Phased Array Imaging Research Articles

A novel adaptive apodization to improve the resolution of phased subarray imaging in medical ultrasound.

Phased subarray imaging (PSA) was previously proposed to extend the receive aperture length. Using overlapped subarrays as transmitters in PSA leads to decrement of sidelobe levels of the overall beam compared to full phased array imaging (PHA). This paper proposes an adaptive compounding of subarray images in PSA to improve both the resolution and contrast compared with PHA. Adaptive apodization (ADAP) is defined proportional to the beamformed responses of subarrays such that the overall energy after compounding is minimized. The simulation and experimental results validate the performance of applying ADAP in PSA. The full width at half maximum (FWHM) at a depth of 30mm in the proposed PSA is about 0.2mm, compared to a FWHM of 0.6mm with PHA imaging. Measuring the contrast ratio index shows that the ADAP method also improves the contrast in PSA imaging at least 25% compared to PHA imaging. Applying the proposed ADAP, besides conventional compounding in PSA imaging, leads to improvement of both the resolution and contrast compared to PHA imaging.

Impact of Bit Errors in Digitized RF Data on Ultrasound Image Quality.

This article quantitatively analyzes the impact of bit errors in digitized RF data on ultrasound image quality. The quality of B-mode images in both linear array and phased array imaging is evaluated by means of three objective image quality metrics: peak signal-to-noise ratio, structural similarity index, and contrast-to-noise ratio, when bit errors are introduced to the RF data with different bit-error rates (BERs). The effectiveness of coding schemes for forward error detection and correction to improve the image quality is also studied. The results show that ultrasound imaging is inherently resilient to high BER. The image quality suffers unnoticeable degradation for BER lower than 1E-6. Simple 1-bit parity coding with 9% added redundancy helps to retain similar image quality for BER up to 1E-4, and Hamming coding with 33.3% added redundancy allows the BER to increase to 1E-3. These results can serve as a guideline in the datalink design for ultrasound probes with in-probe receive digitization. With much more relaxed BER requirements than in typical datalinks, the design can be optimized by allowing fewer cables with higher data rate per cable or lower power consumption with the same cable count.

A new beamforming method and hardware architecture for real time two way dynamic depth focusing

The Total Focusing Method (TFM) yields a focused image in emission and in reception while Phased Array (PA) imaging provides Dynamic Depth Focusing (DDF) in reception only. Besides, most NDE applications have two propagation media, where refraction at the interface complicates time-of-flight (TOF) and focal law computations. This affects especially TFM, which must compute the TOFs from all elements to image pixels and use them to select the data for imaging.A new method with real-time Dynamic Depth Full Focusing (DDFF), in emission and reception, is proposed in this work. It is called Total Focusing Phased Array (TFPA) because it uses concepts of TFM and PA. Omnidirectional emissions are used to create a synthetic aperture as in TFM, while beamforming is carried out along scan lines as in PA, simplifying the delay calculation in the presence of interfaces and providing an efficient hardware implementation.Refraction at the interface between two media is eliminated by a Virtual Array (VA) that converts such scenario into a simple homogeneous medium. Propagation can be considered along scan lines from the virtual array at constant speed, as in homogeneous media. Strict dynamic focusing is performed in real-time, an important difference with other approaches that require iterative Fermat search to get the focal laws for every imaged point. With TFPA only 3 parameters per element and scan line are required to perform this task.Experiments are carried out to compare the three techniques, PA, TFM and TFPA. TFM and TFPA yield similar image quality, offering improved depth of field and resolution over PA. On the other hand, TFPA avoids most of the burden for computing TOFs and operates in real time with one or two media propagation.

Investigation and Analysis of Ultrasound Imaging Based on Linear CMUT Array

In the next generation of ultrasound imaging systems, Capacitive micromachined ultasonic transducer (CMUT) based on microelectromechanical systems (MEMS) is a promising research direction of transducers, which has wide application prospects. In this paper, based on the study of three imaging methods, including classical phased array (CPA) imaging, classical synthetic aperture (CSA) imaging and phased subarray (PSA) imaging, several different imaging schemes are designed for linear CMUT array, after that the performances of these imaging schemes are compared and analyzed. The effects of the three imaging methods are verified and analyzed based on the linear CMUT array. Through analysis, it is found that the image quality of the classical phased array imaging method is the best, the imaging quality of the above three imaging methods can be effectively improved by adopting the amplitude apodization and dynamic focusing method. The research results in this paper will provide theoretical basis and application reference for the design of ultrasonic imaging system based on linear CMUT array in the future.

