Adaptive multi-element synthetic aperture imaging with motion and phase aberration correction

Multi-element synthetic aperture techniques employing subaperture processing over successive firing steps can produce good image quality with simple front-end hardware but are susceptible to motion and phase aberration artifacts. We explore correlation processing using fully common spatial frequencies of overlapping subapertures to adapt beamforming for motion and phase aberrations. Signals derived from the subset of elements representing common spatial frequencies exhibit significantly higher correlation coefficients than those from signals computed using the entire subaperture. In addition, the correlation coefficient decreases linearly with subaperture separation for complete subaperture signals, but remains nearly constant with subaperture separation if only common spatial frequencies are used. Adaptive multi-element synthetic aperture imaging with correlation processing using fully common spatial frequencies is tested on experimental RF data acquired from a diffuse scattering phantom using a 3.5 MHz, 128-element transducer array. The results indicate that common spatial frequencies can be used efficiently for correlation processing to correct motion and phase aberration for adaptive multi-element synthetic aperture imaging     

Subaperture processing for ultrasonic imaging

Ultrasonic subaperture processing using aperture synthesis and beam space interpolation is presented. The number of beam lines scanning the image plane for a given transmit-receive subaperture combination is chosen according to the spatial sampling criteria for that combination. On each beam line, echo signals over the entire array are collected through electronic multiplexing of array channels, where the transmit subaperture at the transducer center is fired K successive times, with K equal to the number of nonoverlapping receive subapertures. For every receive subaperture, the number of beam lines is increased through digital interpolation using a linear filter with spatial frequency band associated with the subaperture. Interpolated beam lines from all receive subapertures are then added to obtain a high resolution sector image. The efficiency of subaperture processing for different system configurations is tested on experimental rf data acquired from two different phantoms using a 3.5 MHz, 128-element transducer array. The proposed subaperture processing reduces the number of firings for data acquisition, and thus allows real-time imaging with very low susceptibility to motion artifacts.     

Synthetic aperture imaging for small scale systems

Multi-element synthetic aperture imaging methods suitable for applications with severe cost and size limitations are explored. Array apertures are synthesized using an active multi-element receive subaperture and a multi-element transmit subaperture defocused to emulate a single-element spatial response with high acoustic power. Echo signals are recorded independently by individual elements of the receive subaperture. Each method uses different spatial frequencies and acquisition strategies for imaging, and therefore different sets of active transmit/receive element combinations. Following acquisition, image points are reconstructed using the complete data set with full dynamic focus on both transmit and receive. Various factors affecting image quality have been evaluated and compared to conventional imagers through measurements with a 3.5 MHz, 128-element transducer array on different gel phantoms. Multielement synthetic aperture methods achieve higher electronic signal to noise ratio and better contrast resolution than conventional synthetic aperture techniques, approaching conventional phased array performance     

A speckle target adaptive imaging technique in the presence of distributed aberrations

Acoustic velocity inhomogeneities in tissue result in aberration of ultrasound images. These aberrations can be modeled as a near field thin phase screen or as a distributed aberrator. The effect of a near field thin phase screen is to time shift the received echo at each element, while distributed aberrators result in both pulse distortions and time shifts from element to element. Most current techniques for the correction of distributed aberrators are limited to application on point targets. A new technique is proposed which uses multiple transmits from spatially shifted transmit apertures (the translating transmit aperture algorithm), in conjunction with phase conjugate filters, to correct for distributed aberrations in the presence of speckle targets. The performance of the translating transmit aperture algorithm in improving the correlation between signals received by elements of different spatial separations is measured, and factors affecting the performance of this technique are investigated in simulation and experiment.     

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.     

