Two-dimensional phased arrays for application in surgery: Scanning by several focuses

Numerical simulation and comparative analysis of acoustic fields formed by two-dimensional phased arrays designed for ultrasonic surgery are conducted for the case of scanning by several focuses (in particular, by nine focuses arranged in a line and also by an array of nine focuses forming a 3×3 square grid). Calculations are performed for arrays with elements positioned at the surface both regularly (in square, ring, or hexagonal patterns) and randomly. Criteria for evaluating the “quality” of the intensity distributions in the field formed by the array in the case of scanning by several focuses are proposed. The quality of the intensity distributions for arrays containing 255 and 256 elements 5 mm in diameter arranged in regular patterns on the array’s surface (in square, ring, or hexagonal patterns) is inferior to that for arrays containing 256 randomly positioned elements. Among the regular arrays, the highest quality of intensity distributions is obtained for ring arrays, and the lowest quality is obtained for arrays with elements arranged in square or hexagonal patterns. The irregularity in the element positioning the array’s surface improves the quality of intensity distributions by reducing the secondary intensity peaks in the field formed by the array and, primarily, in the focal plane.     

Two-dimensional phased arrays for surgical applications: Multiple focus generation and scanning

A numerical simulation and a comparative analysis of the acoustic fields produced by two-dimensional phased arrays intended for ultrasonic surgery are performed for the case of a multiple focus (in particular, 25 foci) generation. The calculations were conducted for arrays (with an operating frequency of 1.5 MHz) consisting of 256 elements 5 mm in diameter, which were positioned on the array surface both regularly and randomly. The array foci can be formed simultaneously, but, in this case, the intensity levels of the secondary peaks in the ultrasonic field can exceed the values that guarantee the safe application of this method in surgery. A much safer way is to synthesize many foci with the use of several configurations, each of which contains a smaller number of foci. The number of foci in individual configurations must be approximately the same. It is demonstrated that randomization of the element distribution over the array surface provides an opportunity to improve the array performance, to reduce the intensity levels of secondary peaks in the acoustic field, and to increase the array capability for multiple focus scanning off the array axis.     

Heating of biological tissues by two-dimensional phased arrays with random and regular element distributions

The effect of an irregularity of the element distribution in a two-dimensional phased array upon the efficiency of heating of biological tissue is studied in an ultrasonic surgery regime. Two arrays of 256 piston elements, which either form a regular square pattern or are positioned randomly on the surface of a spherical segment, are considered as a model. The formation and the steering of a set of nine foci along the array axis and in the direction perpendicular to it are investigated. The theoretical model includes the algorithm of determining a phase set at the array elements that is optimal for the formation of foci with equal intensities and a preset geometry, as well as the calculation of acoustic and temperature fields in a tissue. The results of numerical simulation are presented for the spatial distributions of ultrasonic intensity, temperature, and the corresponding thermal dose in tissue. It is demonstrated that an irregularity of the element distribution reduces the level of secondary intensity peaks in the field produced by the array. This provides an opportunity to avoid the overheating and ablation of tissue outside the target volume, even in the case of steering with the set of foci away from the array axis within a distance of ±7 mm. A nine-foci regime is studied with the parameters necessary to produce uniform thermal ablation in a volume that is evaluated on the basis of the thermal dose distribution.     

Analytical method for evaluating the quality of acoustic fields radiated by a multielement therapeutic array with electronic focus steering

The paper presents an analytical method for calculating and analyzing the quality of 3-D acoustic fields of multielement phased arrays used in noninvasive ultrasound surgical devices. An analytical solution for the far field of each of its elements is used when calculating the array field. This method significantly accelerates calculations while preserving the high accuracy of results as compared to conventional direct numerical integration. Radiation from typical phased arrays is calculated using this approach, and the quality of their dynamic focusing is analyzed. Undesired diffraction effects caused by electronic focus steering are considered: an amplitude decrease in the main maximum and the appearance of grating lobes. The quality of dynamic focusing of the acoustic fields of two practically interesting arrays with a quasi-random element distribution (256 and 1024 elements, respectively), as well as of the regular array consisting of 256 elements is compared. In addition as well, a study is made of how the dimensions of the array elements and their spatial distributions affect the dimensions of the areas in which dynamic focusing is possible without occurrence of strong grating lobes and significant decrease in pressure amplitude at the main focus.

     

Incidence of element distribution on the ultrasonic field of segmented annular arrays

Segmented annular arrays are sometimes used for 3D ultrasonic imaging. However, owing to their geometrical complexity, the acoustic field generated by this type of aperture has not been adequately described. In this work, a method based on the array factor approach is used to describe the field radiated by sector annular arrays. This approach allows one to analyse the influence on the field of several aperture parameters, such as the number of elements per annulus, size and spatial distribution of elements, etc. In addition, strategies to reduce grating lobes are presented.     