Inspection of plate weldments through an interposed layer of viscoelastic material using an elastic-electromagnetic scanning method

In this article, an elastic-microwave based non-destructive evaluation method is presented to inspect for cracks in weldments and thinning of coated steel plates. The approach uses a microwave interferometer operating at 94 GHz to record the total surface displacement of a coated steel plated as it is driven by an incident elastic field. These spatiotemporal data coupled with wavefield processing algorithms provide powerful detection and localization capabilities. From these wavefield data sets, a plate thickness mapping capability has been demonstrated that can detect thickness changes on the order of 0.79 mm (1/32 in.). It is also shown that a topological energy analysis of the wavefield data can detect and locate small flaws on the order of 5-10 mm (0.19-0.40 in.) in the welded joint. Note, all of these results are obtained through a 50.8 mm (2 in.) thick viscoelastic coating without disturbing the coating or the coating bond. At present the algorithm cannot resolve individual flaws within a grid space, just their cumulative effect. Even with the current limitations, this detection approach appears to be a promising alternative to traditional phased array imaging methods where the coating layer must be removed prior to inspection.

교차배열 변환기를 이용한 복합 위상배열 초음파 영상화 기법 : 영상화 알고리즘 및 이중 선형 영상

교차배열 변환기를 이용한 복합 위상배열 초음파 영상화 기법 : 영상화 알고리즘 및 이중 선형 영상

Image analysis applied to Brillouin images of tissue-mimicking collagen gelatins.

Brillouin spectroscopy is an emerging analytical tool in biomedical and biophysical sciences. It probes viscoelasticity through the propagation of thermally induced acoustic waves at gigahertz frequencies. Brillouin light scattering (BLS) measurements have traditionally been performed using multipass Fabry-Pérot interferometers, which have high contrast and resolution, however, as they are scanning spectrometers they often require long acquisition times in poorly scattering media. In the last decade, a new concept of Brillouin spectrometer has emerged, making use of highly angle-dispersive virtually imaged phase array (VIPA) etalons, which enable fast acquisition times for minimally turbid materials, when high contrast is not imperative. The ability to acquire Brillouin spectra rapidly, together with long term system stability, make this system a viable candidate for use in biomedical applications, especially to probe live cells and tissues. While various methods are being developed to improve system contrast and speed, little work has been published discussing the details of imaging data analysis and spectral processing. Here we present a method that we developed for the automated retrieval of Brillouin line shape parameters from imaging data sets acquired with a dual-stage VIPA Brillouin microscope. We applied this method for the first time to BLS measurements of collagen gelatin hydrogels at different hydration levels and cross-linker concentrations. This work demonstrates that it is possible to obtain the relevant information from Brillouin spectra using software for real-time high-accuracy analysis.

An Omnidirectional Near-Field Comprehensive Damage Detection Method for Composite Structures

As one of the active structural health monitoring methods based on the Lamb wave, the ultrasonic phased-array damage detection method can provide information such as damage location and range more intuitively, which is why this method is a research hotspot in the field of Lamb wave-based damage monitoring. However, the ultrasonic phased-array damage detection method intended for the far field is not applicable to near-field damage monitoring. In addition, the traditional one-dimensional piezoelectric phased-array damage imaging method suffers from a blind area in the near field, and the data collection time of its angle scanning is relatively long. In view of these problems, this paper proposes an omnidirectional damage imaging monitoring method, combining the near-field sampling phased-array damage monitoring algorithm and the two-dimensional phased-array. The proposed method is verified by experiments using complex composite materials, and the results obtained show that the proposed omnidirectional near-field sampling phased-array damage imaging method is suitable for real-time damage detection in complex composite materials.

