A comparison between ultrasonic array beamforming and super resolution imaging algorithms for non-destructive evaluation

In this paper the total focusing method, the so called gold standard in classical beamforming, is compared with the widely used time-reversal MUSIC super resolution technique in terms of its ability to resolve closely spaced scatterers in a solid. The algorithms are tested with simulated and experimental array data, each containing different noise levels. The performance of the algorithms is evaluated in terms of lateral resolution and sensitivity to noise. It is shown that for the weak noise situation (SNR > 20 dB), time-reversal MUSIC provides significantly enhanced lateral resolution when compared to the total focusing method, breaking the diffraction limit. However, for higher noise levels, the total focusing method is shown to be robust, whilst the performance of time-reversal MUSIC is degraded. The influence of multiple scattering on the imaging algorithms is also investigated and shown to be small.     

Adaptive field-of-view imaging for efficient receive beamforming in medical ultrasound imaging systems

Quadrature demodulation-based phase rotation beamforming (QD-PRBF) is commonly used to support dynamic receive focusing in medical ultrasound systems. However, it is computationally demanding since it requires two demodulation filters for each receive channel. To reduce the computational requirements of QD-PRBF, we have previously developed two-stage demodulation (TSD), which reduces the number of lowpass filters by performing demodulation filtering on summation signals. However, it suffers from image quality degradation due to aliasing at lower beamforming frequencies. To improve the performance of TSD-PRBF with reduced number of beamforming points, we propose a new adaptive field-of-view (AFOV) imaging method. In AFOV imaging, the beamforming frequency is adjusted depending on displayed FOV size and the center frequency of received signals. To study its impact on image quality, simulation was conducted using Field II, phantom data were acquired from a commercial ultrasound machine, and the image quality was quantified using spatial (i.e., axial and lateral) and contrast resolution. The developed beamformer (i.e., TSD-AFOV-PRBF) with 1024 beamforming points provided comparable image resolution to QD-PRBF for typical FOV sizes (e.g., 4.6% and 1.3% degradation in contrast resolution for 160 mm and 112 mm, respectively for a 3.5 MHz transducer). Furthermore, it reduced the number of operations by 86.8% compared to QD-PRBF. These results indicate that the developed TSD-AFOV-PRBF can lower the computational requirement for receive beamforming without significant image quality degradation.     

一种稳健的柔性阵波束形成方法

由于航向、航速的改变以及水流、风浪等因素的影响,柔性线列阵在实际应用过程中阵型畸变是不可避免的。一方面,这种畸变使得许多假设阵型无畸变的波束形成方法的性能受到严重的影响;另一方面,目前很多的阵型估计方法仍不是很成熟,很难准确估计阵元的实际位置。本文提出将柔性阵列分为一段阵元位置已知的子阵和若干段阵元未知的子阵,利用不同子阵接收信号的互相关即可获得所需要的波束形成加权系数,从而在不需要知道阵元确切位置信息的情况下实现目标方法的估计。     

A new ultrasonic technique for the measurement of liquid level

A new design of pulse-echo ultrasonic liquid level gauge is described in which the ultrasonic pulse (here a Rayleigh or a Lamb wave) travels down a metal bar or strip towards the liquid surface, where, through a double mode-conversion process and a special reflecting structure, a strong echo is generated which travels back up the bar to the receiving transducer.The geometry of the special reflecting structure is outlined, and it is shown how this affects the basic accuracy of measurement. Inaccuracies arising from other effects like temperature variation, viscosity and poor signal-to-noise ratio are also discussed. Some mechanical variants, like using the wall of the tank in place of the metal bar carrying the ultrasonic wave and the use of corner reflectors as part of the reflecting structure, are also considered.A note in the Appendix briefly describes one method which has been used successfully for the generation of Rayleigh and Lamb waves.     

A new delay technique with application to ultrasound phased-array imaging systems.

In ultrasound phased-array imaging systems the receiver directivity pattern is steered by controlling the delay of echoes from the array elements. A new processing technique is introduced that provides the required delay first by properly phasing the echo carrier, and successively by delaying the echo envelope. This latter operation is based on a new analogue sampled-data delay-line. Independent control of phase delay offers a simple means of compensating for random phase errors due to non-uniformity of the array elements response.     