Ultrasound phased arrays for prostate treatment.

The effect of array geometry on the steering performance of ultrasound phased arrays is examined theoretically, in order to maximize array performance under the given anatomical constraints. This paper evaluates the performance of arrays with spherical and cylindrical geometry, determined by using computer simulations of the pressure fields produced at various extremes of steering. The spherical segment arrays were truncated for insertion into the rectum, and contained either annular or linear elements. The cylindrical arrays were either flat or had a variable curvature applied along their length. Fields were computed by dividing the array elements into many point sources. The effectiveness of an array configuration when steered to a particular focal location was assessed by defining a parameter, G, as the ratio of the intensity at the desired focus to the maximum intensity of any unwanted lobes. The performance of truncated spherical arrays with annular elements was evaluated for focal steering along the array axis (in depth, in the z direction). When steered 15 mm toward the source, these truncated spherical annular arrays exhibited excellent performance, with G>5.7 for arrays containing more than 10 elements. Similarly, the spherical arrays with linear elements performed well when steered along the array axis to the same degree, with G>7 (for element widths up to 3 lambda), though many more array elements were required. However, when these arrays were steered 15 mm laterally, along the length of the prostate (the y direction), the value for G fell below 1 for element widths greater than about 1.6 lambda. It was found that the cylindrical arrays performed much better for y-direction steering (G>4, for 60 mm arrays with an element width of 1.75 lambda), but their performance was poorer when steered in the z direction (G approximately 4 for an element width of 1.5 lambda). In order to find a compromise between these extremes, a curved cylindrical array was examined, which was a cylindrical array with additional curvature along its length. These curved cylindrical arrays yielded performance between that of spherical linear arrays and cylindrical arrays, with better steering along the y direction than the spherical arrays and better z-direction steering than the cylindrical arrays.     

Effects of array arrangements in nano-patterned thin film media

In this work, the effect of different arrays arrangements on the magnetic behaviour of patterned thin film media is simulated. The modeled films consist of 80×80 cobalt grains of uniform diameter (20 nm) distributed into two different array arrangement: hexagonal (triangular) or square arrays. In addition to that, for each array arrangement, two cases of anisotropy orientations, random and textured films are considered. For both array arrangements and media orientations, hysteresis loops at different array separation (d) were simulated. Predictions show that for closely packed films, the shearing effects on the magnetization loop are much larger for the square array arrangement than the hexagonal one. According to these predictions, the bit switching field distribution in interacting 2D systems is much narrower for the hexagonal array arrangement. This result could be very important for high-density magnetic recording where a narrow bit switching field distribution is required.     

Simulation of three-dimensional nonlinear fields of ultrasound therapeutic arrays

A novel numerical model was developed to simulate three-dimensional nonlinear fields generated by high intensity focused ultrasound (HIFU) arrays. The model is based on the solution to the Westervelt equation; the developed algorithm makes it possible to model nonlinear pressure fields of periodic waves in the presence of shock fronts localized near the focus. The role of nonlinear effects in a focused beam of a two-dimensional array was investigated in a numerical experiment in water. The array consisting of 256 elements and intensity range on the array elements of up to 10 W/cm² was considered. The results of simulations have shown that for characteristic intensity outputs of modern HIFU arrays, nonlinear effects play an important role and shock fronts develop in the pressure waveforms at the focus.     

Uniform plasmonic near-field nanopatterning by backward irradiation of femtosecond laser

We present localized optical field distribution properties in the vicinity of gold particles on a silicon substrate by backward and forward irradiation. It is technically difficult to fabricate nanostructures on the surface by a conventional forward laser incident to the substrate because gold nanoparticles easily aggregate to form double-layered particle arrays. We calculated enhanced optical field properties in order to pattern the substrate surface only with a template of the bottom-layered particle arrays in the case that the backward irradiation of a femtosecond laser is used in the system of aggregated double-layered gold nanoparticle arrays. With the backward irradiation, the optical field intensity in the substrate for the double-layered hexagonal arrays is found to be only 30% lower than the mono-layered system. Moreover, a near field cannot be generated with the forward irradiation. As a result, only the backward irradiation scheme is found to be effective for uniform surface nanopatterning at enhanced plasmonic near-field zones.     

Investigation of L1_0 FePt-based soft/hard composite bit-patterned media by micromagnetic simulation

The soft/hard composite patterned media have potential to be the next generation of magnetic recording,but the composing modes of soft and hard materials have not been investigated systematically.L10 Fe Pt-based soft/hard composite patterned media with an anisotropic constant distribution are studied by micromagnetic simulation.Square arrays and hexagonal arrays with various pitch sizes are simulated for two composing types:the soft layer that encloses the hard dots and the soft layer that covers the whole surface.It is found that the soft material can reduce the switching fields of bits effectively for all models.Compared with the first type,the second type of models possess low switching fields,narrow switching field distributions,and high gain factors due to the introduction of inter-bit exchange coupling.Furthermore,the readout waveforms of the second type are not deteriorated by the inter-bit soft layers.Since the recording density of hexagonal arrays are higher than that of square arrays with the same center-to-center distances,the readout waveforms of hexagonal arrays are a little worse,although other simulation results are similar for these two arrays.     