Fast Optical Phased Array Calibration Technique for Random Phase Modulation LiDAR

Optical phased array (OPA) imaging technique, which uses electro-optic modulation to achieve beam steering rather than mechanical scanning, is a raster scanning imaging method with great potential due to its noninertia and high speed. However, fabrication imperfection of an OPA causes pre-designed phase modulations not yielding desired steering angles, and a time-consuming calibration is usually required before practical use. Alternatively, it is possible to obtain images with a random phase modulation OPA. In this paper, we propose a fast calibration for the random phase modulation OPA LiDAR. Experimental results demonstrate that, to obtain images of the same quality, the proposed calibration is three times faster than the calibration used in raster scanning scheme. In the meantime, the proposed calibration simultaneously retrieves the point spread function of the imaging system during the process, which can be used to remove the image blurring caused by the sidelobes and further improve the image quality.

Numerical study of comb-based high-accuracy distance measurement utilizing VIPA interferometry

The ultrahigh-precision distance measurement based on a single laser frequency comb as the multi-wavelength source, together with an ultra-high-resolution dispersive spectrometer utilizing a virtually imaged phase array, was studied numerically. The simulations provide the component parameter optimization and the comb spectral images on a charged-coupled device camera. An ameliorated scheme was proposed through achieving real-time comparisons between the reference and interference signals captured on a camera simultaneously to improve both the measurement accuracy and the precision significantly. With the optimized data processing, the proposed approach is especially suitable for serving as a ultra-high precision relative distance measurement, and the measurement resolution can reach the picometer scale in optimal cases.

Eliminating backwall effects in the phased array imaging of near backwall defects.

Ultrasonic array imaging is widely used to provide high quality defect detection and characterization. However, the current imaging techniques are poor at detecting and characterizing defects near a surface facing the array, as the signal scattered from the defect and the strong reflection from the planar backwall will overlap in both time and frequency domains, masking the presence of the defect. To address this problem, this paper explores imaging algorithms and relevant methods to eliminate the strong artefacts caused by the backwall reflection. The half-skip total focusing method (HSTFM), the factorization method (FM) and the time domain sampling method (TDSM) are chosen as the imaging algorithms used in this paper. Then, three methods, referred to as full matrix capture (FMC) subtraction, weighting function filtering, and the truncation method, are developed to eliminate or filter the effects caused by the strong backwall reflection. These methods can be applied easily with few tuning parameters or little prior knowledge. The performances of the proposed imaging techniques are validated in both simulation and experiments, and the results show the effectiveness of the developed methods to eliminate the artefacts caused by the backwall reflections when imaging near backwall defects.

Fast, Low-Frequency Plane-Wave Imaging for Ultrasound Contrast Imaging

Plane-wave ultrasound contrast imaging offers a faster, less destructive means for imaging microbubbles compared with traditional ultrasound imaging. Even though many of the most acoustically responsive microbubbles have resonant frequencies in the lower-megahertz range, higher frequencies (>3 MHz) have typically been employed to achieve high spatial resolution. In this work we implement and optimize low-frequency (1.5-4 MHz) plane-wave pulse inversion imaging on a commercial, phased-array imaging transducer in vitro and illustrate its use in vivo by imaging a mouse xenograft model. We found that the 1.8-MHz contrast signal was about four times that acquired at 3.1 MHz on matched probes and nine times greater than echoes received on a higher-frequency probe. Low-frequency imaging was also much more resilient to motion. In vivo, we could identify sub-millimeter vasculature inside a xenograft tumor model and easily assess microbubble half-life. Our results indicate that low-frequency imaging can provide better signal-to-noise because it generates stronger non-linear responses. Combined with high-speed plane-wave imaging, this method could open the door to super-resolution imaging at depth, while high power pulses could be used for image-guided therapeutics.

