Conventional Synthetic Aperture Focusing Technique Research Articles

Resolution Enhancement in Ultrasonic TOFD Imaging by Combining Sparse Deconvolution and Synthetic Aperture Focusing Technique (Sparse-SAFT)

The shallow subsurface defects are difficult to be identified and quantified by ultrasonic time-of-flight diffraction (TOFD) due to the low resolution induced by pulse width and beam spreading. In this paper, Sparse-SAFT is proposed to improve the time resolution and lateral resolution in TOFD imaging by combining sparse deconvolution and synthetic aperture focusing technique (SAFT). The mathematical model in the frequency domain is established based on the l1 and l2 norm constraints, and the optimization problem is solved for enhancing time resolution. On this basis, SAFT is employed to improve lateral resolution by delay-and-sum beamforming. The simulated and experimental results indicate that the lateral wave and tip-diffracted waves can be decoupled with Sparse-SAFT. The shallow subsurface defects with a height of 3.0 mm at the depth of 3.0 mm were detected quantitatively, and the relative measurement errors of flaw heights and depths were no more than 10.3%. Compared to conventional SAFT, the time resolution and lateral resolution are enhanced by 72.5 and 56% with Sparse-SAFT, respectively. Finally, the proposed method is also suitable for improving resolution to detect the defects beyond dead zone.

Three-dimensional synthetic aperture focusing photoacoustic microscopy based on the acoustic simulation generated delay time and weighted factor.

Acoustic resolution photoacoustic microscopy (ARPAM) is a promising imaging tool in biomedical applications for its advantage of penetration over other optical imaging techniques. However, the lateral resolution of ARPAM deteriorates significantly in the out-of-focus region. The synthetic aperture focusing technique (SAFT) is required to restore this kind of focus-related imaging distortion. The conventional SAFT method is based on the virtual detector (VD) conception, in which the phase of the received photoacoustic (PA) signal is calculated by assuming the focus of the transducer as a VD. Nevertheless, the phase of the received PA signal is not only determined by the geometrical parameters of the transducer, but also by the transducer's electromechanic response and the original PA signal. Ignoring these two factors will reduce the quality of the imaging results. In this work, a new SAFT method, which is based on acoustic simulation, is proposed for ARPAM. The measured PA signal from a point target at the focus is employed to evaluate the convolution of the transducer's electromechanic response and the original PA signal. This measured signal is used as the excitation in an acoustic simulation. The simulation, which is based on the geometrical and acoustic parameters of the transducer, is employed to calculate the delay time and weighted coefficient for the SAFT calculation. The phantom experiments with point and line targets indicate that the proposed method obtains imaging results with better lateral resolution and improved signal-noise ratio compared with the widely used VD-based SAFT method.

Frequency domain synthetic aperture focusing technique for variable-diameter cylindrical components.

Ultrasonic non-destructive testing (UNDT) plays an important role in ensuring the quality of cylindrical components of equipment such as pipes and axles. As the acoustic beam width widens along propagation depths, the diffraction of acoustic wave becomes serious and the images of defects will be interfered with. To precisely evaluate the dimensions of defects and flaws concealed in components, the synthetic aperture focusing technique (SAFT) is introduced to enhance the image resolutions. Conventional SAFTs have been successfully implemented for the ultrasonic imaging of normal cylinders, while solutions for complex ones, such as variable-diameter cylinders, are still lacking. To overcome this problem, a frequency-domain SAFT for variable-diameter cylindrical components is proposed. This algorithm is mainly based on acoustic field extrapolation, which is modified from cylindrical phase shift migration with the aid of split-step Fourier. After a series of extrapolations, a high-resolution ultrasound image can be reconstructed using a particular imaging condition. According to the experimental results, the proposed method yields low side lobes and high resolutions for flat transducers. Its attainable angular resolution relies on the transducer diameter D and scanning radius R and approximates D/(2R).

Circular Radon Transform Inversion Technique in Synthetic Aperture Ultrasound Imaging: an Ultrasound Phantom Evaluation

Abstract The paper presents an overview of theoretical aspects of ultrasound image reconstruction techniques based on the circular Radon transform inversion. Their potential application in ultrasonography in a similar way as it was successfully done in the x-ray computer tomography is demonstrated. The methods employing Radon transform were previously extensively explored in the synthetic aperture radars, geophysics, and medical imaging using x-ray computer tomography. In this paper the main attention is paid to the ultrasound imaging employing monostatic transmit-receive configuration. Specifically, a single transmit and receive omnidirectional source placed at the same spatial location is used for generation of a wide-band ultrasound pulse and detection of back-scattered waves. The paper presents derivation of the closed-form solution of the CRT inversion algorithms by two different approaches: the range-migration algorithm (RMA) and the deconvolution algorithm (DA). Experimentally determined data of ultrasound phantom obtained using a 32-element 5 MHz linear transducer array with 0.48 mm element pitch and 0.36 mm element width and 5 mm height, excited by a 2 sine cycles burst pulse are used for comparison of images reconstructed by the RMA, DA, and conventional synthetic aperture focusing technique (SAFT). It is demonstrated that both the RMA and SAFT allow better lateral resolution and visualization depth to be achieved as compared to the DA approach. Comparison of the results obtained by the RMA method and the SAFT indicates slight improvement of the lateral resolution for the SAFT of approximately 1.5 and 1.6% at the depth of 12 and 32 mm, respectively. Concurrently, however, the visualization depth increase for the RMA is shown in comparison with the SAFT. Specifically, the scattered echo amplitude increase by the factor of 1.36 and 1.12 at the depth of 22 and 32 mm is demonstrated. It is also shown that the RMA runs about 30% faster than the SAFT and about 12% faster than the DA method

Improved optoacoustic microscopy through three-dimensional spatial impulse response synthetic aperture focusing technique.