A forward model and conjugate gradient inversion technique for low-frequency ultrasonic imaging

Emerging methods of hyperthermia cancer treatment require noninvasive temperature monitoring, and ultrasonic techniques show promise in this regard. Various tomographic algorithms are available that reconstruct sound speed or contrast profiles, which can be related to temperature distribution. The requirement of a high enough frequency for adequate spatial resolution and a low enough frequency for adequate tissue penetration is a difficult compromise. In this study, the feasibility of using low frequency ultrasound for imaging and temperature monitoring was investigated. The transient probing wave field had a bandwidth spanning the frequency range 2.5-320.5 kHz. The results from a forward model which computed the propagation and scattering of low-frequency acoustic pressure and velocity wave fields were used to compare three imaging methods formulated within the Born approximation, representing two main types of reconstruction. The first uses Fourier techniques to reconstruct sound-speed profiles from projection or Radon data based on optical ray theory, seen as an asymptotical limit for comparison. The second uses backpropagation and conjugate gradient inversion methods based on acoustical wave theory. The results show that the accuracy in localization was 2.5 mm or better when using low frequencies and the conjugate gradient inversion scheme, which could be used for temperature monitoring.     

Adaptive beamforming for photoacoustic imaging

An adaptive photoacoustic image reconstruction technique that combines coherence factor (CF) weighting and the minimum variance (MV) method is introduced. The backprojection method is widely used to reconstruct photoacoustic tomography images. Owing to the scattering of light, the quality of the photoacoustic imaging can be degraded. CF, an adaptive weighting technique, is known to improve the lateral resolution of photoacoustic images. In addition, an MV adaptive beamforming method can further improve the image quality by suppressing signals from off-axis directions. Experimental studies are performed to quantify the spatial resolution and contrast of the adaptive photoacoustic beamforming methods.     

A new method for determining deformation amplitudes in ultrasonic fatigue systems

An experimental procedure is described, which allows determination of the deformation parameters of materials excited in resonance with high amplitude ultrasound. The theory concerning resonance in the case of hysteresis phenomena is outlined briefly. By this procedure the maximum stress in the sample and the plastic strain can be determined from measurements of strain and particle velocity obtained in the region where the sample remains elastic. Experimentally obtained data on aluminium samples are discussed and compared with respect to predicted theory.     

A new phase plate for noise-free coherent imaging systems

A new phase plate has been developed for noise-free coherent imaging system. The phase plate consists of a bleached holographic lens whose frequency is modulated in such a way that the information from each point of the object is distributed in a small three dimensional space at the Fourier plane. This introduces redundancy in the hologram and helps in smoothing out the amplitude variations in the Fourier plane. Experimental results are presented which show the capabilities of the phase plate.     

A new technique for the identification of ultrasonic flaw signals using deconvolution

The identification of ultrasonic flaw signals is a difficult task in the angle beam ultrasonic testing of welded joints due to the presence of non-relevant signals from the geometric reflectors such as weld-roots and counter-bores. This paper describes a new approach to identify ultrasonic flaw signals in such a problematic situation. A similarity function is defined as the deconvolution of a target signal by a reference signal. The similarity functions for the same type of flaws/references are symmetric bandlimited impulse-like patterns with larger amplitudes while those for different types of flaws/references are asymmetrical broad patterns with relatively smaller amplitudes. Therefore, ultrasonic signals could be identified by the pattern of the similarity function. In the initial experiments, the proposed technique showed great potential for identifying ultrasonic flaw signals in the inspection of weld joints.     

