Wideband focused transducer array for optoacoustic tomography

The calculation procedure of the parameters of a multielement transducer array for the optoacoustic tomography of biological objects with high spatial resolution values is proposed. A multielement transducer with given spatial resolution values in three dimensions has been developed based on the proposed procedure for the early detection of breast cancer. The transducer array consists of a set of 8 linear PVDF piezoelectric films located on a plane and a focusing cylindrical acoustic lens. A map of the transducer’s focal area and point spread function have been measured using the constructed transducer array. Spatial resolutions of the transducer array obtained experimentally are in agreement with their calculated values.     

A new method of spatial compounding imaging

A new method of spatial compound imaging is presented that improves image quality without the usual requirement to decrease the frame rate. The new method of imaging utilizes three transducers for data acquisition. The transducer located at the center of the transducer system is a phased array probe that acts as both transmitter and receiver. The other transducers are unfocused pistons that act only as receivers. Envelope data acquired by each transducer are combined to form a final image with improved quality (speckle contrast, target detectability and lateral resolution). It is shown that the improvement in speckle contrast depends on the correlation between individual images acquired by the transducers. The effective aperture approach is used for analytic estimation of the correlation between images in order to optimize the lateral separation between transducers. Using simulations, several compounding strategies have been performed to find the strategy that maximizes image quality. The central frequency of 2.5 MHz is used in simulations. Quantitative analysis of simulated B-mode images shows that the new method of imaging efficiently improves visibility, detectability, and lateral resolution of low contrast regions. The image frame rate is preserved because multiple scans are not required for the spatial compounding.     

Focused array transducer for two-dimensional optoacoustic tomography

Optoacoustic (OA) imaging utilizes short laser pulses to create acoustic sources in tissue and time resolved detection of generated pressure profiles for image reconstruction. The ultrasonic transients provide information on the distribution of optical absorption coefficient that can be useful for early cancer diagnostics. In this work a new design of wide-band array transducer is developed and tested. The array consists of 32 focused piezo-elements made of PVDF slabs imposed on a cylindrical surface. A single array element response to an OA signal coming from arbitrarily located point source is investigated theoretically and experimentally. The measured signals correspond well to numerically calculated ones. Focal zone maps of the elements with aperture angles 30 degrees and 60 degrees are presented and discussed; the resolution in direction perpendicular to the imaging plane is determined. Point spread function of the whole array is calculated using experimentally obtained signals from the sources located at different distances from the array. Backprojection algorithm is employed for reconstruction of the optoacoustic images. It is shown that the spatial resolution of the images yielded by the proposed array increases significantly compared to previous transducer designs.     

Optoacoustic imaging with synthetic aperture focusing and coherence weighting

Optoacoustic imaging takes advantage of high optical contrast and low acoustic scattering and has found several biomedical applications. In the common backward mode a laser beam illuminates the image object, and an acoustic transducer located on the same side as the laser beam detects the optoacoustic signal produced by thermoelastic effects. A cross-sectional image is formed by laterally scanning the laser beam and the transducer. Although the laser beam width is generally narrow to provide good lateral resolution, strong optical scattering in tissue broadens the optical illumination pattern and thus degrades the lateral resolution. To solve this problem, a combination of the synthetic aperture focusing technique with coherence weighting is proposed. This method synthesizes a large aperture by summing properly delayed signals received at different positions. The focusing quality is further improved by using the signal coherence as an image quality index. A phantom comprising hair threads in a 1% milk solution was imaged with an optoacoustic imaging system. The results show that the proposed technique improved lateral resolution by 400-800% and the signal-to-noise ratio by 7-23 dB over conventional techniques.     

高分辨率快速数字化光声CT乳腺肿瘤成像

提出了一种基于聚焦线性阵列探测器的快速光声计算机断层成像技术（光声CT）.在光声二维图像重建中,根据阵列探测器机械扫描和电子扫描相结合的组合扫描模式,提出了改进的有限场滤波反投影重建算法.一方面该算法适合多元探测器旋转扫描模式,另一方面探测器的指向性函数作为反投影的权重因子提高了系统的横向分辨率.同时,该成像系统还利用柱面声透镜实现Z轴方向上的聚焦扫描以实现三维层析成像.实验中,这套成像系统空间分辨率达到0.2mm,Z轴方向分辨率为1.5mm,扫描一幅二维图像仅需150s,得到 关键词： 光声CT 有限场滤波反投影算法 声透镜聚焦 乳腺肿瘤检测     

Deconvolution of vibroacoustic images using a simulation model based on a three dimensional point spread function

