Shape-based ultrasound tomography using a Born model with application to high intensity focused ultrasound therapy

A shaped-based ultrasound tomography method is proposed to reconstruct ellipsoidal objects using a linearized scattering model. The method is motivated by the desire to detect the presence of lesions created by high intensity focused ultrasound (HIFU) in applications of cancer therapy. The computational size and limited view nature of the relevant three-dimensional inverse problem renders impractical the use of traditional pixel-based reconstruction methods. However, by employing a shape-based parametrization it is only necessary to estimate a small number of unknowns describing the geometry of the lesion, in this paper assumed to be ellipsoidal. The details of the shape-based nonlinear inversion method are provided. Results obtained from a commercial ultrasound scanner and a tissue phantom containing a HIFU-like lesion demonstrate the feasibility of the approach where a 20 mm x 5 mm x 6 mm ellipsoidal inclusion was detected with an accuracy of around 5%.     

Compensation of fat layer effects in ultrasound imaging using an inclined-fat-layer model derived from magnetic resonance images

When cutaneous fat layers are in the ultrasound imaging region, the phase aberration caused by the fat layers induce image distortion as well as spatial resolution degradation. The phase aberration may complicate clinical procedures particularly when ultrasound imaging is employed for spatial positioning of medical devices like a biopsy needle or HIFU. To compensate the fat layer effects more precisely in beamforming, an inclined-fat-layer model has been established from the magnetic resonance images of the same imaging region as in the ultrasound scanning. We have verified utility of the fat layer model by taking images of a metal needle put into an inclined-fat-layer mimicking phantom. The ultrasound images taken with a 128-element linear phase array operating at 6 MHz have shown better resolution and less distortion when receive beamforming was performed with the phase delay data derived from the inclined-fat-layer model.     

Acoustic tomography of temperature distribution in a medium heated by a focused ultrasound beam

A new method is proposed for measuring temperature in the focal region of a high-intensity ultrasonic emitter on the assumption of axially symmetric temperature field. This method is based on solving the integral equation relating the temperature of the medium to the delay times of the probe ultrasound pulse intersecting the heated region in the transverse direction at different distances from the beam axis. The accuracy of the algorithm for calculating temperature with allowance for the finite experimental data set is analyzed. The calculation results are compared with the experimental data. Original Russian Text ? S.A. Tsysar’, O.A. Sapozhnikov, V.G. Andreev, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 4, pp. 558–561.     

Cathode spot energy transfer simulated by a focused laser beam

Minimum conditions for the formation of surface craters by laser irradiation were studied experimentally and theoretically for various metals. The critical power density for crater formation within 20 ns was about 10¹¹ W/m². It is therefore concluded that crater formation by ion bombardment requires an ion current density on the order of 10¹⁰ A/m²     

Ultrasonic beam profiles and beam propagation in an austenitic weld using a theoretical ray tracing model

A model for ultrasonic wave propagation in anisotropic and inhomogeneous materials is applied to the case of ultrasonic inspection of an austenitic V-butt weld manufactured by the downhand Manual Metal Arc technique. We examine the propagation behaviour of waves within the weld region and, in addition, model beam divergence behaviour. From this work we predict directions of low inspection sensitivity and also identify regions of material to which no ultrasound penetrates. The relative merits of the three different wave modes are examined, showing clearly the advantages of horizontally polarized shear waves for austenitic steel inspection. Vertically polarized shear waves are shown to be the least effective for such inspections. We discuss the relevance of this work to the ultrasonic non-destructive testing of austenitic steel components, concluding that care is needed over the choice of wave modes and angles, to ensure sensitive inspection of the whole weld material.     

Enhancement of photoacoustic tomography in the tissue with speed-of-sound variance using ultrasound computed tomography

The speed-of-sound variance will decrease the imaging quality of photoacoustic tomography in acoustically inhomogeneous tissue. In this study, ultrasound computed tomography is combined with photoacoustic tomography to enhance the photoacoustic tomography in this situation. The speed-of-sound information is recovered by ultrasound computed tomography. Then, an improved delay-and-sum method is used to reconstruct the image from the photoacoustic signals. The simulation results validate that the proposed method can obtain a better photoacoustic tomography than the conventional method when the speed-of-sound variance is increased. In addition, the influences of the speed-of-sound variance and the fan-angle on the image quality are quantitatively explored to optimize the image scheme. The proposed method has a good performance even when the speed-of-sound variance reaches 14.2%. Furthermore, an optimized fan angle is revealed,which can keep the good image quality with a low cost of hardware. This study has a potential value in extending the biomedical application of photoacoustic tomography.     

Multiple trapping using a focused hybrid vector beam

We propose a simple and efficient method that uses a single focused hybrid vector beam to confine metallic Rayleigh particles at multiple positions.We study the force mechanisms of multiple trapping by analyzing the gradient and scattering forces.It is observed that the wavelength and topological charges of the hybrid vector beam regulate the trapping positions and number of optical trap sites.The proposed method can be implemented easily in three-dimensional space, and it facilitates both trapping and organization of particles.Thus, it can provide an effective and controllable means for nanoparticle manipulation.     