Compressed Sensing Based Synthetic Transmit Aperture for Phased Array Using Hadamard Encoded Diverging Wave Transmissions

Previously, we proposed compressed sensing based synthetic transmit aperture (CS-STA) to improve the contrast and frame rate of STA while maintaining its spatial resolution in linear array by choosing uniform random matrix as the measurement matrix and transmitting the plane waves (PWs). In this paper, to extend CS-STA for phased array imaging and further improve its performance, we design four types of CS-STA implementations with different combinations of measurement matrices (i.e., uniform random and Hadamard matrices) and transmitted waves [i.e., PW and diverging wave (DW)]. Through simulations and phantom experiments with a 3 MHz, 64-element phased array, we find that type-IV CS-STA with the combination of a Hadamard matrix and DW outperforms the other three implementations including the previously proposed type-I CS-STA in terms of image quality and reconstruction time. Specifically, PW transmission produces visible discontinuity and the reconstruction time with uniform random matrix is about 100-fold longer than that with the Hadamard matrix. Compared with STA, with eightfold higher frame rate, type-IV CS-STA achieves 8.2 and 12.3 dB higher contrast-to-noise ratio and signal-to-noise ratio in the simulations, respectively. These improvements are slightly lower in the phantom experiments, which are 6.2 and 6.6 dB, respectively. In addition, CS-STA does not deteriorate the spatial resolution of STA, with the maximum deterioration being smaller than 1/8 wavelength. These results demonstrate that type-IV CS-STA can achieve phased array imaging with high image quality at high frame rate and may be beneficial to cardiac imaging.

Full non-contact laser-based Lamb waves phased array inspection of aluminum plate

Lamb waves defect detection technique is suitable to detect long-range defects and multi-defects in plates because of its attractive properties such as multi-modes, low attenuation, and multi-defect sensitivity. As the laser beams have the tiny size and noncontact property, in this paper a full non-contact laser-based Lamb waves phased array inspection technique was proposed for defect detection in an aluminum plate. A Q-switched Nd: YAG pulse laser generating Lamb waves and a continuous laser inspection system was used to inspect the out-of-plane displacements. The laser-generated Lamb waves have the broadband property and low temporal resolution that are not suitable for defects detection directly. So the continuous wavelet transform was used to extract narrowband signals from laser-generated Lamb wave and a linear mapping algorithm was adopted to compensate for the dispersion of the extracted narrowband signals. Two experiments were carried out to verify the applicability and effectiveness of the developed full non-contact laser-based Lamb waves phased array inspection technique. Two compact phased array imaging algorithms, the total focusing method and the sign coherence factor algorithm, were adopted to image the defects with processed signals. The experimental results accurately located the positions of the defects.

Reactive-element based decoupling network for a two-element MRI phased array

A reactive-element based Decoupling Network (DN) for 7-Tesla Magnetic Resonance Imaging (MRI) phased arrays is presented to compensate for the effect of mutual coupling between array elements. The DN network consists of two reactive elements connected in parallel between a two-element array, each element in the array is a rectangular loop-shape microstrip transmission line Radio Frequency (RF) coil. Integrated with an L-shaped tunable matching network, the decoupled elements can be tuned easily to operate at the working frequency of the 7-Tesla MRI system. Numerical modeling is carried out using CST to design, characterize and verify the performance of the decoupling and matching network for a two-element MRI array.

Ultrasonic Phased Array Compressive Imaging in Time and Frequency Domain: Simulation, Experimental Verification and Real Application.

Embracing the fact that one can recover certain signals and images from far fewer measurements than traditional methods use, compressive sensing (CS) provides solutions to huge amounts of data collection in phased array-based material characterization. This article describes how a CS framework can be utilized to effectively compress ultrasonic phased array images in time and frequency domains. By projecting the image onto its Discrete Cosine transform domain, a novel scheme was implemented to verify the potentiality of CS for data reduction, as well as to explore its reconstruction accuracy. The results from CIVA simulations indicate that both time and frequency domain CS can accurately reconstruct array images using samples less than the minimum requirements of the Nyquist theorem. For experimental verification of three types of artificial flaws, although a considerable data reduction can be achieved with defects clearly preserved, it is currently impossible to break Nyquist limitation in the time domain. Fortunately, qualified recovery in the frequency domain makes it happen, meaning a real breakthrough for phased array image reconstruction. As a case study, the proposed CS procedure is applied to the inspection of an engine cylinder cavity containing different pit defects and the results show that orthogonal matching pursuit (OMP)-based CS guarantees the performance for real application.