Vibro-acoustography (VA) is a medical imaging method based on the difference-frequency generation produced by the mixture of two focused ultrasound beams. VA has been applied to different problems in medical imaging such as imaging bones, microcalcifications in the breast, mass lesions, and calcified arteries. The obtained images may have a resolution of 0.7–0.8 mm. Current VA systems based on confocal or linear array transducers generate C-scan images at the beam focal plane. Images on the axial plane are also possible, however the system resolution along depth worsens when compared to the lateral one. Typical axial resolution is about 1.0 cm. Furthermore, the elevation resolution of linear array systems is larger than that in lateral direction. This asymmetry degrades C-scan images obtained using linear arrays. The purpose of this article is to study VA image restoration based on a 3D point spread function (PSF) using classical deconvolution algorithms: Wiener, constrained least-squares (CLSs), and geometric mean filters. To assess the filters’ performance on the restored images, we use an image quality index that accounts for correlation loss, luminance and contrast distortion. Results for simulated VA images show that the quality index achieved with the Wiener filter is 0.9 (when the index is 1.0 this indicates perfect restoration). This filter yielded the best result in comparison with the other ones. Moreover, the deconvolution algorithms were applied to an experimental VA image of a phantom composed of three stretched 0.5 mm wires. Experiments were performed using transducer driven at two frequencies, 3075 kHz and 3125 kHz, which resulted in the difference-frequency of 50 kHz. Restorations with the theoretical line spread function (LSF) did not recover sufficient information to identify the wires in the images. However, using an estimated LSF the obtained results displayed enough information to spot the wires in the images. It is demonstrated that the phase of the theoretical and the experimental PSFs are dissimilar. This fact prevents VA image restoration with the current theoretical PSF. This study is a preliminary step towards understanding the restoration of VA images through the application of deconvolution filters.     

Wave guide imaging through Time Domain Topological Energy

Time Domain Topological Energy (TDTE), uses the reflected ultrasonic field recorded by an array of transducers placed on the boundary of the inspected medium. Two numerical determinations (forward and adjoint problems) of the acoustical field inside a reference medium are necessary to compute an image. Topological Energy is defined as a variation of topological sensitivity or gradient and comes from the field of mathematical optimisation. Recent developments for Non-Destructive Testing have shown the analogy with Time Reversal concepts. Time Reversal mirrors have been employed for various applications in a wide number of situations including wave guides where very good re-focalization performances have been obtained with a reduced number of transducers instead of an array of transducers. Moreover recent works have enlightened that the reverberation properties of a wave guide allow to re-focalize using Time Reversal with only one transducer. For TDTE imaging a single transducer placed at one end of a wave guide has been modelled. The boundaries of the wave guide create virtual sources that can be understood as a virtual array of transducers. Numerical and experimental results are presented using TDTE and a single transducer in a wave guide for targets and conditions of increasing complexity.     

基于二维声子晶体的大尺寸夹心式换能器的优化设计

夹心式换能器应用极为广泛,但当其横向尺寸过大时,存在耦合振动,影响其辐射面的位移分布.本文通过在大尺寸夹心式换能器的前盖板中加工周期排列的槽,来形成一种二维声子晶体结构.随后,采用有限元法对基于二维声子晶体的大尺寸夹心式换能器的振动传输特性、共振频率以及发射电压响应进行仿真模拟,讨论了开槽高度和开槽宽度对其带隙、共振与反共振频率、带宽以及辐射面位移分布的影响.研究结果表明,通过在大尺寸夹心式换能器中应用声子晶体结构可对其进行优化设计.当大尺寸夹心式换能器的工作频率位于其带隙范围内时,二维声子晶体结构能有效地抑制其横向振动,从而改善换能器辐射面位移分布的均匀程度.此外,在大尺寸夹心式换能器的前盖板中加工二维声子晶体结构,能有效提升换能器的带宽,进而拓宽大尺寸夹心式换能器的工作频带.     

A high frequency amplitude-steered array for real-time volumetric imaging

Real-time three-dimensional acoustic imaging is difficult in water or tissue because of the slow speed of sound in these media. Conventional pulse-echo data collection, which uses at least one transmit pulse per line in the image, does not allow for the real-time update of a volume of data at practical ranges. Recently, a linear amplitude-steered array was presented that allows the collection of a plane of data with a single transmit pulse by spatially separating frequencies in the lateral direction. Later, by using a linear array with frequency separation in the vertical direction and rotating the array in the horizontal direction, volumetric data were collected with a small number of transmit pulses. By expanding the linear array to a two-dimensional array, data can now be collected for volumetric imaging in real time. In this study, the amplitude-steered array at the heart of a real-time volumetric sonar imaging system is described, giving the design of the array and describing how data are collected and processed to form images. An analysis of lateral resolution in the vertical and horizontal directions shows that resolution is improved in the direction of frequency separation over systems that use a broad transmit beam. Images from simulated data are presented.     

