Ultrasonic phased arrays for nondestructive testing

Although ultrasonic phased arrays are potentially ideally suited to many applications in NDT, their use in practice is limited by the complexity of array probe manufacture and the bulk and cost of the associated electronic control instrumentation. In this article a number of probe designs are discussed, with particular emphasis being placed on those having a simpler construction. The requirements of the electronic systems for both transmission and reception of array signals are reviewed. A compact, low-cost, phased-array instrument which compromises between array probe scanning speed and the rate of data acquisition is also described.     

Intracavitary ultrasound phased arrays for noninvasive prostatesurgery

The feasibility of using intracavitary ultrasound phased arrays for thermal surgery of the prostate was investigated. A simulation study was performed which demonstrated the ability of phased arrays to generate necrosed tissue volumes over anatomically appropriate ranges (2-6 cm deep and >6 cm axially) and investigated the effects of varying frequency, sonication time, maximum temperature, and blood perfusion on the necrosed tissue volume. An advantage that phased arrays have over geometrically focused transducers is that they are able to electronically scan a single focus over a specified range very quickly. This study demonstrated that the necrosed tissue volume may be increased by more than a factor of 100 by using electronic scanning. Scan parameters that were investigated included foci spacing, scan width, perfusion, maximum temperature, and unequal weighting of the foci. An optimization was performed to select the foci weighting parameters such that a uniform thermal dose was achieved at the focal depth, providing a more uniformly heated target volume. Finally, the ability of linear ultrasound phased arrays to create necrosed tissue lesions was demonstrated experimentally in fresh beef liver using a single stationary focus and single focus scans generated by an aperiodic 0.83-MHz 57-element linear ultrasound phased array     

Synthetic phased arrays for intraluminal imaging of coronaryarteries

A 64-element, high efficiency, ceramic piezoelectric array transducer operating at 20 MHz has been constructed for ultrasonic intraluminal imaging. The array is mounted on the surface of a 1.2 mm diameter catheter appropriate for coronary artery applications. Integrated into the catheter tip is a custom analog chip set permitting complete data capture from the array. That is, on each firing any combination of array elements can be selected independently as transmitter or receiver. Using data acquired in this way, a complete phased array aperture (i.e., independent transmit and receive apertures) can be synthesized. Reconstruction hardware based on a custom application specific integrated circuit (ASIC) has been designed and built to produce real-time images. Beam forming coefficients are derived using an optimal filtering approach accounting for the circular geometry of the array. Simulated and measured beam patterns for this system are compared. In addition, images of coronary anatomy acquired with the real-time system are displayed demonstrating the marked image quality improvement compared to previous synthetic aperture intraluminal systems     

Optically produced true-time delays for phased antenna arrays

A device is described for generating true-time delays optically for microwave signals used in beam steering and beam shaping in phased-array antennas. The device can be adapted to provide delays from picoseconds to nanoseconds. A single, compact unit should provide parallel delays for more than 64 independent antenna elements with a greater than 6-bit resolution. The time delays are produced by multiple reflections in a mirror configuration with continuous refocusing. A single spatial light modulator selects independent optical path lengths for each of the parallel antenna elements. Amplitude control for beam shaping can be integrated into the device. The unit can be made rugged for harsh environments by use of solid-block construction. The operation of the true-time delay device is described, along with the overall system configuration. Preliminary experimental data are given.     

Treatment planning for hyperthermia with ultrasound phased arrays

Treatment planning for ultrasound phased arrays suggests a strategy for hyperthermia therapy which satisfies therapeutic conditions at the target and spares other sensitive anatomical structures. To predict both desirable and harmful interactions between ultrasound and important structures such as the tumor, bones, and air pockets, a hyperthermia treatment planning system has been developed for ultrasound phased arrays. This collection of treatment planning routines consists of geometric and thermal optimization procedures specific to ultrasound phased arrays, where geometric treatment planning, combined with thermal treatment planning and three-dimensional visualization, provides essential information for the optimization of individual patient treatments. A patient image data set for cancer of the prostate, a difficult target situated in the midst of multiple pelvic bone obstructions, illustrates the geometric treatment planning algorithm and other tools for treatment analysis. The results indicate that the analysis of complex three-dimensional relationships between the applicator, anatomical structures, and incident fields provides an important means of predicting treatment limiting conditions, thereby allowing the hyperthermia applicator to electronically adapt to individual patients and specific sites     