Damage localization with fiber Bragg grating Lamb wave sensing through adaptive phased array imaging

Fiber Bragg gratings are known being immune to electromagnetic interference and emerging as Lamb wave sensors for structural health monitoring of plate-like structures. However, their application for damage localization in large areas has been limited by their direction-dependent sensor factor. This article addresses such a challenge and presents a robust damage localization method for fiber Bragg grating Lamb wave sensing through the implementation of adaptive phased array algorithms. A compact linear fiber Bragg grating phased array is configured by uniformly distributing the fiber Bragg grating sensors along a straight line and axially in parallel to each other. The Lamb wave imaging is then performed by phased array algorithms without weighting factors (conventional delay-and-sum) and with adaptive weighting factors (minimum variance). The properties of both imaging algorithms, as well as the effects of fiber Bragg grating’s direction-dependent sensor factor, are characterized, analyzed, and compared in details. The results show that this compact fiber Bragg grating array can precisely locate damage in plates, while the comparisons show that the minimum variance method has a better imaging resolution than that of the delay-and-sum method and is barely affected by fiber Bragg grating’s direction-dependent sensor factor. Laboratory tests are also performed with a four–fiber Bragg grating array to detect simulated defects at different directions. Both delay-and-sum and minimum variance methods can successfully locate defects at different positions, and their results are consistent with analytical predictions.

基于编码激励的内镜超声相控阵成像算法

基于编码激励的内镜超声相控阵成像算法

Two-Stage Beamforming for Phased Array Imaging Using the Fast Hankel Transform.

An ultrasound scan generates a huge amount of data. To form an image, this data has to be transferred to the imaging system. This is an issue for applications where the data transfer capacity is limited such as hand-held systems, wireless probes, and miniaturized array probes. Two-stage beamforming methods can be used to significantly reduce the data transfer requirements. In the first stage, which is applied in-probe, the amount of data is reduced from channel to scanline data. In the imaging system, the data are then beamformed to obtain images, which are synthetically focused over the entire image. Currently, two approaches exist for the second stage. The first approach is a time-of-flight (TOF) approach called synthetic aperture sequential beamforming (SASB), which has been developed for both linear and phased arrays. SASB does, however, introduce artifacts in the image that can be reduced by tapering the first-stage scanlines at the cost of lateral resolution. The second approach is based on the wave equation, but a computationally efficient method for phased arrays that is producing sector scan data is lacking. Here, we propose an algorithm that uses the fast Hankel transform to obtain a fast algorithm. The imaging performance of this method is evaluated with simulations and experiments. Compared with PSASB, which is an adaption of SASB for phased arrays, our method requires a similar amount of operations to construct the entire image and there is no tradeoff between resolution and artifacts. These results show the advantage of using the wave equation instead of a TOF approach.

Phased Array Imaging of Complex-Geometry Composite Components.

Progress in computational fluid dynamics and the availability of new composite materials are driving major advances in the design of aerospace engine components which now have highly complex geometries optimized to maximize system performance. However, shape complexity poses significant challenges to traditional nondestructive evaluation methods whose sensitivity and selectivity rapidly decrease as surface curvature increases. In addition, new aerospace materials typically exhibit an intricate microstructure that further complicates the inspection. In this context, an attractive solution is offered by combining ultrasonic phased array (PA) technology with immersion testing. Here, the water column formed between the complex surface of the component and the flat face of a linear or matrix array probe ensures ideal acoustic coupling between the array and the component as the probe is continuously scanned to form a volumetric rendering of the part. While the immersion configuration is desirable for practical testing, the interpretation of the measured ultrasonic signals for image formation is complicated by reflection and refraction effects that occur at the water-component interface. To account for refraction, the geometry of the interface must first be reconstructed from the reflected signals and subsequently used to compute suitable delay laws to focus inside the component. These calculations are based on ray theory and can be computationally intensive. Moreover, strong reflections from the interface can lead to a thick dead zone beneath the surface of the component which limits sensitivity to shallow subsurface defects. This paper presents a general approach that combines advanced computing for rapid ray tracing in anisotropic media with a 256-channel parallel array architecture. The full-volume inspection of complex-shape components is enabled through the combination of both reflected and transmitted signals through the part using a pair of arrays held in a yoke configuration. Experimental results are provided for specimens of increasing complexity relevant to aerospace applications such as fan blades. It is shown that PA technology can provide a robust solution to detect a variety of defects including porosity and waviness in composite parts.