Sequential beamforming for synthetic aperture imaging

Synthetic aperture sequential beamforming (SASB) is a novel technique which allows to implement synthetic aperture beamforming on a system with a restricted complexity, and without storing RF-data. The objective is to improve lateral resolution and obtain a more depth independent resolution compared to conventional ultrasound imaging. SASB is a two-stage procedure using two separate beamformers. The initial step is to construct and store a set of B-mode image lines using a single focal point in both transmit and receive. The focal points are considered virtual sources and virtual receivers making up a virtual array. The second stage applies the focused image lines from the first stage as input data, and take advantage of the virtual array in the delay and sum beamforming. The size of the virtual array is dynamically expanded and the image is dynamically focused in both transmit and receive and a range independent lateral resolution is obtained. The SASB method has been investigated using simulations in Field II and by off-line processing of data acquired with a commercial scanner. The lateral resolution increases with a decreasing F#. Grating lobes appear if F# ⩽ 2 for a linear array with λ-pitch. The performance of SASB with the virtual source at 20 mm and F# = 1.5 is compared with conventional dynamic receive focusing (DRF). The axial resolution is the same for the two methods. For the lateral resolution there is improvement in FWHM of at least a factor of 2 and the improvement at −40 dB is at least a factor of 3. With SASB the resolution is almost constant throughout the range. For DRF the FWHM increases almost linearly with range and the resolution at −40 dB is fluctuating with range. The theoretical potential improvement in SNR of SASB over DRF has been estimated. An improvement is attained at the entire range, and at a depth of 80 mm the improvement is 8 dB.     

AN-aided beamforming for IRS-assisted SWIPT systems

This paper studies artificial noise (AN)-aided beamforming design in an intelligent reflecting surface (IRS)-assisted system empowered by simultaneous wireless information and power transfer (SWIPT) technique. Multiple power splitting (PS) single-antenna receivers simultaneously receive information and energy from a multi-antenna base station (BS). Although all users are legitimate, in each transmission interval only one receiver is authorized to receive information and the others are only allowed to harvest power which are considered as unauthorized receivers (URs). To prevent information decoding by URs, AN signal is transmitted from the BS. We adopt a non-linear model for energy harvesting. In the optimization problem, we minimize the total transmit power, and for this purpose, we utilize an alternating optimization (AO) algorithm. For the non-convex rank-one constraint for IRS phase shifts, we utilize a sequential rank-one constraint relaxation (SROCR) algorithm. In addition to single antenna URs scenario, we investigate multi-antenna URs scenario and evaluate their performance. Simulation results validate the effectiveness of using IRS.     

A new subdivision technique for grating based on CMOS microscopic imaging

We propose a new subdivision technique directly subdividing the grating stripe by using complementary metal-oxide semiconductor (CMOS) microscopic imaging system combined with image processing. The corresponding optical system, subdivision principle, and image processing methods are illuminated. The relations of systemic resolution to subdivision number, grating period, magnifying power and tilt angle are theoretically discussed and experimentally checked on the Abbe comparator. The measurement precision for displacement of the proposed subdivision system is tested in the range of 5 mm and the maximum displacement error is less than 0.4μm. The factors contributing to the systemic error are also discussed.     

High-resolution deconvolved beamforming for three-dimensional imaging sonars

Three-dimensional(3D) imaging sonars based on the conventional beamforming(CBF) suffers from relatively wide main-lobes and high sidelobe level.To improve the spatial resolution of 3D imaging sonars,a deconvolved beamforming method is proposed with the iterative Richardson-Lucy algorithm in this paper.At first,each distance slice can be processed to obtain images by the CBF.For the near field,the Fresnel approximation is used.Then,the deconvolution technique is applied to the CBF outputs to obtain high-resolution images and suppress the sidelobe level.The simulations demonstrate that the proposed algorithm is able to improve the spatial resolution significantly and suppress the sidelobes for 3D imaging sonars.Meanwhile,the algorithm shows similar robustness with the CBF in the case of wideband and sparse array.The priority of the proposed algorithm is also validated by the tank experiment data.The presented results indicate that the spatial resolution is increased by one time and the average sidelobe level is reduced by 20 dB.     

