A comparative study of systems used for dynamic focusing of ultrasound

A comparative study of two methods used for dynamic focusing of ultrasound: the conventional phased arrays and a new method based on time reversal of acoustic signals is carried out. A laboratory model of the focusing system based on time reversal is developed and manufactured. One of the principal elements of the system is a reverberator with several piezoelectric transducers attached to its walls. Experiments are carried out to demonstrate the ability of such a system to generate one focus or several foci and to steer them electronically at considerable distances (50 mm at minimum) off the axis of the focusing system without causing the appearance of any grating lobes or other secondary intensity maxima. The focusing properties of the system are compared with the results of numerical simulation of two-dimensional phased arrays, whose parameters are taken to be typical for the arrays used in extracorporeal surgery. The important role of randomization is demonstrated for both of the aforementioned focusing methods. The prospects of practical application of the two methods are discussed.     

平面相控阵的尺寸对声波聚焦效果的影响

对平面相控阵加上一定的时间延迟即可实现声波的聚焦的性质进行了研究，推出了平面相控阵的空间声压分布表达式，并采用数值分析法，探讨了相控阵的尺寸对声波聚焦效果的影响。为进一步研究相控阵打下了基础。     

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.     

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.

     

Broadband acoustic focusing by symmetric Airy beams with phased arrays comprised of different numbers of cavity structures

We realize broadband acoustic focusing effect by employing two symmetric Airy beams generated from phased arrays,in which the units of the phased arrays consist of different numbers of cavity structures, each of which is composed of a square cavity and two inclined channels in air. The exotic phenomenon arises from the energy overlapping of the two symmetric Airy beams. Besides, we demonstrate the focusing performance with high self-healing property, and discuss the effects of structure parameters on focusing performance, and present the characteristics of the cavity structure with straight channels. Compared with other acoustic lenses, the proposed acoustic lens has advantages of broad bandwidth(about 1.4 kHz), high self-healing property of focusing performance, and free adjustment of focal length. Our finding should have great potential applications in ultrasound imaging and medical diagnosis.     

非整数维超声相控阵探测方法研究

研究了非整数维超声相控阵聚焦特性以及在石油井下的扫描探测方法,首先对1.25D,1.5D和1.75D相控阵聚焦声场进行了分析,对如何提高仰角方向分辨率及减少动态阵元数等问题进行了深入的讨论;然后,研究了环形和分割环形相控阵列聚焦声场及声束偏转等特性,并和2D面阵进行了对比分析;论文还对1.25D、1.5D、1.75D、环形和分割环形相控阵列在石油测井中的应用方法进行了研究和讨论,为超声相控阵技术在井下应用提供了理论基础。     

Ultrasonic phased array controller for hyperthermia applications

Multiple and mechanically scanned ultrasound transducer systems have demonstrated the efficacy of using ultrasound to produce deep localized hyperthermia. The use of ultrasonic phased arrays has been proposed as an alternative to these systems. A phased array offers a more flexible approach to heating tumours in that the size, shape, and position of its focal region can be altered during the course of treatment in order to achieve the desired temperature distribution. This added flexibility comes at the cost of increased complexity of the hardware necessary to drive the transducer because each element requires its own amplifer with both phase and amplitude control. In order for phased arrays with large numbers of elements to be feasible for hyperthermia applications, the complexity of this circuitry must be minimized. This paper describes a circuit design which simplifies the electronics required to control a phased array transducer system for hyperthermia applications. The design is capable of controlling virtually any type of phased array transducer operating at frequencies less than 2 MHz. The system performance was verified through beam profile measurements using a 48-element tapered phased array transducer.     

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.     

Two-dimensional phased arrays for surgery: Movement of a single focus

Numerical simulation and comparative analysis of acoustic fields generated by two-dimensional phased arrays designed for ultrasonic surgery is conducted. The case of movement of a single focus by an array with the surface shaped as a part of a spherical shell with the curvature radius 120 mm is considered. The influence of the number of elements (varying from 64 to 1024), their diameter (from 2.5 to 10 mm), frequency (from 1 to 2 MHz), and the degree of sparseness of the elements at the array surface on the field characteristics is studied. The calculations are performed for arrays with elements positioned at the surface both regularly (in square, annular, or hexagonal patterns) and randomly. Criteria for the evaluation of the “quality” of intensity distributions in the field generated by an array in the case of movement of a single focus are suggested. Of all arrays studied, the best quality of distributions is obtained for an array containing 256 elements of diameter 5 mm randomly positioned at the array surface. The quality of the intensity distributions for arrays consisting of 255, 256, and 1024 elements positioned regularly (in square, annular, and hexagonal patterns) is inferior to the corresponding quality for arrays with randomly positioned elements. The irregularity in elements’ positioning considerably improves the distribution quality by suppressing the secondary intensity peaks in the field generated by the array; or, alternatively, it provides an opportunity to obtain the same distribution quality with a fraction of the number of elements in the array. The effects of the number and shape of elements, errors in phase setting, frequency modulation of signals, and non-uniform distribution of amplitudes over the array surface on the distribution quality are analyzed.     

