Sparse random ultrasound phased array for focal surgery

Ultrasound phased arrays offer several advantages over single focused transducer technology, enabling electronically programmable synthesis of focal size and shape, as well as position. While phased arrays have been employed for medical diagnostic and therapeutic (hyperthermia) applications, there remain fundamental problems associated with their use for surgery. These problems stem largely from the small size of each array element dictated by the wavelength employed at surgical application frequencies (2-4 MHz), the array aperture size required for the desired focal characteristics, and the number of array elements and electronic drive channels required to provide RF energy to the entire array. The present work involves the theoretical and experimental examination of novel ultrasound phased arrays consisting of array elements larger than one wavelength, minimizing the number of elements in an aperture through a combination of geometric focusing, directive beams, and sparse random placement of array elements, for tissue ablation applications. A hexagonally packed array consisting of 108 8-mm-diameter circular elements mounted on a spherical shell was modeled theoretically and a prototype array was constructed to examine the feasibility of sparse random array configurations for focal surgery. A randomly selected subset of elements of the prototype test array (64 of 108 available channels) was driven at 2.1 MHz with a 64-channel digitally controlled RF drive system. The performance of the prototype array was evaluated by comparing field data obtained from theoretical modeling to that obtained experimentally via hydrophone scanning. The results of that comparison, along with total acoustic power measurements, suggest that the use of sparse random phased arrays for focal surgery is feasible, and that the nature of array packing is an important determinant to observed performance     

A concentric-ring equivalent phased array method to model fields of large axisymmetric ultrasound transducers

An equivalent concentric-ring ultrasound phased array method was developed to estimate ultrasonic continuous wave fields generated by axisymmetric single-source transducers. The method models a given source as a concentric-ring phased array by mathematically segmenting it into many rings and subsequently finding the amplitude and phase for each ring that produces an acoustic field similar to the field of the single-source transducer. The excitation source of each ring was calculated using an inverse technique based on complex pressure measurements along a radial line close to the source. The predicting abilities of the method are evaluated by comparing measured and estimated ultrasound fields for six different transducers. The results show that the concentric-ring equivalent phased array method (CREPAM) is able to estimate quantitatively the ultrasound fields generated by large axisymmetric planar and focused transducers.     

Two-step hybrid virtual array ray (VAR) technique for focusing through the rib cage

A new methodology for focusing ultrasonic beams noninvasively in the presence of the rib cage is investigated. This investigation is motivated by the need to employ high intensity focused ultrasound (HIFU) using phased array applicators for the treatment of liver tumors partially shadowed by the rib obstacles. This approach enables us to efficiently perform the ultrasound computational analysis and pattern synthesis in the interior region of the rib cage. The proposed technique consists of two main steps. First, a virtual array is introduced along the intercostal spacings between the solid ribs to generate the prespecified intensity levels at a set of control points within the target region. The second step involves the design of the actual feed array that induces the virtual sources between the intercostal spacings. This design optimization is carried out via the pseudo-inverse technique (minimum norm least squares solution) and by enforcing a constrained preconditioned pseudo-inverse method. The proposed procedure calculates the required primary sources (feed array) while maintaining minimal power deposition over the solid obstacles.     

Phase-aberration correction using signals from point reflectors and diffuse scatterers: measurements

A method for phase-aberration correction of phased-array images is tested using a model of near-field velocity inhomogeneities. A set of grooved room-temperature vulcanizing plates was constructed to simulate near-field aberrations encountered in clinical ultrasound imaging. As expected, large image distortion was experienced when grooved plates producing significant aberrations were placed near the surface of the array. An iterative aberration correction procedure based on cross-correlation measures between neighboring elements in a phased array, using signals reflected from diffuse scatterers, significantly reduced the effects of these aberrations, producing images nearly identical to those generated in the absence of aberrations. The results suggest that a practical phase-aberration correction system can be constructed for medical ultrasound imaging and possibly all coherent imaging systems by using a sampled aperture.     