无参考Lamb波相控阵结构损伤监测成像方法

超声相控阵结构健康监测中通过计算差信号获取损伤散射信号的方法易受环境和结构变化的影响,针对这一问题,提出无参考信号的相控阵结构健康监测成像定位方法。采用窗函数截取散射信号,剔除直达波和边界反射信号。基于相控阵方法,对结构损伤状态时的传感器响应信号进行处理,无需结构健康状态时传感信号作为参考。该方法在碳纤维复合材料板结构上的实验研究证明,采用无参考信号的超声相控阵方法能够精确实时地监测并清晰表征结构损伤;同时,该方法可以大大缩短监测时间,且不受环境条件变化的影响,为该方法的工程化应用奠定基础。     

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

Methods for correction of phase aberrations induced by near-field variations in the index of refraction are explored. Using signals obtained from a sampled aperture (i.e. transducer array), phase aberrations can be accurately measured with a correlation approach similar to methods used in adaptive optics and radar. However, the method presented here has no need for a beacon or an ideal point reflector to act as a source for estimating phase errors. It uses signals from random collections of scatterers to determine phase aberrations accurately. Because there is no longer a need for a beacon signal, the method is directly applicable not only to medical ultrasound imaging but also to any coherent imaging system with a sampled aperture, such as radar and sonar.     

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.     

Resolution of synthetic-aperture imaging through turbulence

A theory is developed for the resolution of an optical synthetic-aperture imaging system viewing an object through an inhomogeneous refractive medium. The inhomogeneities of the propagation medium create errors in the phase history data with resultant space-variant image effects, including geometric distortions and broadening of the impulse response or point-spread function. I relate the intensity-impulse response to the usual wave structure function. I determine the modulation transfer function for synthetic apertures of any size and exposure time, valid whenever the optical bandwidth is small compared with the carrier frequency, and derive the resolution for monostatic and bistatic synthetic apertures, valid whenever the real sampling aperture is small compared with the medium's coherence length. The results take the same form as the well-known turbulence-limited resolution of incoherent, real-aperture imaging with short exposure. Turbulence-limited synthetic-aperture resolution is somewhat better than incoherent real-aperture resolution under the same conditions. Autofocus processing improves synthetic-aperture resolution beyond this limit, and adaptive correction of higher-order phase history errors would improve it further.     

A closed loop ML algorithm for phase aberration correction in phased array imaging systems. I. Algorithm synthesis and experimental results [Ultrasound medical imaging

A new closed loop algorithm is described, based on the maximum likelihood theory, that corrects phase aberrations in phased array coherent imaging systems using phase estimates which are directly obtained from measured data (and not from estimated correlation functions). It has the advantage of estimating phase differences with respect to the same reference, and not phase differences between signals in adjacent array elements, eliminating the need to perform an integration to get the phase aberration profile. It allows for a real time implementation, in the sense that corrected data flows out of the circuit at the same rate that input data is fed into it. Being a closed loop algorithm, it is able to track time variations that may occur in the phase aberrations and, as in any phase lock loop circuit, it shows a trade-off between acquisition time and phase jitter.     

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.     

基于超声相控阵的水下船体表面成像方法研究

针对当前水下船体表面成像设备较少、成本较高、成像误差较大的问题,提出一种基于超声相控阵技术的水下船体成像方法。以超声相控阵技术为基础,控制一维线型超声相控阵列换能器发射的聚焦声波对水下船体表面进行相控扫描,声波作用于船体表面后发生散射,回波被换能器接收后利用双曲线交汇法计算出各反射点的点云数据,最终利用点云数据重构出水下船体表面的三维模型。通过搭建水池船模成像实验对方法进行了验证,结果表明,重构图像最大误差小于5mm,具有较高的成像分辨力。     

超声相控阵全聚焦无损检测技术概述

超声相控阵全聚焦成像技术基于接收信号后处理的思想,对检测回波数据进行离线分析成像,是超声检测领域里的一项新技术。因其成像分辨率高、覆盖面广、对小缺陷灵敏度高等优势,在航空航天、高铁、石油管道、核电站等工业领域已有初步应用。文章介绍了全聚焦成像技术的检测原理及优缺点,介绍了其国内外发展历程,总结了研究热点及发展趋势。     

Digital phased array beamforming using single-bit delta-sigma conversion with non-uniform oversampling