Perspectives in clinical uses of high-intensity focused ultrasound

Focused ultrasound holds promise in a large number of therapeutic applications. It has long been known that high intensity focused ultrasound can kill tissue through coagulative necrosis. However, it is only in recent years that practical clinical applications are becoming possible, with the development of high power ultrasound phased arrays and noninvasive monitoring methods. These technologies, combined with more sophisticated treatment planning methods allow noninvasive focusing in areas such as the brain, that were once thought to be unreachable. Meanwhile, exciting investigations are underway in microbubble-enhanced heating which could significantly reduce treatment times. These developments have promoted an increase in the number of potential applications by providing valuable new tools for medical research. This paper provides an overview of the scientific and engineering advances that are allowing the growth in clinical focused ultrasound applications. It also discusses some of these prospective applications, including the treatment of brain disorders and targeted drug delivery.     

On the possibility of using multi-element phased arrays for shock-wave action on deep brain structures

A noninvasive ultrasound surgery method that relies on using multi-element focused phased arrays is being successfully used to destroy tumors and perform neurosurgical operations in deep structures of the human brain. However, several drawbacks that limit the possibilities of the existing systems in their clinical use have been revealed: a large size of the hemispherical array, impossibility of its mechanical movement relative to the patient’s head, limited volume of dynamic focusing around the center of curvature of the array, and side effect of overheating skull. Here we evaluate the possibility of using arrays of smaller size and aperture angles to achieve shock-wave formation at the focus for thermal and mechanical ablation (histotripsy) of brain tissue taking into account current intensity limitations at the array elements. The proposed approach has potential advantages to mitigate the existing limitations and expand the possibilities of transcranial ultrasound surgery.     

Arbitrary shaped, liquid filled reverberators with non-resonant transducers for broadband focusing of ultrasound using Time Reversed Acoustics

The ability to generate short focused ultrasonic pulses with duration on the order of one period of carrier frequency depends on the bandwidth of the transmitter as the pulse duration is inversely proportional to the bandwidth. Conventional focusing arrays used for focusing ultrasound have limited bandwidth due to the resonant nature of the piezoelements generating ultrasound. Theoretically it is possible to build a broadband phased array composed of “non-resonant” elements: wedge-shaped or flat-concave piezotransducers, though there are numerous technical difficulties in designing arrays with hundreds of elements of complex shape. This task is much easier to realize in an alternative technique of ultrasound focusing based on the principles of Time Reversed Acoustics (TRA) because in TRA systems, effective focusing can be achieved with just a few, or even one, transducers. The goal of this study is to demonstrate the possibility of broadband focusing of ultrasonic waves using a TRA system with non-resonant transducers and to explore the factors affecting the performance of such a system. A new type of TRA reverberators, such as water-filled thin-wall plastic vessels, which can be used with the submersible piezotransducers fixed internally in the reverberator, are proposed and tested. The experiments are conducted in a water tank with the walls and bottom covered by a sound absorbing lining. A needle hydrophone mounted on a 3D positioning system is used as a beacon for the TRA focusing and then for measuring the spatial distribution of the focused ultrasound field. The bandwidth and spatial distribution of the signal focused by the TRA system using a single channel with the resonant versus non-resonant transducers have been analyzed. Two types of non-resonant transducers were tested: a flat-concave transducer with a diameter of 30 mm, and a thickness varying from 2 mm in the center to 11 mm at the edge, and a specially designed submersible transducer having an uneven shape with a diameter of about 25 mm and a thickness varying from 2 to 6 mm. It was shown that TRA focusing system using non-resonant transducer had a bandwidth at 10 dB of 500 kHz while the resonant transducer provided about 100 kHz bandwidth. Correspondingly, the extended bandwidth of the TRA focusing system, especially toward higher frequencies, provides a 50% sharper spatial distribution. Furthermore, the relative level of the background ultrasound was reduced by a factor up to 3 as more frequencies were added coherently in focus and incoherently out of focus. Advantages of water-filled reverberators made of thin-wall plastic vessels include easy manufacturing, low costs, extreme simplicity, and good acoustical matching with soft tissues, important for biomedical applications.     