Effects of Broadband Laser Generated Ultrasound on Array Gain

Array gain is a common parameter used in laser phased array research. This paper will present a new parameter called the frequency modulation of laser phased array (FMLPA). The array gain model for laser phased arrays was derived using an assumption that ultrasound from each array member interferes with each other. This would be always true if laser generated ultrasound is narrow band. However, laser generation of ultrasound is broad band. Broad band ultrasound signals have short duration in the time domain. If the time delay between generated wave fronts from each array member is longer than the duration of the broad band ultrasound signal from each array member, the ultrasound signals from each array member will not interfere with each other. The time delay between generated wave fronts from each array member is 0 s at a laser phased array’s beam steering angle and increases away from the beam steering angle. Therefore, ultrasound from each array member always interfere at angles close to the beam steering angle. However, ultrasound from each array member may not interfere at angles away from the beam steering angle depending on the time delay between generated wave fronts and duration of the broad band ultrasound signal. A theoretical model of the FMLPA was developed and experimentally verified for use when ultrasound from each array member does not interfere with each other. It was experimentally verified that current array gain equations still apply when ultrasound from array members interfere with each other. The FMLPA can be used to create new techniques for measuring weld penetration depth, crack location, and dimensions of objects.     

Trans-skull ultrasound therapy: the feasibility of using image-derived skull thickness information to correct the phase distortion

Recent papers have shown that focused ultrasound therapy may be feasible in the brain through an intact human skull by using phased arrays to correct the phase distortion induced by the skull bone. The hypothesis of this study is that the required phase shifts for the phased array can be calculated from the skull shape and thickness provided by modern imaging techniques. The shape and thickness of a piece of human skull was traced from the serial images and used in a theoretical model to calculate the phase distribution for a phased array. A 76-element phased array was manufactured and used in the tests. The piece of skull and the transducer array were positioned in a waterbath, and the ultrasound field distributions were mapped with and without the phase correction. The image-derived phase correction produced a sharp focus through the skull. These results showed that ultrasound brain therapy may be executed completely noninvasively through an intact skull by using a phased array and the skull thickness information derived from MRI scans.     

A 32 x 32 capacitive micromachined ultrasonic transducer array manufactured in standard CMOS

As ultrasound imagers become increasingly portable and lower cost, breakthroughs in transducer technology will be needed to provide high-resolution, real-time 3-D imaging while maintaining the affordability needed for portable systems. This paper presents a 32 x 32 ultrasound array prototype, manufactured using a CMUT-in-CMOS approach whereby ultrasonic transducer elements and readout circuits are integrated on a single chip using a standard integrated circuit manufacturing process in a commercial CMOS foundry. Only blanket wet-etch and sealing steps are added to complete the MEMS devices after the CMOS process. This process typically yields better than 99% working elements per array, with less than ±1.5 dB variation in receive sensitivity among the 1024 individually addressable elements. The CMUT pulseecho frequency response is typically centered at 2.1 MHz with a -6 dB fractional bandwidth of 60%, and elements are arranged on a 250 μm hexagonal grid (less than half-wavelength pitch). Multiplexers and CMOS buffers within the array are used to make on-chip routing manageable, reduce the number of physical output leads, and drive the transducer cable. The array has been interfaced to a commercial imager as well as a set of custom transmit and receive electronics, and volumetric images of nylon fishing line targets have been produced.     

A parametric study of ultrasonic beam profiles for a linear phased array transducer

A numerical simulation model is presented to investigate the influences of design parameters of linear phased array transducers on beam focusing and steering performance. The characteristic of ultrasonic beam profiles has been simulated on the basis of the Huygen's superposition principle. For the simulation, a linear phased array is considered as the composition of finite number of elements separated by equidistance. Individual elements are considered as two-dimensional point sources. The waves generated from piezoelectric elements are considered as simplified transient ultrasonic waves that are constructed with the cosine function enveloped with a Hanning window. The characteristic of ultrasonic wave propagation into a medium from the phased array transducer is described. The effects of the number, the interelement spacing, steering angle, the focal length, and frequency bandwidth of the piezoelectric elements on beam directivity and ultrasonic pressure field in a linear phased array transducer are systematically discussed.     