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.     

基于FPGA的高强度聚焦超声相位控制系统设计*

多阵元相控阵换能器具有焦距可调和可实现经颅聚焦等优势,近来受到众多研究者的关注,其相位控制系统是决定多阵元相控换能器能否应用于临床的关键技术之一。在输出信号稳定的同时提高高强度聚焦超声相位控制系统精度是设计过程中的重点与难点。本文基于现场可编程门阵列设计了一种高强度聚焦超声相位控制系统。实测结果表明,多通道延时分辨率为1 ns,延时误差小于1 ns,可满足多阵元高强度聚焦超声治疗相控聚焦换能器延时精度的需要。     

Modified synthetic transmit aperture algorithm for ultrasound imaging

The modified synthetic transmit aperture (STA) algorithm is described. The primary goal of this work was to assess the possibility to improve the image quality achievable using synthetic aperture (SA) approach and to evaluate the performance and the clinical applicability of the modified algorithm using phantoms. The modified algorithm is based on the coherent summation of back-scattered RF echo signals with weights calculated for each point in the image and for all possible combinations of the transmit-receive pairs. The weights are calculated using the angular directivity functions of the transmit-receive elements, which are approximated by a far-field radiation pattern of a narrow strip transducer element vibrating with uniform pressure amplitude over its width. In this way, the algorithm takes into account the finite aperture of each individual element in the imaging transducer array. The performance of the approach developed was tested using FIELD II simulated synthetic aperture data of the point reflectors, which allowed the visualization (penetration) depth and lateral resolution to be estimated. Also, both simulated and measured data of cyst phantom were used for qualitative assessment of the imaging contrast improvement. The experimental data were obtained using 128 elements, 4 MHz, linear transducer array of the Ultrasonix research platform. The comparison of the results obtained using the modified and conventional (unweighted) STA algorithms revealed that the modified STA exhibited an increase in the penetration depth accompanied by a minor, yet discernible upon the closer examination, degradation in lateral resolution, mainly in the proximity of the transducer aperture. Overall, however, a considerable (12 dB) improvement in the image quality, particularly in the immediate vicinity of the transducer’s surface was demonstrated. The modified STA method holds promise to be of clinical importance, especially in the applications where the quality of the “near-field” image, that is the image in the immediate vicinity of the scanhead is of critical importance such as for instance in skin- and breast-examinations.     

Harmonic analysis of lossy, piezoelectric composite transducers using the plane wave expansion method

Periodic composite ultrasonic transducers offer many advantages but the periodic pillar architecture can give rise to unwanted modes of vibration which interfere with the piston like motion of the fundamental thickness mode. In this paper, viscoelastic loss is incorporated into a three-dimensional plane wave expansion model (PWE) of these transducers. A comparison with experimental and finite element data is conducted and a design to damp out these lateral modes is investigated. Scaling and regularisation techniques are introduced to the PWE method to reduce ill-conditioning in the large matrices which can arise. The identification of the modes of vibration is aided by examining profiles of the displacements, electrical potential and Poynting vector. The dispersive behaviour of a 2-2 composite transducer with high shear attenuation in the passive phase is examined. The model shows that the use of a high shear attenuation filler material improves the frequency band gap surrounding the fundamental thickness mode.     

采用超声导波阵列技术研究板类结构大面积检测

超声导波阵列检测技术可以实现板类结构的快速、大面积检测,是工业领域中最有前景的技术之一。针对目前此技术中存在的单一模态传感器难制作、检测频率高(一般在200 kHz以上)、需非常密集的传感器阵列和成像分辨率低等问题,本文采用一种圆环形线圈EMAT (Electromagnetic Acoustic Transducer),在较低频率15 kHz激励产生单一A0模态Lamb波,由8个传感器组成环形阵列,利用TFM (Total Focus Method)进行成像,实现了超声Lamb波对板类结构的快速、大面积检测。对一块包含直径为27 mm通孔的3 mm厚铝板进行实验研究,结果表明,此系统具有较高的测量稳定性和成像分辨率,能快速定位缺陷,适用于对板类结构进行大面积粗检。在对检测系统盲区形成原因分析的基础上,建立了一种计算盲区范围的方法。      