Nanoscale effects in focused ion beam processing

Focused ion beams with diameters of 8 to 50 nm are used for material processing in the nanoscale regime. In this paper, effects of the ion beam–solid interaction determining the formation of small structures by ion-beam sputtering and chemically assisted material deposition and etching are investigated. In the case of decreasing feature size, angle-dependent sputtering, a non-constant sputter rate, and scattered ions play an important role. The impact on side-wall angle, aspect ratio, and shape of the bottom of the etched structures is discussed. In beam tail regions, these effects will be especially pronounced, leading to material swelling instead of material removal. Ion beam assisted etching and deposition will face additional effects. For small structures, gas depletion becomes a significant drawback. The impact on gas depletion and the competition with sputtering are discussed. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-9131/761360, E-mail: frey@iis-b.fhg.de     

基于射线模型的超声声速层析算法研究进展

得益于超声换能器硬件的进步和数据采集速度的提升,超声层析成像近年来得到了越来越多的关注,特别是在乳腺成像领域.超声层析成像中的声速图像分布可以为乳腺组织表征提供良好的定量分析数据,进而使得超声层析成像在乳腺癌的诊断方面有着良好的应用前景.目前,超声声速层析重建方法主要包括基于射线模型和全波模型的两大类算法.基于射线模型...     

Promoting inertial cavitation by nonlinear frequency mixing in a bifrequency focused ultrasound beam

Enhancing cavitation activity with minimal acoustic intensities could be interesting in a variety of therapeutic applications where mechanical effects of cavitation are needed with minimal heating of surrounding tissues. The present work focuses on the relative efficiency of a signal combining two neighbouring frequencies and a one-frequency signal for initiating ultrasound inertial cavitation. Experiments were carried out in a water tank, using a 550 kHz piezoelectric composite spherical transducer focused on targets with 46 μm roughness. The acoustic signal scattered, either by the target or by the cavitation bubbles, is filtered using a spectral and cepstral-like method to obtain an inertial cavitation activity measurement. The ultrasound excitations consist of 1.8 ms single bursts of single frequency f₀ = 550 kHz excitation, in the monofrequency case, and of dual frequency f₁ = 535 kHz and f₂ = 565 kHz excitation, in the bifrequency case. It is shown that depending on the value of the monofrequency cavitation threshold intensity the bifrequency excitation can increase or reduce the cavitation threshold. The analysis of the thresholds indicates that the mechanisms involved are nonlinear. The progress of the cavitation activity beyond the cavitation threshold is also studied. The slope of the cavitation activity considered as a function of the acoustic intensity is always steeper in the case of the bifrequency excitation. This means that the delimitation of the region where cavitation occurs should be cleaner than with a classical monofrequency excitation.     

Redeposition effects in transmission electron microscope specimens of FeAl-WC composites prepared using a focused ion beam

Artifacts associated with transmission electron microscope (TEM) specimens prepared using a focused ion beam (FIB) are not well understood, especially those in non-semiconductor materials. In this paper the extent and origins of artifacts associated with redeposition of milled material in TEM specimens of a FeAl--WC metal matrix composite prepared by FIB were investigated. Cross-sections were prepared normal to an initial FIB cut that allowed direct observation of any damage layers, which are believed to be associated with both redeposition of sputtered material and amorphisation of the surface of the specimen by the ion beam. Techniques for the minimisation of redeposition using either final cleaning mills at low accelerating voltages or plasma cleaning were also investigated and found to be ineffective in removing or reducing these damaged layers. TEM cross-sections of specimens treated using low energy mills and plasma cleaning, further confirmed that these techniques did little to reduce any redeposited or amorphous material.     

利用秩和条件数分析透射式超声CT的数据完备性

为了优化探头设计、估计重构图像误差,提出了一种利用秩和条件数分析透射式超声CT数据完备性的方法。首先分析了透射式超声CT的原理,透射式超声CT的测量方程可以表示成线性方程组,进而透射式超声CT的重构问题可以转化为线性方程组的求解问题。对于所选用的设计方案,描述线性方程组的可解性和解的稳定性的秩和条件数可以用来评估其数据完备性、估计重构图像误差。数学分析和试验结果表明,只有在测量方程组满秩时才可以求得唯一解,在测量误差和介质不均匀性相同时,重构误差近似正比与方程组的条件数,因此可以使用测量方程组的秩和条件数来衡量设计方案的数据完备性,指导实际装置的设计。     

Hemorrhage control in arteries using high-intensity focused ultrasound: a survival study