Diagnostic of phased arrays with faulty elements using the mutual coupling method

The ability to calibrate phased array antennas by utilising the mutual coupling method (MCM), which takes advantage of the mutual coupling effect between adjacent elements, is addressed. The basic assumption of the method is that the mutual coupling between adjacent elements is equal for all elements in the array and its major deficiency is its failure in the case of faulty elements. A parametric study to identify the effect of faulty elements in the array has been conducted. It has been shown that displacement of one element in the array may cause a significant error in the calibration, which affects its radiation characteristics (increase in the far side lobe level). The main contribution is the presentation of the effect of faulty elements in the calibration process and the proposal of a way to detect and bypass the faulty elements in a phased array calibrated by the MCM.     

Design and evaluation of a feedback based phased array system for ultrasound surgery

A driving system has been designed for phased array ultrasound applicators. The system is designed to-operate in the bandwidth 1.2 to 1.8 MHz, with independent channel power control up to 60 W (8 bit resolution) for each array element. To reduce power variation between elements, the system utilizes switching regulators in a feedback loop to automatically adjust the DC supply of a class D/E power converter. This feedback reduces the RF electrical power variation from 20% to 1% into a 16 element array. DC-to-RF efficiencies close to 70% for all power levels eliminates the need for large heat sinks. In addition to power control, each channel may be phase shifted 360 degrees with a minimum of 8 bit resolution. To ensure proper operation while driving ultrasound arrays with varying element sizes, each RF driving channel implements phase feedback such that proper phase of the driving signal is produced either at the amplifier output before the matching circuitry or after the matching circuitry at the transducer face. This feedback has been experimentally shown to increase the focal intensities by 20 to 25% of two tested phased arrays without array calibration using a hydrophone.     

超声阵列换能器设计及声场模拟

聚焦超声采用的聚焦方式有许多种,电子聚焦是其中之一,与其它聚焦方式相比,电子聚焦技术有很多优点,然而一些相互矛盾的条件对其设计有很大影响。通过用快速算法对阵元声场、以及频率对场的影响进行模拟,对阵列设计中的几个主要考虑方面进行了分析,提供了一般设计思路。采用伪逆矩阵算法,对变迹技术和多焦点技术进行计算机模拟,为焦域控制提供了新的思路,结果表明：将电子聚焦阵列换能器用于高强度聚焦超声治疗设备,比其它     

Eigenspace-based blind pattern optimisations of steerable antenna array for interference cancellation

A novel adaptive algorithm for an array using directional elements called a hybrid smart antenna system is proposed. The algorithm controls the element patterns on the basis of an objective function composed of eigenvalues of a covariance matrix. A high and stable array output signal-to-interference-plus-noise ratio is achieved by improving both the received powers and the spatial correlation coefficient between incident waves, without prior knowledge such as directions-of-arrival, channel state information or training signals. The characteristics of the proposed algorithm are theoretically and numerically clarified for a simple case involving two incident waves. Convergence with least mean squares algorithm is found to be as fast as that with recursive least squares algorithm in this system. Also, simulation for statistical performance evaluation is carried out in comparison with a conventional system. Furthermore, a method to implement the proposed eigenspace control algorithm without having to solve the eigenvalue problem is shown.     

基于Voronoi最小邻近点集的Delaunay三角化方法

针对局部条件下网格生成的需求,提出一种基于节点的Delaunay三角化生成算法,该算法以Delaunay三角形及其对偶Voronoi图的局部性特征为基础,通过在局部搜索最小Voronoi邻近点集,来生成约束点附近的局部网格,通过建立背景索引网格,来提高算法效率。给出算法的原理证明、程序实现、效率分析和测试结果,并给出了算法的应用领域。     