Field-of-view limitations of phased telescope arrays

The optical performance of imaging phased telescope arrays is degraded by various design, manufacturing, and operational errors. Perhaps the most basic and fundamental of these error sources are the residual aberrations of the optical design chosen for the individual telescopes. We show that third-order field curvature and distortion, which are rather benign aberrations in a conventional telescope, result in relative phase and tilt errors between the individual telescopes making up the array. The field-dependent image degradation caused by these relative phase and tilt errors is then predicted for different subaperture configurations and telescope design parameters. For phased arrays made up of simple two-mirror telescopes, distortion limits the field of view to less than 5 arcmin for small subapertures (D < 0.5 m), and field curvature limits the field of view to less than 1 arcmin for subaperture diameters greater than 2 m. Quantitative parametric results yielding tolerances for residual field curvature as the phased array is scaled up in size are presented graphically. If a 0.5-deg field of view is desired for telescope diameters greater than 2 m, complex telescope configurations are necessary to satisfy the rather tight tolerances on both field curvature and distortion.     

Simulations of astronomical imaging phased arrays

We describe a theoretical procedure for analyzing astronomical phased arrays with overlapping beams and apply the procedure to simulate a simple example. We demonstrate the effect of overlapping beams on the number of degrees of freedom of the array and on the ability of the array to recover a source. We show that the best images are obtained using overlapping beams, contrary to common practice, and show how the dynamic range of a phased array directly affects the image quality.     

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.     

Feasibility of using ultrasound phased arrays for MRI monitorednoninvasive surgery

The purpose of this paper was to evaluate the in vivo feasibility of using phased arrays for MRI guided ultrasound surgery. Two different array concepts were investigated: a spherically curved concentric ring array to move the focus along the central axis and a spherically curved 16 square element array to make the focus larger. Rabbit thigh muscles were exposed in vivo in a 1.5 T MRI scanner to evaluate the array performance. The results showed that both of the arrays performed as expected, and the focus could be moved and enlarged. In addition, adequate power could be delivered from the arrays to necrose in vivo muscle tissue in 10 s. This study was the first implementation of phased arrays for MRI guided ultrasound surgery. The results demonstrate that phased arrays have significant potential for noninvasive tissue coagulation     

Sparse 2-D arrays for 3-D phased array imaging--design methods

One of the most promising techniques for limiting complexity for real-time 3-D ultrasound systems is to use sparse 2-D layouts. For a given number of channels, optimization of performance is desirable to ensure high quality volume images. To find optimal layouts, several approaches have been followed with varying success. The most promising designs proposed are Vernier arrays, but also these suffer from high peaks in the sidelobe region compared with a dense array. In this work, we propose new methods based on the principles of suppression of grating lobes to form symmetric and non-symmetric regular sparse periodic and radially periodic designs. The proposed methods extend the concept of sparse periodic layouts by exploiting either an increased number of symmetry axes or radial symmetry. We also introduce two new strategies to form designs with nonoverlapping elements. The performance of the new layouts range from the performance of Vernier arrays to almost that of dense arrays. Our designs have simplicity in construction, flexibility in the number of active elements, and the possibility of trade off sidelobe peaks against sidelobe energy.     

Sparse 2-d arrays for 3-D phased array imaging--experimental validation

To be able to describe more precisely the behavior of a real-time 3-D ultrasound system with either a dense array or various sparse designs, experimental data from a 2-D fully connected array prototype with 50/spl times/50 elements have been collected. The data have been processed off line to form synthetic aperture 3-D volume images. Simulated and experimental results are compared and show good correlation. The performance of the best sparse designs, all thinned to more than 50%, offer performances comparable to a dense array.     

Optical phased array configuration for an extremely large telescope

Extremely large telescopes are currently under consideration by several groups in several countries. Extrapolation of current technology up to 30 m indicates a cost of over dollars 1 billion. Innovative concepts are being explored to find significant cost reductions. We explore the concept of an Optical Phased Array (OPA) telescope. Each element of the OPA is a separate Cassegrain telescope. Collimated beams from the array are sent via an associated set of delay lines to a central beam combiner. This array of small telescope elements offers the possibility of starting with a low-cost array of a few rings of elements, adding structure and additional Cass elements until the desired diameter telescope is attained. We address the salient features of such an extremely large telescope and cost elements relative to more conventional options.     