256阵元高强度聚焦超声相控阵系统误差与多焦点模式精确控制

对一种256阵元中心开孔凹球面二维相控阵合成三维多焦点声场进行了系统的误差分析,总结了阵元激励信号的幅度和相位误差对声场参数的影响规律,为上百阵元相控阵驱动控制提供了设计容差依据.分析表明:上百阵元相控阵在声场合成能力方面具有很强的鲁棒性,5位相位量化精度足以保证合成声场的有效性,误差主要影响声场焦域能量的分布,通过提高相控阵发射总声功率等途径降低幅度误差百分比可以有效减弱固定方差的幅度误差的影响.     

基于微透镜阵列匀束的激光二极管面阵抽运耦合系统分析

为了提升高功率固体激光器中激光二极管(LD)面阵抽运场性能,采用几何光学和数理统计分析的方法,建立了基于微透镜阵列匀束的LD面阵抽运耦合系统的数学与物理模型,对微透镜阵列参数与最终耦合输出抽运场参数之间的关系进行分析,明确了微透镜单元F数、微透镜通光单元数以及微透镜阵列空间周期参数的设计原则.经实验测试,优化设计完成的LD面阵抽运耦合系统光场不均匀度为7.9%,耦合效率为90.7%.     

纳米银六角阵列在掺氧氮化硅中的局域表面等离激元共振特性仿真

通过COMSOL Multiphysics 和 Lumerical FDTD solution对不同尺寸纳米银六角阵列在非晶态掺氧氮化硅(a-SiN_x:O)介质中的局域表面等离激元共振(LSPR)特性进行仿真, 计算结果表明半径为25 nm的纳米银六角阵列形成的局域表面等离激元(LSP)与厚度为70 nm的a-SiN_x:O的蓝光发射(460 nm)的共振效果最为显著, 随着纳米银颗粒尺寸的增大其消光共振峰红移. 在460 nm波长激发下半径为25 nm的纳米银阵列在a-SiN_x:O中的极化强度和表面极化电荷的分布模拟证明了该阵列在460 nm激发下形成的LSP为偶极子极化模式, 通过对该尺寸的纳米银阵列的LSP 在a-SiN_x:O中的最强垂直辐射空间计算, 获得了银颗粒上方a-SiN_x:O的最佳厚度为30 nm, 仿真结果对硅基蓝光发射器件(450–460 nm)的设计提供了重要的理论参考.     

随机相控高强度聚焦超声换能器的栅瓣抑制

采用3种随机排列策略形成相控阵元线性排列结构抑制高强度聚焦超声(HIFU)相控阵栅瓣。第1种和第2种策略中阵元基于规则排列随机移动,而第3种策略中阵元则直接进行随机移动,阵元可移动范围依次为:第1种<第2种<第3种。采用瑞利积分和非线性Westervelt方程分别计算了3种策略对应随机相控阵产生的线性和非线性声场,并通过归一化栅瓣最大声压、归一化栅瓣平均声强和归一化旁瓣平均声强3个参量,对栅瓣抑制效果进行评价。结果表明:线性声场中,阵元可移动范围的增加有利于栅瓣抑制,3种随机策略的归一化栅瓣最大声压相比规则排列分别降低30.7%,53.8%和55.8%;非线性声场中对于同一种随机排列策略,随机度的增加可以改善栅瓣抑制效果。例如,第3种随机策略在随机度为0.9时正负压的归一化栅瓣最大声压相比规则排列分别降低55.6%和54.8%。进一步讨论了焦点偏移时随机相控阵的非线性声场,以-8 dB作为栅瓣的安全标准,第2种和第3种随机策略可以满足要求,横向偏移分别为6 mm和10 mm。本文的工作为抑制栅瓣提供了新思路,有利于随机HIFU相控阵的设计优化。      

Observation of large arrays of plasma filaments in air breakdown by 1.5-MW 110-GHz gyrotron pulses

We report the observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-micros pulses. Each element is a plasma filament elongated in the electric field direction and regularly spaced about one-quarter wavelength apart in the plane perpendicular to the electric field. The development of the array is explained as a result of diffraction of the beam around the filaments, leading to the sequential generation of high intensity spots, at which new filaments are created, about a quarter wavelength upstream from each existing filament. Electromagnetic wave simulations corroborate this explanation and show very good correlation to the observed pattern of filaments.     