Digital beamforming based on oversampled delta-sigma (ΔΣ) analog-to-digital (A/D) conversion can reduce the overall cost, size, and power consumption of phased array front-end processing. The signal resampling involved in dynamic ΔΣ beamforming, however, disrupts synchronization between the modulators and demodulator, causing significant degradation in the signal-to-noise ratio. As a solution to this, we have explored a new digital beamforming approach based on non-uniform oversampling ΔΣ A/D conversion. Using this approach, the echo signals received by the transducer array are sampled at time instants determined by the beamforming timing and then digitized by single-bit ΔΣ A/D conversion prior to the coherent beam summation. The timing information involves a nonuniform sampling scheme employing different clocks at each array channel. The ΔΣ coded beamsums obtained by adding the delayed 1-bit coded RF echo signals are then processed through a decimation filter to produce final beamforming outputs. The performance and validity of the proposed beamforming approach are assessed by means of emulations using experimental raw RF data     

A comparative evaluation of several algorithms for phase aberrationcorrection

A common framework is presented to classify several phase correction techniques. A subset of these techniques are evaluated through simulations which utilize 2-D phase aberration profiles measured in the breast. The techniques are compared based on their ability to reduce phase errors, stability, and sensitivity to noise and missing elements in the transducer array. Significant differences are observed in these measures of performance when the size and location of the aperture area used to generate a phase reference signal are varied. Techniques that utilize a small correction reference region are more susceptible to noise and missing elements than techniques which use larger reference regions. The algorithms encounter problems in 2-D phase correction when making the transition from one row to the next, due to the low interelement correlation at the transition points. It is shown that the magnitude of the interelement correlation is the key parameter governing phase correction performance     

A closed loop ML algorithm for phase aberration correction in phased array imaging systems. II. Performance analysis [Ultrasound medical imaging

For pt.I see ibid., vol.44, pp.259-70 (1997). The performance of the maximum likelihood closed loop circuits, proposed in part I for phase aberration correction in phased array imaging systems, is analysed. This analysis is helpful in designing and also in determining the tracking mode phase error variance performance of both closed loop circuits (1-D and 2-D). It is an approximate analysis, suitable under small errors conditions, and considered accurate for high values of the signal to noise ratio.     

A fundamental limit on delay estimation using partially correlatedspeckle signals

Delay estimation is used in ultrasonic imaging to estimate blood or soft tissue motion, to measure echo arrival time differences for phase aberration correction, and to estimate displacement for tissue elasticity measurements. In each of these applications delay estimation is performed using speckle signals which are at least partially decorrelated relative to one another. Delay estimates which utilize such data are subject to large errors known as false peaks and smaller magnitude errors known as jitter. While false peaks can sometimes be removed through nonlinear processing, jitter errors place a fundamental limit on the performance of delay estimation techniques. The authors apply the Cramer-Rao Lower Bound to derive an analytical expression which predicts the magnitude of jitter errors incurred when estimating delays using radio frequency (RF) data from speckle targets. The analytical expression presented includes the effects of signal decorrelation due to physical processes, corruption by electronic noise, and a number of other factors. Simulation results are presented which show that the performance of the normalized cross correlation algorithm closely matches theoretical predictions. These results indicate that for poor signal to noise ratios (0 dB) a small improvement in signal to noise ratio can dramatically reduce jitter magnitude. At high signal to noise ratios (30 dB) small amounts of signal decorrelation can significantly increase the magnitude of jitter errors     

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     

A phase aberration correction method for ultrasound imaging

A computationally efficient method for phase aberration correction in ultrasound imaging is presented. The method is based on time delay estimation via minimization of the sum of absolute differences between radio frequency samples of adjacent array elements. Effects of averaging estimated aberration patterns over scan angle and truncation to a single bit wordlength are examined. Phase distortions due to near-field inhomogeneities are simulated using silicone rubber aberrators. Performance of the method is tested using experimental data. Simulation studies addressing different factors affecting efficiency of the method, such as the number of iterations, window length, and the number of scan angles used for averaging, are presented. Images of a standard resolution phantom are reconstructed and used for qualitative testing.