Synthetic Aperture Focusing Technique Algorithm Research Articles

Enhancing vascular network visualization in 3D photoacoustic imaging: in vivo experiments with a vasculature filter

Filter-based vessel enhancement algorithms facilitate the extraction of vascular networks from medical images. Traditional filter-based algorithms struggle with high noise levels in images with false vessel extraction, and a low standard deviation (σ) value may introduce gaps at the centers of wide vessels. In this paper, a robust technique with less sensitivity to parameter tuning and better noise suppression than other filter-based methods for two-dimensional and three-dimensional images is implemented. In this study, we propose a filter that employs non-local means (NLM) for denoising, applying the vesselness function to suppress blob-like structures and filling the gaps in wide vessels without compromising edge quality or details. Acoustic resolution photoacoustic microscopy (AR-PAM) systems generate high-resolution volumetric photoacoustic images, but their vascular structure imaging suffers from out-of-focal signal-to-noise ratio (SNR) and lateral resolution loss. Implementing a synthetic aperture focusing technique (SAFT) based on a virtual detector (VD) improves out-of-focal region resolution and SNR. Combining the proposed filter with the SAFT algorithm enhances vascular structural imaging in AR-PAM systems. The proposed method is robust and applicable for animal tissues with less error of vasculature structure extraction in comparison to traditional fliter-based methods like Frangi and Sato filter. Also, the method is faster in terms of processing speed and less tuning parameters. We applied the method to a digital phantom to validate our approach and conducted in vivo experiments to demonstrate its superiority for real volumetric tissue imaging.

Restoring the imaging quality of circular transducer array-based PACT using synthetic aperture focusing technique integrated with 2nd-derivative-based back projection scheme

Circular-array-based photoacoustic computed tomography (CA-PACT) is a promising imaging tool owing to its broad acoustic detection coverage and fidelity. However, CA-PACT suffers from poor image quality outside the focal zone along both elevational and lateral dimensions. To address this challenge, we proposed a novel reconstruction strategy by integrating the synthetic aperture focusing technique (SAFT) with the 2nd derivative-based back projection (2nd D-BP) algorithm to restore the image quality outside the focal zone along both the elevational and lateral axes. The proposed solution is a two-phase reconstruction scheme. In the first phase, with the assistance of an acoustic lens, we designed a circular array-based SAFT algorithm to restore the resolution and SNR along the elevational axis. The acoustic lens pushes the boundary of the upper limit of the SAFT scheme to achieve enhanced elevational resolution. In the second phase, we proposed a 2nd D-BP scheme to improve the lateral resolution and suppress noises in 3D imaging results. The 2nd D-BP strategy enhances the image quality along the lateral dimension by up-converting the high spatial frequencies of the object’s absorption pattern. We validated the effectiveness of the proposed strategy using both phantoms and in vivo human experiments. The experimental results demonstrated superior image quality (7-fold enhancement in elevational resolution, 3-fold enhancement in lateral resolution, and an 11-dB increase in SNR). This strategy provides a new paradigm in the PACT system as it significantly enhances the spatial resolution and imaging contrast in both the elevational and lateral dimensions while maintaining a large focal zone.

The Feasibility of Shadowed Image Restoration Using the Synthetic Aperture Focusing Technique

The phenomenon of acoustic shadowing on ultrasonography is characterized by an echo signal void behind structures that strongly absorb or reflect ultrasonic energy. In medical ultrasonography, once the ultrasound energy is shielded, acoustic shadowing makes it difficult to create an image, leading to misinterpretations and obscure diagnoses. Hence, instead of dealing with the defocused problem encountered in an ultrasound scan (US), this current research focuses on revealing the existence of an acoustically shadowed target (or a potential lesion) using a well-known restoration algorithm, i.e., the synthetic aperture focusing technique (SAFT). To demonstrate the effects of an acoustic shadow on an ultrasound scan (US), a forward model study is carried out. In laboratory manipulations, a purposely designed physical model is created and then scanned using B-mode and pitch/catch arrangements to carry out shadowed and shadow-free scans in a water tank. Thereafter, making use of a delay-and-sum (DAS) operation, the echo signals are processed by the synthetic aperture focusing technique (SAFT) to perform image restoration. The results of the restoration process show that the SAFT algorithm performs well with respect to directional shadowing. Once the target or lesion is positioned in a total anechoic zone, or even in a multi-channel scan, it will fail.