High refractive index sensitivity sensing in gold nanoslit arrays

The extraordinary optical transmission(EOT) phenomenon of nano-periodic aperture array in metallic film has been widely investigated and used in biosensors. The surface plasmon resonance and cavity mode in some periodic nanostructures, such as nanohole and nanoslit, cause EOTs at certain wavelengths. This resonance wavelength is sensitive to the refractive index on the surface of periodic nanostructures. Therefore, the metallic nanostructures are expected to be good sensing elements. The sensing performances of gold nanoslit arrays are experimentally and theoretically investigated.Three-dimensional finite difference time domain(FDTD) simulations are utilized to explore their transmission spectra and steady-state field intensity distributions. The electron beam evaporation, electron beam lithography, and ion milling are applied to the gold nanoslit arrays with different widths and periods. The sensing performances of the gold nanoslit array are characterized via transmission spectra in four kinds of refractive index samples. The highest sensitivity reaches726 nm/RIU when the width of the gold nanoslit array is 38.5 nm.     

Evolution of high-intensity focusing systems for different applications in medicine (Review)

The evolution of high-intensity focusing systems for application in medicine has been outlined from their introduction (in the 1950s) to date. This work is in fact a review with two sections. The focusing systems for medical application discussed under the first section are based on using single focusing transducers with a radiating element shaped as a section of a spherical shell. Attention is mainly given to the devices developed at the Andreev Acoustics Institute in the 1970s–1980s; focusing systems developed abroad in the past few decades and most widely clinically used are also briefly discussed there. The second section of the review is devoted to focusing systems based on phased arrays which allow users not only to electronically steer the focus within a selected range, but also, especially importantly for various medical applications, to simultaneously create several foci. The main attention under the second section is given to the two-dimensional phased arrays with randomized location of elements on the surface, which the Acoustics Institute developed in active collaboration with others. It has been shown that irregularity in the location of elements either substantially improves the quality of spatial intensity distributions by reducing the level of secondary maxima of intensity in the array-induced fields or allows to considerably reduce the number of elements in the array with the same quality of distributions. The paper discusses principles of designing such arrays, methods of their calculations and possible applications.     

The possibility of generating focal regions of complex configurations in application to the problems of stimulation of human receptor structures by focused ultrasound

Studies of the stimulating effect of ultrasound on human receptor structures have recently become more intensive in connection with the development of promising robotic techniques and systems, sensors, and automated control systems, as well as with the use of taction in the design of a human-machine interface. One of the promising fields of research is the development of tactile displays for transmission of sensory data to a human by an acoustic method based on the effect of radiation pressure. In this case, it is necessary to generate rapidly changing patterns on a display (symbols, letters, digits, etc.), which may often have a complex shape. It is demonstrated that such patterns can be created by the generation of multiple-focus ultrasonic fields with the help of two-dimensional phased arrays whose elements are randomly positioned on the surface. The parameters for such an array are presented. It is shown that the arrays make it possible to form the regions of action by focused ultrasound with various necessary shapes and the sidelobe (or other secondary peak) intensity level acceptable for practical purposes. Using these arrays, it is possible to move the set of foci off the array axis to a distance of at least ±5 mm, which corresponds to the display dimensions. It is possible, on the screen of a tactile display, to generate the regions of action with a very complex shape, for example, Latin letters. This opportunity may be of interest, for example, for the development of systems that enable a blind person to perceive the displayed text information by using the sense of touch.     

Non-integral dimensions ultrasonic phased arrays in a borehole

The non-integral dimensions ultrasonic phased arrays and their scanning and testing methods in a borehole are studied. First, the focusing acoustic fields excited by the 1.25D, 1.5D, and 1.75D phased arrays are analyzed, and then the imaging resolution in the elevation direction and the influence of the dynamic elements are investigated. Second, the focusing and deflexion characteristics of the acoustic fields excited by the annular and segmented annular phased arrays are studied, and they are compared with those excited by the 2D surface array. The application method of the 1.25D, 1.5D, and 1.75D, annular and segmented annular phased arrays in acoustic logging are analyzed and discussed. It provides a theoretical foundation for the application of the ultrasonic phased arrays in acoustic logging.     

Bounds and exact theories for the transport properties of inhomogeneous media

We investigate the bounds of Milton on the transport coefficient of a two-component composite, in their application to the square and hexagonal arrays of cylinders, and the three cubic lattices of spheres. We show that, in all five cases, as more information is supplied about the geometry of the composite, the bounds converge to the precise point obtained from an exact theory specific to the geometry in question. We illustrate the use of the bounds in determining whether a set of known values of the transport coefficient adequately specifies the general behaviour of that quantity. We determine the values of two structure-dependent parameters for cell materials with spheroidal cells and the value of one parameter for hexagonal and square arrays of cylinders with missing array elements. These parameters determine bounds both on the transport and on the elastic properties of the respective materials.