固体中线性相控阵列的瞬态聚焦特性

针对线性相控阵列在固体介质中的声场聚焦特性及参数优化问题,该文给出了一个线性阵列的纵波瞬态聚焦声场模型.数值结果表明,当阵元被短时脉冲信号激励时,瞬态聚焦声场中不会形成栅瓣,突破了传统稳态理论模型中对阵元间距的限制;同时由于横纵波在聚焦区域内可完全分离,声束旁瓣的幅值也得到了抑制.其次,增大阵元间距能够显著提高聚焦性能...     

Examination of time-reversal acoustics in shallow water and applications to noncoherent underwater communications

The shallow water acoustic communication channel is characterized by strong signal degradation caused by multipath propagation and high spatial and temporal variability of the channel conditions. At the receiver, multipath propagation causes intersymbol interference and is considered the most important of the channel distortions. This paper examines the application of time-reversal acoustic (TRA) arrays, i.e., phase-conjugated arrays (PCAs), that generate a spatio-temporal focus of acoustic energy at the receiver location, eliminating distortions introduced by channel propagation. This technique is self-adaptive and automatically compensates for environmental effects and array imperfections without the need to explicitly characterize the environment. An attempt is made to characterize the influences of a PCA design on its focusing properties with particular attention given to applications in noncoherent underwater acoustic communication systems. Due to the PCA spatial diversity focusing properties, PC arrays may have an important role in an acoustic local area network. Each array is able to simultaneously transmit different messages that will focus only at the destination receiver node.     

Development and investigation of ultrasound linear phased arrays for transrectal treatment of prostate

We used the rectangular radiator method as a numerical solution to the Rayleigh-Sommerfeld diffraction integral for calculating the acoustic fields produced by linear arrays. The appropriate phases and amplitudes of voltages applied to the elements were computed using the pseudo-inverse method. We have developed and acoustically evaluated several constructions of planar linear ultrasound phased arrays for transrectal thermotherapy of prostate diseases. The designs of the linear phased arrays used in this work were the result of compromise in terms of the choice of frequency and number and size of elements, in order to investigate a means of improving array performance which might lead to increased efficacy and safety of the thermal treatment. The results obtained are in agreement with the data of other studies and show that a linear plane phased array may be potentially useful for thermal therapy.     

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.     

稀疏光学相控阵设计方法研究

为克服光学相控阵单元间隔必须小于工作波长二分之一的限制,构建了一种稀疏光学相控阵模型。分析了一维稀疏光学相控阵在近场和远场条件下的扫描原理,并提出了一种设计方法。对其相关参数进行仿真的结果表明：稀疏光学相控阵所用的单元数目较少,单元间隔远大于工作波长,扫描范围较大,波束宽度较窄,且在整个扫描空间内没有栅瓣。因此,稀疏光学相控阵单元间隔不受工作波长的限制,同时具有较好的扫描性能。     

The radiated fields of focussing air-coupled ultrasonic phased arrays

This paper presents an investigation into the fields radiated into air by ultrasonic phased arrays under transient excitation. In particular, it includes a theoretical prediction of spatial variations in amplitude throughout the both the near-field and far-field of such arrays. The approach has been used to predict the result of phasing to produce a focus in air, which can be seen to be particularly effective in the near-field of the array. Interesting features are observed, which are then described in terms of the performance of both individual elements and the resulting array. It is shown how some elements of design can be used to improve performance in focussing. The predictions are compared to the results of experiments in air using electrostatic arrays, where good focussing could be achieved provided the appropriate design principles were followed. The approach has been developed specifically for use in air, but the results would also hold for modelling in certain medical arrays where a focussing requirement might be needed close to the array itself.     

Optimal focusing by spatio-temporal inverse filter. II. Experiments. Application to focusing through absorbing and reverberating media

To focus ultrasonic waves in an unknown heterogeneous medium using a phased array, one has to calculate the optimal set of signals to be applied on the transducers of the array. (In most applications of ultrasound, medical imaging, medical therapy, nondestructive testing, the first step consists of focusing a broadband ultrasound beam deeply inside the medium to be investigated.) Focusing in a homogeneous medium simply requires to compensate for the varying focus-array elements geometrical distances. Nevertheless, heterogeneities in the medium, in terms of speed of sound, density, or absorption, may strongly degrade the focusing. Different techniques have been developed in order to correct such aberrations induced by heterogeneous media (time reversal, speckle brightness, for example). In the companion to this paper, a new broadband focusing technique was investigated: the spatio-temporal inverse filter. Experimental results obtained in various media, such as reverberating and absorbing media, are presented here. In particular, intraplate echoes suppression and high-quality focusing through a human skull, as well as hyper-resolution in a reverberating medium, will be shown. It is important to notice that all these experiments were performed with fully programmable multichannel electronics whose use is required to fully exploit the spatio-temporal technique.