Ultrasonic measurements of grouted tendon ducts in the concrete structure using Elop Insight Scanner

Infrastructures such as bridges, tunnels, dams, retaining walls, docks, parking garages and buildings are constructed from reinforced concrete. These structures deteriorate due to cyclic temperature and loading variations, aging, as well as physical causes such as accidents and aggressive chemical attack from the environment. Post tension tendon ducts commonly used in bridges are filled with grout and steel tendons. The voids in the grout inside the tendon ducts can occur due various reasons such as improper filling, material mixing, blockage due to tendons etc. The water can penetrate in these voids inside the tendon ducts which can initiate corrosion. It is therefore vital to detect voids/ unfilled or defective portions of the tendon ducts at an early stage of the construction phase to ensure the strength and safety of the structure. Different types of NDT (Non-Destructive Testing) methods are frequently used for monitoring such infrastructures during regular operation to prevent failure, increase life span and improve safety. One of these NDT techniques, ultrasonic testing, is relatively simple in operation and gives more accurate description of air pockets, voids, cracks, and delaminations in the concrete structures compared to other NDT methods. ELOP has developed a rolling ultrasonic scanner COBRI (COncrete and BRidge Inspection) together with a complete software solution for asset management of structures for faster and better inspection and monitoring of the internal condition of the concrete. It will provide asset owners with information about when, where and how to inspect, maintain and more accurately assess damages and overall condition of their structures. The COBRI scanner contains an array of transducers assembled in two separate rollers used in pitch-catch mode. It uses a patented elastomer solution to provide dry coupling to the concrete surface during scanner movement, without the need for any added fluid couplant. The system uses SAFT (Synthetic Aperture Focusing Technique) algorithm for image reconstruction to increase image quality and signal to noise ratio. It will provide users with improved tomographic (B, C and D-scan) image quality and 3D imaging. In the current work, we have used the COBRI scanner to explore its ability to image grouting defects inside tendon ducts used in concrete structures. The concrete specimen used in the evaluation has two tendon ducts positioned at 8 cm from the top surface with dimensions of 70 cm in length, 55 cm in width and 25 cm in thickness with average aggregate size of 22 mm. One of the tendon ducts is completely empty. The second one is half-length fully filled with grouting along the prestressed steel and other half only three quarters filled, leaving an air cavity on the top. The scanner is rolled approximately 50 cm over the concrete surface and the tomographic (B, C and D-scan) SAFT images are generated at different locations of the concrete specimen together with a 3D volume image. The results showed that the COBRI scanner is able to differentiate the filled and partially filled tendon ducts in the concrete structure. As COBRI is a rolling ultrasound scanner, it will help the bridge owners to scan and assess the grouting defects in the tendon ducts much more efficiently than other scanners available in the market.

Evaluation of retroreflective corner echo for detection of surface breaking cracks in concrete by ultrasound

The retroreflective corner echo is used, for example, in ultrasonic non-destructive testing of metals to find fatigue cracks in tubes or shafts. If the much weaker crack tip signal is additionally detected, the crack length can also be determined. A corner reflection occurs in cases of surface breaking cracks with predominantly perpendicular orientation to the surface. The intensity of the corner reflection depends on the angle of incidence and on the ultrasonic wave mode used. For the reliable detection of vertical surface breaking cracks in metals, transversal waves are commonly used, which propagate at an angle of 37° to 53° to the inspection surface. As shown in this contribution, the wide spread low frequency ultrasonic arrays with dry point contact sources available for ultrasonic testing of concrete also allow to receive corner echoes. These devices generate transversal waves in concrete structures with a large divergence of the sound field. A series of experiments was carried out with such dry point contact arrays on concrete specimens with artificial test defects and controlled induced cracks of different depths. The ultrasonic time-of-flight signals were recorded, exported and reconstructed utilising the SAFT (Synthetic Aperture Focusing Technique) algorithm. The SAFT reconstruction parameters were adjusted to visualize the corner echo indication. As will be shown, with this targeted processing, the reproducible detection of surface breaking cracks in concrete is possible. The retroreflective corner echo can thus be exploited in civil engineering for non-destructive inspection of concrete.