次声相控阵列的聚焦特性研究

研究了提高非致命次声武器聚束能力和声强增益的方法。利用抛物面本身的自聚焦性能优化设计了一种新型的抛物面不等间距阵列。基于声场理论,计算出阵列声压分布;采用伪逆矩阵算法,数值仿真了与抛物面阵列中心轴垂直截面上以及中心轴上不同位置的声压分布。研究结果表明,不等间距抛物面相控阵列相对于传统平面阵列而言,声强分布的半峰宽度减小了16.7%,并且声强增益提高了48.6%,是较为理想的组合声源。通过仿真模拟论证了抛物面阵列的优越性,进而为次声武器的设计及研究提供了理论基础。     

Phase aberration correction and motion compensation for ultrasonichyperthermia phased arrays: experimental results

In ultrasound hyperthermia, focal patterns generated by phased arrays can be degraded by phase errors due to tissue inhomogeneities, digitization of the driving signals, and imperfect fabrication of the transducers. The degree of degradation depends on the severity of phase aberrations. As predicted by simulation and verified by experimental results, focal degradation scales with the circular variance of phase errors. However, degraded power deposition patterns can be significantly improved after phase aberration correction, especially where patterns are complicated and the aberrations are severe. Also, as shown in motion compensation experiments, an aberration corrected pattern can be particularly sensitive to aberrator movement greater than the correlation length of the aberrator. After motion compensation, new sharply focused patterns can be accomplished, thus reducing the unwanted influence of “body” movement by stabilizing the positions of foci with respect to patient anatomy     

Biologically inspired structural diagnostics through beamforming with phased transducer arrays

This paper discusses research in the use of biologically inspired spatial phased transducer arrays for the nondestructive evaluation of homogeneous and heterogeneous structural components. It is shown that beamforming, which is used by orb web spiders to locate their prey in a network of web fibers, can be achieved by applying weights and time delays to the tapped signals from a transducer array in a narrow frequency band to obtain desired directional sensitivities and optimal array gains. The resulting spatio-temporal filters are then used to detect, locate and quantify structural damage. The theory of beamsteering and beamforming for processing propagating wave data in damaged elastic media is discussed. Experimental results for homogeneous and heterogeneous plates are given to verify the theoretical discussions. Design considerations for the phased arrays are examined as are the benefits of nonlinear array geometries for better spatial coverage. The advantage of using adaptive over conventional beamforming is demonstrated with a Frost Constraint adaptive technique.     

Advanced reflector characterization with ultrasonic phased arrays in NDE applications

Ultrasonic arrays are increasingly widely used in nondestructive evaluation (NDE) due to their greater flexibility and potentially superior performance compared to conventional monolithic probes. The characterization of small defects remains a challenge for NDE and is of great importance for determining the impact of a defect on the integrity of a structure. In this paper, a technique for characterizing reflectors with subwavelength dimensions is described. This is achieved by post-processing the complete data set of time traces obtained from an ultrasonic array using two algorithms. The first algorithm is used to obtain information about reflector orientation and the second algorithm is used to distinguish between point-like reflectors that reflect uniformly in all directions and specular reflectors that have distinct orientations. Experimental results are presented using a commercial 64-element, 5-MHZ array on two aluminum test specimens that contain a number of machined slots and side-drilled holes. The results show that the orientation of 1-mm-long slots can be determined to within a few degrees and that the signals from 1-mm-long slots can be distinguished from that from a 1-mm-diameter circular hole. Techniques for quantifying both the orientation and the specularity of measured signals are presented and the effect of processing parameters on the accuracy of results is discussed.     

Algorithm for the nonlinear propagation of acoustic beams from phased arrays and nonplanar sources

A multistage algorithm is presented to allow for the convenient and accurate computation of the finite amplitude-distorted acoustic fields produced by phased arrays and nonplanar sources. The algorithm also allows for non-normal (angled) beam computations.     

A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3x3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.     

Harmonic ultrasonic field of medical phased arrays: simulations and measurements

With the introduction of harmonic imaging, the design of new array transducers for tissue and contrast imaging became indispensable. Hence, prior knowledge of harmonic beams is essential to attain optimal harmonic performances. For that purpose, a new numerical algorithm that solves the parabolic nonlinear wave equation is developed. The algorithm is based on finite differences and performs exclusively in time domain. Pulsed harmonic fields emitted by a medical transducer were measured and computed at different mechanical indices. Simulations and measurements showed very good agreement for all the harmonic components.