提高阿基米德螺旋相控阵换能器焦点声压的仿真研究

针对阿基米德螺旋相控阵换能器焦点声压不足的问题,通过有限元仿真,分别讨论了换能器阵元形状和频率改变与声场的变化关系,从焦点声压、偏转范围、栅瓣水平、焦域大小等方面进行了研究。结果表明,相同频率下,扇形阵元阿基米德螺旋相控阵换能器的焦点声压比圆形阵元时的焦点声压提高了32.28%,偏转范围为20 mm×20 mm×40 mm;在扇形阵元形状的情况下,频率选择0.9~1.0 MHz,不仅能获得较高的焦点声压,同时还能保证改善聚焦性能。文章的仿真结果为阿基米德螺旋相控阵换能器提高焦点声压,满足深部组织消融治疗需要的高功率要求提供了有用的设计参考。     

Omni-directional guided wave transducer arrays for the rapid inspection of large areas of plate structures

Omni-directional guided wave array transducers contain a circular pattern of elements that individually behave as omni-directional point transmitters or receivers. The data set acquired from such an array contains time-domain signals from each permutation of transmitter and receiver. A phased addition algorithm is developed that allows an omni-directional, B-scan image of the surrounding plate to be synthesized from any geometry of array. Numerically simulated data from a single reflector is used to test the performance of the algorithm. The results from an array containing a fully populated circular area of elements (Type I array) are found to be good, but those from an array containing a single ring of elements (Type II array) contain many large side-lobes. An enhancement to the basic phased addition algorithm is presented that uses deconvolution to suppress these side-lobes. The deconvolution algorithm enables a Type II array to equal the performance of a Type I array of the same overall diameter. The effect of diameter on angular resolution is investigated. Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.     

拉曼激光相位噪声对原子干涉重力仪测量分辨力的影响

为了保证相控阵雷达的性能,它的天线现场校准越来越受到重视。本文首先介绍相控阵雷达天线现场校准的现状, 提出目前存在的问题。然后分析光学电磁场探测技术优势,阐述基于光子技术的相控阵雷达天线现场校准实现的可行性,建立了现场校准装置。通过实验验证,该技术能实现对相控阵雷达天线现场校准,具有高精度,对天线的辐射场的影响较小,能满足相控阵雷达天线现场校准要求。     

精密水槽收缩段流场模拟与收缩曲线选型评估

通过求解RANS方程,数值模拟了精密水槽收缩段的流场特性。针对两种不同出口尺寸不同收缩比的收缩段,选取了三种不同的收缩曲线来进行选型评估。根据壁面是否分离、逆压梯度变化缓和程度、出口速度均匀度,来判断不同收缩曲线的优劣。研究表明:对于出口尺寸为250 mm×20 mm、收缩比为30的收缩段,双三次收缩曲线是最优的;对于出口尺寸为250 mm×10 mm、收缩比为60的收缩段,维氏收缩曲线是最优的。本次数值模拟计算的工作可为精密水槽收缩段优化设计工作提供借鉴。     

A method of reducing grating lobes associated with an ultrasoundlinear phased array intended for transrectal thermotherapy

Some practical aspects of planar linear ultrasound phased arrays for transrectal thermotherapy of prostate diseases are discussed. Several regimens for driving the array are investigated and spatial distributions of ultrasound intensities are measured in water and compared with computer simulations. Practical recommendations for suppressing grating lobes based on the use of subsets of elements and de-activation of several elements in the array are given. Treatment safety could be increased by adopting these measures since the relative intensities and power in grating lobes and other secondary intensity peaks are decreased, as is the overall ultrasound energy introduced into the body without significant reduction in the maximum power at the focus     

Ultrasonic phased arrays with variable geometric focusing for hyperthermia applications

An ultrasonic applicator, which utilizes both electronic and variable geometric focusing, for deep-localized hyperthermia is investigated. The applicator is based around a linear phased array that furnishes its electronic focusing capability. The output of the array radiates through a spherical liquid-lens that provides the applicator a variable geometric focusing capability as well. A lens of this type adds dynamic focusing to the elevation dimension of the linear phased array. By controlling the volume of liquid in the lens (and thus the radius of curvature of its membrane), dynamic control of the geometrical focus can be achieved. Comparisons of computer simulations and experimental measurements of the field intensity distribution of a small-scale prototype applicator are presented. Important design parameters, such as the choice of the liquid for the lens and the size and number of array elements, are examined.