High-intensity focused ultrasound (HIFU) has been shown to provide an effective method for hemorrhage control of blood vessels in acute animal studies. The objective of the current study was to investigate the long-term effects of HIFU-induced hemostasis in punctured arteries. The femoral arteries ( approximately 2mm in diameter) of 25 adult anesthetized rabbits were surgically exposed, and either punctured and treated with HIFU (n=15), served as control (no puncture and no HIFU application: n=7), or were punctured and left untreated (n=3). Treated animals were allowed to recover, and examined and/or sacrificed on days 0, 1, 3, 7, 14, 28, and 60 after treatment to obtain ultrasound images and samples of blood and tissue. Hemostasis (arrest of bleeding) was achieved in all 15 of the HIFU-treated arteries. Eleven of the arteries were patent after HIFU treatment, and four arteries were occluded, as determined by Doppler ultrasound. The median HIFU application time to achieve hemostasis was 20s (range 7-55 s) for the patent arteries and 110 s (range 50-134 s) for the occluded arteries. In untreated animals, bleeding had not stopped after 120 s. One of the occluded arteries had reopened by day 14. No immediate or delayed re-bleeding was observed after HIFU treatment. Maximal blood flow velocities were similar in HIFU-treated patent vessels and control vessels. No significant difference in hematocrits was found between HIFU-treated and control groups at different time points after the procedure. Light microscopy observations of the HIFU-treated arteries showed disorganization of adventitia, and coagulation and thinning of the tunica media. The general organization of the adventitia and tunica media recovered to normal appearance within 28 days, with some thinning of the tunica media observed up to day 60. Neointimal hyperplasia was observed on days 14 and 28. The results show that HIFU can produce effective and long-term (up to 60 days) hemostasis of punctured femoral arteries while preserving normal blood flow and vessel wall structure in the majority of vessels.     

MR monitoring of focused ultrasound surgery in a breast tissue model in vivo

The objective of this study was to investigate MRI methods for monitoring focused ultrasound surgery (FUS) of breast tumors. To this end, the mammary glands of sheep were used as tissue model. The tissue was treated in vivo with numerous single sonications which covered extended target volumes by employing a scanning technique. The ultrasound focus position was controlled by online temperature mapping based on the temperature dependence of the relaxation time T(1). This approach proved to be reliable and offers thus an alternative to proton resonance frequency methods, whose application is hampered in fatty tissues. FUS-induced tissue changes were visible on T(2)- as well as on pre- and post-contrast T(1)-weighted images. According to our initial experience, noninvasive MRI-guided FUS of breast tumors is feasible.     

Imaging characterization of carbon nanotube tips modified using a focused ion beam

A carbon nanotube (CNT) tip, which assembled on the sharp end of a Si tip by dielectrophoresis, was structurally modified using focused ion beam (FIB). We described the imaging characterization of the FIB-modified CNT tip in noncontact AFM mode in terms of wear, deep trench accessibility, and imaging resolution. Compared to a conventional Si tip, the FIB-modified CNT tip was superior, especially for prolonged scanning over 10 h. We conclude that modified CNT tips have the potential to obtain high-quality images of nanoscale structures.     

Application of computed tomography images in the evaluation of magnetic nanoparticles biodistribution

In this work we evaluate the effectiveness of computed tomography images as a tool to determine magnetic nanoparticle biodistribution over biological tissues. For this purpose, tomography images for magnetic nanoparticles, composed of Fe₃O₄, coated with 2,3-dimercaptosuccinic acid (DMSA), were generated at several material concentrations. The comparison of CT numbers, calculated from these images generated at clinical conditions, with typical CT numbers for biological tissues, shows that the detection of nanoparticle in most tissues is only possible for high material concentrations.     

Electron acceleration in a tightly focused laser beam

Electron acceleration in a tightly focused ultra-intensity linear polarized laser beam is investigated numerically. It has been found that the acceleration is strong phase dependent and is periodic to the variety of the initial laser field phase. When optimal initial parameters are chosen, the electron can be accelerated effectively. The accelerated electrons are emitted in pulses of which the full width is less than the half period of the laser field.     

漂移对聚焦高斯光束闪烁影响的数值模拟

采用非自适应坐标变换对聚焦高斯光束在湍流大气中的传输进行了数值模拟,结果显示轴闪烁指数并没有出现如Rytov理论所预言的随初始光束半径的增大而明显减小的现象,其原因在于Rytov近似理论未考虑大尺度湍涡产生的漂移效应对闪烁的贡献。对比数值模拟结果与漂移理论结果以及相关实验结果,三者相吻合,表明未考虑漂移效应的Rytov近似理论不能完全准确地描述聚焦光束的闪烁特征,在研究聚焦光束的闪烁时,应当考虑漂移的影响。     

漂移对聚焦高斯光束闪烁影响的数值模拟

 采用非自适应坐标变换对聚焦高斯光束在湍流大气中的传输进行了数值模拟,结果显示轴闪烁指数并没有出现如Rytov理论所预言的随初始光束半径的增大而明显减小的现象,其原因在于Rytov近似理论未考虑大尺度湍涡产生的漂移效应对闪烁的贡献。对比数值模拟结果与漂移理论结果以及相关实验结果,三者相吻合,表明未考虑漂移效应的Rytov近似理论不能完全准确地描述聚焦光束的闪烁特征,在研究聚焦光束的闪烁时,应当考虑漂移的影响。