Synthetic aperture focusing technique (SAFT) is effective in restoring lateral resolution of ultrasonic images for scans with focusing-related distortions. Although successfully applied in pulse-echo ultrasonics, the physical nature of an optoacoustic modality requires a modified algorithm to return accurate results. The SIR-SAFT method reported here uses the spatial impulse response (SIR) of the transducer to weight the contributions to the SAFT and is tailored to provide significant resolution and signal gains for out-of-focus sources in scanning optoacoustic microscopy systems. Furthermore, the SIR-SAFT is implemented in full three dimensions, applicable to signals both far of and at the focus of the ultrasonic detector. The method has been further shown to outperform conventional SAFT algorithms for both simulated and experimental optoacoustic data.

Multiple virtual source aperture imaging for non-destructive testing

In recent years, the field of non-destructive testing has seen an expansion in the use of synthetic aperture imaging techniques, from the conventional synthetic aperture focusing technique (SAFT) and the total focusing method (TFM) through to the virtual source aperture (VSA) technique. Each technique shares a common imaging principle, which allows for improved lateral resolution and signal-to-noise ratio (SNR) by synthesising a larger aperture and the averaging effect. However, the computational burden of the advanced imaging algorithm limits the speed at which such systems can operate, which is further compounded by the acquisition of a larger number of raw amplitude data. Previous work has sought to limit transmission cycles in an effort to achieve a real-time data acquisition and imaging system, but at the detriment of image quality. This paper introduces several improvements to the VSA technique that allow for real-time imaging at lateral resolution and SNR levels comparable to and exceeding those of TFM, which is often referred to as the ‘gold standard’ in image quality. For this, three improvements are presented: firstly, by applying an apodisation function on transmit it is shown that improvements in side-lobe suppression can be achieved while maintaining a wide main lobe; secondly, two VSA scanning strategies are introduced, allowing for a greater averaging effect due to the additional virtual sources; and finally, a self-adaptive weighting function is applied to each pixel in the B-scan image based on the principles of incoherent and coherent summation.

Time Domain/Frequency Domain SAFT Imaging in Thin-Diameter Rod

The ultrasonic flaw reflected signal of the thin-diameter rod was acquired by the general ultrasonic C-scan testing device. And synthetic aperture focusing technique be used in ultrasonic imaging of thin-diameter rod. Respectively, Comparing B-scan imaging and Conventional synthetic aperture focusing technique imaging as well as Frequency domain synthetic aperture focusing technique imaging . The final results show that both time domain and frequency domain synthetic aperture method could obtain a higher signal-to-noise ratio and testing resolution.

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.

Improved synthetic aperture focusing technique by Hilbert-Huang transform for imaging defects inside a concrete structure

A useful nondestructive testing tool for civil engineering should immediately reveal defects inside concrete structures at the construction sites. To date, there are few effective methods to image defects inside concrete structures. In this paper, a new nondestructive testing method using elastic waves for imaging possible defects inside concrete is developed. This method integrates the point-source/point receiver scheme with the synthetic aperture focusing technique (SAFT) to increase functioning depth and enhance received signals. To improve image quality, received signals are processed by Hilbert-Huang transform (HHT) to get time-frequency curves for the SAFT process. Compared with conventional SAFT method processing with time-amplitude signals, this new method is capable of providing a better image of defects not only in the numerical simulation but also in the experimental result. The image can reveal the number of defects and their locations and front-end profiles. The results shown in this paper indicate that this new elastic-wave-based method exhibits high capability in imaging the defects of in situ concrete structures.

Application of a matched filter approach for finite aperture transducers for the synthetic aperture imaging of defects

The suitability of the synthetic aperture imaging of defects using a matched filter approach on finite aperture transducers was investigated. The first part of the study involved the use a finite-difference time-domain (FDTD) algorithm to simulate the phased array ultrasonic wave propagation in an aluminum block and its interaction with side-drilled hole-like defects. B-scans were generated using the FDTD method for three active aperture transducer configurations of the phased array (a) single element and (b) 16-element linear scan mode, and (c) 16-element steering mode. A matched filter algorithm (MFA) was developed using the delay laws and the spatial impulse response of a finite size rectangular phased array transducer. The conventional synthetic aperture focusing technique (SAFT) algorithm and the MFA were independently applied on the FDTD signals simulated with the probe operating at a center frequency of 5 MHz and the processed B-scans were compared. The second part of the study investigated the capability of the MFA approach to improve the SNR. Gaussian white noise was added to the FDTD generated defect signals. The noisy B-scans were then processed using the SAFT and the MFA and the improvements in the SNR were estimated. The third part of the study investigated the application of the MFA to image and size surface-crack-like defects in pipe specimens obtained using a 45 degrees steered beam from a phased array probe. These studies confirm that MFA is an alternative to SAFT with little additional computational burden. It can also be applied blindly, like SAFT, to effect synthetic focusing with distinct advantages in treating finite transducer effects, and in handling steered beam inspections. Finally, limitations of the MFA in dealing with larger-sized transducers are discussed.