Detection of defect in ballastless track based on impact echo method combined with improved SAFT algorithm

Impact echo method is a superior method for structural health monitoring in concrete, but it has the disadvantages of low accuracy in detecting multi-layer structures. To overcome this limitation, an improved synthetic aperture focusing technique (SAFT) algorithm was proposed and combined with impact echo method for the detection of ballastless track. The improved SAFT algorithm makes full use of time domain echo signals containing multiple conversion elastic waves to image ballastless track defect. Simulation analysis based on finite difference time-domain (FDTD) method and a laboratory experiment were conducted to verify the effectiveness of improved SAFT algorithm combined with impact echo method for the identification of ballastless track defect. The results show that the decrease of sensor spacing and contact time in impact echo method contributes to the visual identification of defects by the improved SAFT algorithm, the visualization effect of defects can be improved by adjusting the minimum threshold level of the color scale of reconstructed image. Except for defect near the boundary, impact echo method combined with the improved SAFT algorithm has good applicability for the detection of ballastless track defects with random spatial location and number distribution.

Multi-mode laser-ultrasound imaging using Time-domain Synthetic Aperture Focusing Technique (T-SAFT)

The Synthetic Aperture Focusing Technique (SAFT) is an imaging algorithm used in laser ultrasonics (LU) to visualise the appearance of defects. However, ultrasound excited by a pulsed laser has the characteristics of wide bandwidth and multi-mode directivity patterns, leading to common problems in the SAFT process, such as low utilisation of ultrasound information and possible artefacts. To solve these problems, a Multi-mode Time-domain SAFT (MMT-SAFT) algorithm is proposed in this paper. The influence of ultrasonic directivity is discussed according to the imaging depth range, and imaging with multiple LU modes is performed to reduce artefacts. Simulations and experimental results prove the feasibility of the MMT-SAFT algorithm, which not only presents a clearer image of the upper part of defects but also improves image quality compared with time-domain SAFT using a single ultrasonic mode. The proposed technique can provide feasible directions for laser ultrasonic defect imaging.

3D Internal Visualization of Concrete Structure Using Multifaceted Data for Ultrasonic Array Pulse-Echo Tomography.

This research proposes a 3D internal visualization using ultrasonic pulse-echo tomography technique to evaluate accurately the state of concrete structures for their efficient maintenance within a limited budget. Synthetic aperture focusing technique (SAFT) is used as a post-processing algorithm to manipulate the data measured by the ultrasonic pulse-echo technique. Multifaceted measurements improve the weakness of the existing ultrasonic pulse-echo tomography technique that cannot identify the area beyond a reflector as well as the area located far away from measuring surfaces. The application of apodization factor, pulse peak delay calibration and elimination of trivial response not only complements the weaknesses of the SAFT algorithm but also improves the accuracy of the SAFT algorithm. The results show that the proposed method reduces the unnecessary surface noise and improves the expressiveness of the reflector’s boundaries on the resulting images. It is expected that the proposed 3D internal visualization technique will provide a useful non-destructive evaluation tool in combination with another structure evaluation method.

Study on PCA-SAFT imaging using leaky Rayleigh waves

Efficient and accurate nondestructive testing of surface or sub-surface flaws is essential for metal components. This study focuses on the high-quality image reconstruction of these flaws. An imaging detection method is introduced which combines the leaky Rayleigh waves testing and the synthetic aperture focusing technique (SAFT) with an immersion pulse-echo scanning. The principal component analysis (PCA) is used to reduce the noise of the leaky Rayleigh waves, and then the imaging is obtained using the SAFT algorithm. The experimental results show that the PCA-SAFT imaging method can reduce structural noise and imaging artifacts comparing with the B-scan imaging method. The lateral resolution of defects is improved and the mean error of defects sizing can be reduced by 48.81%. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the ultrasonic image are increased by 6.95 dB and 4.20 dB respectively. The proposed method can be a new choice for effective evaluating the surface quality of key components.

