化试验装置,进行典型试验条件下的气溶胶再夹带试验,验证高速摄影拍摄法对百微米级含气溶胶液滴进行测量的可行性;通过对液滴进行拍摄和分析,研究液池中气溶胶种类、质量浓度、水温对气泡破碎后液滴粒径分布的影响。结果表明:气溶胶种类、质量浓度、水温均对液滴粒径分布产生影响;升高温度和提高气溶胶悬浊液质量浓度,均使得液滴粒径分布变小; BaSO_4悬浊液较TiO_2悬浊液更不易生成夹带液滴;不同试验条件下的液滴粒径分布公式拟合结果较好。     

Numerical study on the dynamics of an electrical discharge generated bubble in EDM

In this paper, the influence of hydrodynamic behaviour of dielectric liquid media around a rapidly growing small spherical bubble in the process of electrical discharge machining (EDM) between the tool and the workpiece on material removal from the workpiece is investigated. The tool and the workpiece are assumed as two parallel rigid boundaries with dielectric liquid between them. The boundary integral equation method is applied for numerical solution of the problem. Results illustrate the time dependent shapes of the bubble generated between the two parallel rigid boundaries due to the electrical discharge. Results also indicate that continuous growth of the gas bubble leads to a sharp drop of pressure within the bubble which results in the sharp pressure drop over the surfaces of the tool and the workpiece. This pressure drop over the surfaces of the tool and the workpiece causes expelling of the gas dissolved in the molten metal and helps the molten material to escape.     

Debris clouds generated by hypervelocity impact of cylindrical projectiles with thin aluminum plates

Orthogonal, flash x rays were used to observe the debris clouds produced by the hypervelocity impact of cylindrical aluminum projectiles with thin aluminum sheets or bumpers. Three major structural features were observed in the debris clouds--a front cone, a bulbous main debris cloud, and an inner cone. Inclination of the projectile at impact changed the orientation of these features and the severity of damage to the rear wall of a double-sheet structure; projectiles with the greatest inclination produced the most damage. Two experiments, using aluminum and copper as projectile and target or target and projectile, respectively, were performed to determine the source of material in each of the three structural features of the debris clouds. The front cone and main cloud were shown to consist of bumper debris while the inner cone was composed of projectile fragments.     

Computations of bubble formation and pulsations generated by impacting cylindrical water jets

The impact of a water jet onto a water surface can entrain air-bubbles whose pulsations provide acoustic sources. Such impacts can occur during the breaking of a wave, or on a smaller scale, when a raindrop strikes a puddle of water. A better understanding of this phenomenon can lead to improved characterizations of the ambient noise and acoustic detection algorithms. Liquid on liquid impacts correspond to a breakdown of classical hydrodynamic theory which assumes that the free surface remains smooth and topologically invariant. A computational model using a generalized theory of hydrodynamics designed to rigorously treat liquid collisions is described in this paper. Numerical simulations are compared to experiments of a liquid cylinder impacting a still-water surface The simulations provide details not only of the initial formation of the air-entrained bubble at the time of the cavity collapse, but also the subsequent pulsations of this bubble until it rises back to the free surface. Computed initial bubble sizes and natural frequencies are compared to the experimental results for different cylinder lengths.     

Transient bubble domain configuration in garnet materials observed using high speed photography

The dynamic domain configurations of bubbles in garnet materials have been studied using a sampling optical microscope capable of single exposure photographs with a 10 nsec exposure time. The microscope is an integral part of a sampling system so that the transient shape of the bubble is recorded at various times after a field pulse or, for bubbles in field access devices, during a clock cycle. A triggerable flowing nitrogen gas laser pumping a low Q Rodamine 6G Dye laser is used as an illumination source giving light pulses of ∼1.5 KW for 10 nsec. This light is sufficient to expose Kodak 4 × 16 mm movie film. Standard pulse generators (HP 214A) are used to make free bubble radial velocity measurements. A modified generator allows free bubble collapse measurement to be made. For bubbles propagating at operating frequency within field access devices, a standard bubble exerciser is used, synchronized to the sampling system. In this case, special samples with an internal mirror and epi-mode illumination are used. Illustrative results of bubble domain shapes in a chevron propagating structure and a 90° chevron expander detector are included.     

Fast ultrasound imaging simulation in K-space

Most available ultrasound imaging simulation methods are based on the spatial impulse response approach. The execution speed of such a simulation is of the order of days for one heart-sized frame using desktop computers. For some applications, the accuracy of such rigorous simulation approaches is not necessary. This work outlines a much faster 3-D ultrasound imaging simulation approach that can be applied to tasks like simulating 3-D ultrasound images for speckle-tracking. The increased speed of the proposed simulation method is based primarily on the approximation that the point spread function is set to be spatially invariant, which is a reasonably good approximation when using polar coordinates for simulating images from phased arrays with constant aperture. Ultrasound images are found as the convolution of the PSF and an object of sparsely distributed scatterers. The scatterers are passed through an anti-aliasing filter before insertion into a regular beam-space grid to reduce the bandwidth and significantly reduce the amount of data. A comparison with the well-established simulation software package field II has been made. A simulation of a cyst image using the same input object was found to be in the order of 7000 times slower than the presented method. Following these considerations, the proposed simulation method can be a rapid and valuable tool for working with 3-D ultrasound imaging and in particular 3-D speckle-tracking.     

Use of modulated excitation signals in medical ultrasound. Part III: High frame rate imaging

This paper, the last from a series of three papers on the application of coded excitation signals in medical ultrasound, investigates the possibility of increasing the frame rate in ultrasound imaging by using modulated excitation signals. Linear array-coded imaging and sparse synthetic transmit aperture imaging are considered, and the trade-offs between frame rate, image quality, and SNR are discussed. It is shown that FM codes can be used to increase the frame rate by a factor of two without a degradation in image quality and by a factor of 5, if a slight decrease in image quality can be accepted. The use of synthetic transmit aperture imaging is also considered, and it is here shown that Hadamard spatial encoding in transmit with FM emission signals can be used to increase the frame rate by 12 to 25 times with either a slight or no reduction in signal-to-noise ratio and image quality. By using these techniques a complete ultrasound-phased array image can be created using only two emissions.