A novel ultrasonic micro-dissection technique for biomedicine

Molecular techniques are transforming our understanding of cellular function and disease. However, accurate molecular analysis methods will be limited if the input DNA, RNA, or protein is not derived from pure population of cells or is contaminated by the wrong cells. A novel Ultrasonic Vibration Micro-dissection (UVM) method was proposed to procure pure population of targeted cells from tissue sections for subsequent analysis. The principle of the ultrasonic vibration cutting is analyzed, and a novel micro-tool is designed. A multilayer piezoelectric actuator is used to actuate a sharp needle vibrating with high frequency and low amplitude (approx. 16-50 kHz, and 0-3 microm) to cut the tissue. Contrast experiment was done to test the feasibility of UVM method. Experimental results show that the embedded tissue can be quickly and precisely cut with the ultrasonic vibration micro-dissection method.     

A new technique for Raman spectroscopy in highly absorbing liquid systems

In this paper we describe a new technique for Raman spectroscopy in highly absorbing liquid samples. Such a technique, called Jet Flow Raman Spectroscopy (JFRS) is used in the case of saturated solutions of strong electrolytes such as CuCl₂ and CoCl₂. A comparison of spectra obtained with JFRS and a semicapillary cell is shown. The power of the JFRS technique is also proved with the evidence of a resonant Raman effect in saturated aqueous solutions of CuCl₂.     

Characterization of ultrasonic imaging systems using transfer functions

The transfer functions for focussed and defocussed, coherent and confocal optical imaging systems have been applied to the equivalent ultrasonic imaging systems. The transfer functions with varying degrees of defocus were calculated to show the defocus effects for ultrasonic imaging systems. Assuming that the acoustic waves are reflected perfectly on the surface of the step edge, the theoretical line-scans for small amplitude signals across a step edge, with various degrees of defocus, were generated. The first derivative of the line-scan for a step edge is shown theoretically to yield the same impulse response as that calculated using the inverse Fourier transform applied to the original transfer function. These results show how the real and imaginary parts of the transfer functions contribute to ultrasonic image formation. A method for the experimental determination of the impulse response, and the transfer functions for the characterization of an ultrasonic imaging system, such as an acoustic microscope, is provided.     

3D beamforming with ultrasonic divided-ring arrays

Conventional 2D arrays have a set of squared elements whose inter-element spacing is around lambda/2. This arrangement requires an excessive amount of electronic resources for the generation and processing of ultrasonic signals. In this work, the beam properties of a single divided-ring array are analysed theoretically with the goal of producing volumetric images. Divided-ring arrays are based on a circular pattern, which has a lower periodicity than square arrays, and this property allows increasing the element size while keeping the amplitude of the grating lobes at a reasonably low level. The paper emphasises several advantages of ring arrays, suggesting that these apertures are useful for 3D ultrasonic imaging. First, as the element size may increase, the number of elements can be reduced with little loss of emitting area. Second, ring arrays produce beams of large depth of field in both transmission and reception. This can be used to avoid the complexity associated with dynamic focusing.     

A simulation model for ultrasonic pulse-echo systems

Echo patterns obtained by using an ultrasonic pulse-echo system are mainly determined by the structure of an object, but are influenced by many other factors such as the directivity and damping characteristics of the ultrasonic transducer, the dynamic range of the display unit and so on. A simulation model has been developed in which if these factors are specified numerically and fed to the simulator, then an A- or B-mode display pattern is obtained as its output. In this paper the principles of the simulator are presented first and some results of the simulation are given and they are compared with echo patterns obtained experimentally.     

Correlation-based imaging technique using ultrasonic transmit–receive array for Non-Destructive Evaluation

This paper describes a novel array post-processing method for Non-Destructive Evaluation (NDE) using phased-array ultrasonic probes. The approach uses the capture and processing of the full matrix of all transmit–receive time-domain signals from a transducer array as in the case of the Total Focusing Method (TFM), referred as the standard of imaging algorithms. The proposed technique is based on correlation of measured signals with theoretical propagated signals computed over a given grid of points. In that case, real-time imaging can be simply implemented using discrete signal product. The advantage of the present technique is to take into account transducer directivity, dynamics and complex propagation patterns, such that the number of required array elements for a given imaging performance can be greatly reduced. Numerical and experimental application to contact inspection of isotropic structure is presented and real-time implementation issues are discussed.