SAFT Imaging of Transverse Cracks in Austenitic and Dissimilar Welds

Up to now there is no sufficient technique to detect transverse cracks in austenitic and dissimilar welds which recently are of increasing interest in the integrity surveillance of nuclear power plants as well as in quality control of longitudinally welded pipes. Weld inspection by interpretation of single A-scans will lead to erroneous results due to effects caused by anisotropy and in worst case might leave flaws undetected. Therefore, imaging techniques such as the synthetic aperture focusing technique (SAFT) should be used. If the SAFT algorithm is applied on data taken from austenitic welds, the inhomogeneous, anisotropic structure of these welds has to be taken into account in order to properly attribute amplitudes measured in A-scans to the corresponding coordinates in the region of interest. While this has been investigated in the past, all attempts so far were limited to the imaging of longitudinal cracks which requires a less complicated setup than the imaging of transverse cracks. In this paper we give an outline of our attempts to reconstruct images of transverse cracks in different welds. For this purpose a SAFT program based on ray tracing and a layered structure weld model derived from an empirical model of grain orientations in welds are used. The results of the image reconstruction on experimental data are shown and compared to images obtained by assuming an isotropic homogeneous model. Root reflection and crack tip echo are clearly visible which allows an estimation of size and position of the crack with good accuracy.

Synthetic aperture focusing technique with virtual transducer for immersion inspection of solid objects

The conventional synthetic aperture focusing technique (SAFT) can improve the lateral resolution and increase signal-to-noise ratio (SNR) in ultrasonic imaging. However, the use of SAFT in immersed inspection is limited due to the issues related to the refraction effects at the boundary between the water and a solid object. The virtual transducer (VT) method employing a focused transducer or phased array is proposed as a solution to this problem. In the VT method, the focal point of a focused aperture is used as a virtual source of ultrasonic waves. If the virtual source is placed on or below the surface of an inspected solid object, SAFT can be applied in the same way as for contact inspection. Combining VT with the new phase-shift migration SAFT (PSM-SAFT) algorithm allows one to reconstruct images of multilayer objects with high resolution and high SNR. What is more, the computation efficiency of PSM-SAFT can significantly reduce the time needed for image reconstruction. Experimental validation of the proposed technique is presented in this paper. We present the results of experiments where the VT was created using a focused transducer and a phased array in the immersion inspection of a stainless steel block containing artificial defects. The performance of four different focused apertures used for generating a virtual transducer is evaluated in relation to the PSM-SAFT algorithm implemented in the frequency domain.

Defect imaging with elastic waves in inhomogeneous–anisotropic materials with composite geometries

Imaging of defects in composite structures plays an important role in non-destructive testing (NDT) with elastic waves, i.e., ultrasound. Traditionally the imaging of such defects is performed using the synthetic aperture focusing technique (SAFT) algorithm assuming homogeneous isotropic materials. However, if parts of the structure are inhomogeneous and/or anisotropic, this algorithm fail to produce correct results that are needed in order to asses the lifetime of the part under test. Here we present a modification of this algorithm which enables a correct imaging of defects in inhomogeneous and/or anisotropic composite structures, whence it is termed InASAFT. The InASAFT is based on the exact modelling of the structure in order to account for the true nature of the elastic wave propagation using travel time ray tracing techniques. The algorithm is validated upon several numerical and real life examples yielding satisfactory results for imaging of cracks. The modified algorithm suffers, though, from the same difficulties encountered in the SAFT algorithm, namely “ghost” images and eventual lack of clear focused images. However, these artifacts can be identified using a forward wave propagation analysis of the structure.

Synthetic aperture imaging using sources with finite aperture: deconvolution of the spatial impulse response.

A method for ultrasonic synthetic aperture imaging using finite-sized transducers is introduced that is based on a compact, linear, discrete model of the ultrasonic measurement system developed using matrix formalism. Using this model a time-domain algorithm for deconvolution of the transducer's spatial impulse responses (SIRs) is developed that is based on a minimum mean square error (MMSE) criterion. The algorithm takes the form of a spatiotemporal filter that compensates for the SIRs associated with a finite-sized transducer at every point of the processed image. A major advantage of the proposed method is that it can be used for any transducer, provided that its associated SIRs are known. This is in contrast to the synthetic aperture focusing technique (SAFT), which treats the transducer as a point source. The performance of the method is evaluated with simulations and experiments, performed in water using a linear phased array. The results obtained using the proposed method are compared to those obtained with a classical time-domain SAFT algorithm. For a finite aperture source, it is clearly shown that the resolution obtained using the proposed method is superior to that obtained using the SAFT algorithm.

A SAFT algorithm for lamb wave imaging of isotropic plate-like structures

Synthetic aperture focusing technique (SAFT) algorithm in the Fourier domain (F-SAFT) has been modified to take into account the dispersive nature of Lamb waves. This new algorithm can be used to produce good ultrasonic images of a plate structure for both dispersive and non-dispersive parts of the Lamb wave dispersion curve. This imaging technique has been tested by performing an inspection in a highly dispersive area of the fundamental flexural S 0 mode in order to demonstrate the usefulness of the new algorithm. Results are presented as a comparison between standard B-scan, original F-SAFT algorithm and new Lamb-SAFT algorithm.

Two-dimensional synthetic focusing using large apertures—Deconvolution of the spatial impulse response

The synthetic aperture focusing technique (SAFT) has been used successfully in both medical and nondestructive evaluation (NDE) applications of ultrasound. Transducer size is an important issue when implementing SAFT algorithms. The classical SAFT method is based on the assumption that a point source is used for emitting ultrasonic waves, which means in practice that the transducer aperture is so small compared to the wavelength that it emits spherical waves. If the transducer aperture is large its spatial impulse response (SIR) significantly differs from a Dirac pulse and nonspherical waves will be generated. Therefore, to successfully perform synthetic focusing using large apertures, the transducer’s SIR must be taken into account. A new method for compensating transducer SIR in synthetic focusing is presented in the paper. The method employs a stochastic, time-domain Wiener filtering technique for two-dimensional synthetic focusing. Results of the experiments performed in water using a linear phased array are presented to demonstrate the performance of the proposed method. The use of an array enabled altering the aperture without changing the electrical characteristics of the whole system. The results obtained using the proposed technique for finite apertures are compared to those obtained with a classical SAFT algorithm.

Three-dimensional imaging system based on Fourier transform synthetic aperture focusing technique

For planar scan surfaces, digitized ultrasonic RF-data can be adequately processed in terms of the Fourier transform synthetic aperture focusing technique algorithm, i.e. in terms of synthetic aperture pulse-echo backpropagation utilizing Fourier transforms only, to yield a quantitative three-dimensional image of defects residing in the homogeneous and isotropic bulk material. The implementation of this algorithm into an ultrasonic imaging system is described, which mainly comprises an array processor and high-resolution graphics to display the three-dimensional reconstruction volume as a walk-through along three orthogonal planes. To enhance the signal-to-noise ratio and the axial resolution of the system, controlled ultrasonic signals are transmitted as complementary Golay-sequences, cross-correlated with the received signals and deconvolved with similarly obtained reference signals.

A nondestructive ultrasonic imaging system for detection of flaws in metal blocks

An ultrasonic imaging system is presented for the nondestructive evaluation of internal defects in metal blocks. The surfaces of the blocks are coarsely scanned by a single ultrasonic transducer to record multiple A-scans. The recorded A-scans are processed by a synthetic-aperture focusing technique (SAFT) algorithm to search for defects in a metal specimen. The processed information is then sent to a graphics package to be displayed on a CRT screen of a personal computer. The current linear imaging technique is presented and is compared to the SAFT algorithm. The theory and limitations of the SAFT algorithm as well as the hardware implementation are discussed. Images of defects in aluminum blocks are illustrated with varying operating parameters showing the performance of the imaging system. >

SAFT — the evolution of a signal processing technology for ultrasonic testing

This paper is a review of the development of the synthetic aperture focusing technique (SAFT) for use in ultrasonic testing. The equations for processing the data using the SAFT algorithm in several modes are described. This paper also compares the results obtained using full 3D processing against line SAFT processing. It then briefly discusses ways to reduce the processing time for the SAFT algorithm.

Ultrasonic synthetic aperture focusing using planar pulse-echo transducers

The lateral resolution of digital data from the planar (unfocused) pulse-echo transducers used in conventional ultrasonic inspections can be improved using the synthetic aperture focusing technique (SAFT). For practical applications it is important to minimize the level of sidelobes (artefacts) introduced by SAFT, without significant loss of resolution. This may be achieved by the inclusion of a suitable aperture-weighting function in the SAFT algorithm, combined with a synthetic aperture size related to the width of the transducer beam-spread. The properties of the resulting optimized SAFT algorithm are quantified using experimental data from a series of artificial flaws (slots) of different sizes.