Sound speed correction in ultrasound imaging

A constant sound speed of 1.54 mm/micros is generally used by ultrasound imaging systems for delay and timing. However, the body's sound speed in-homogeneity can lead to defocusing and increased clutter. To provide an improvement using standard transducers, the sound speed used in delay and timing was computed using different sound speeds. We observed improvement in lateral resolution and clutter in phantom, OB, abdominal, and breast imaging. In OB and abdominal imaging using a 4 MHz curved array, 1.48 mm/micros provided higher image quality in many situations. In breast with an 8 MHz linear array, 1.44 mm/micros provided better images in some cases. To provide an automated way to determine and adjust the sound speed used by the imaging system, an algorithm was developed that determines the sound speed that produces the best overall lateral image quality by analyzing the spatial frequency content in a single B-mode frame of channel data using images reconstructed using various trial sound speeds. The metric produced correlates well with the observed best lateral image quality.     

微纳相控线阵超声换能器参数的理论分析*

微纳相控线阵超声换能器参数(阵元数目、阵元宽度及阵元间距)直接影响其横向声场分布,而其横向声场分布是能否实现高成像分辨率、大探测深度的决定性因素,也是制备换能器的主要依据。该文利用数值模拟研究微纳相控线阵超声换能器阵元参数对其横向声场中主瓣强度、-3 dB主瓣宽度、第一级旁瓣及栅瓣的影响。结果表明,主瓣强度随着阵元数目增加而增大,随阵元间距减小而增大,随着阵元宽度的增大呈现先增大后减小的趋势;-3 dB主瓣宽度随着阵元数目和阵元间距的增大而减小,随着阵元宽度的减小而减小;此外,减小阵元数目、减小阵元间距或增大阵元宽度均可以抑制旁瓣;栅瓣在阵元间距满足一定条件时可以完全消除。通过这些研究为微纳相控线阵超声换能器的优化设计与制备提供理论参考。     

高分辨率扇形阵列超声激发振动声成像研究

对扇阵换能器的振动声成像进行了研究.在相关振动声成像和扇阵换能器的理论基础上,经过推导修正并得到了精确的速度势函数.进一步计算并仿真了系统点扩散函数（point spread function, PSF）,构造了组织模型,进行了成像仿真.最后分析了影响扇阵换能器系统分辨率的因素.研究结果表明：用扇阵换能器进行振动声成像可以获得很高的图像分辨率及对比度,在高强度聚焦超声（HIFU）治疗的热损伤检测等方面有重要的应用价值.分析指出通过提高换能器工作频率,减小几何焦距,增大口径可以提高系统的分辨率. 关键词： 振动声成像 扇形阵 点扩散函数     

Ultrasound fields in attenuating media

For medical ultrasonic imaging and for nondestructive testing, the attenuation of pressure waves and the resulting shift in wave velocity are important features in commonly used transmission media such as biological tissue. An algorithm for the numerical evaluation of pressure field distributions generated by ultrasonic transducers is presented. The attenuation and dispersion of the sound transmission medium are taken into consideration. The sound fields are computed numerically for continuous wave as well as pulse excitation. The transducer has plane or gently curved geometry and is embedded in a plane rigid baffle. The numerically determined pressure fields are presented as 3D plots, as gray-scale images for a fixed time stamp (like a snapshot), or as isobars regarding the maximum values over time for each local point in the area under investigation. The algorithm described here can be utilized as a tool for design of ultrasound transducers, especially array antennas.     

Elastic guided wave propagation in a periodic array of multi-layered piezoelectric plates with finite cross-sections

In this study, we present a model study of guided wave dispersion and resonance behavior of an array of piezoelectric plates with arbitrary cross-sections. The objective of this work is to analyze the influence of the geometry of an element of a 1D-array ultrasound transducer on generating multi-resonance frequency so as to increase the frequency bandwidth of the transducers. A semi-analytical finite-element (SAFE) method is used to model guided wave propagation in multi-layered 1D-array ultrasound transducers. Each element of the array is composed of LiNbO₃ piezoelectric material with rectangular or subdiced cross-section. Four-node bilinear finite-elements have been used to discretize the cross-section of the transducer. Dispersion curves showing the dependence of phase and group velocities on the frequency, and mode shapes of propagating modes were obtained for different geometry consurations. A parametric analysis was carried out to determine the effect of the aspect ratio, subdicing, inversion layer and matching layers on the vibrational behavior of 1D-array ultrasound transducers. It was found that the geometry with subdiced cross-section causes more vibration modes compared with the rectangular section. Modal analysis showed that the additional modes correspond to lateral modes of the piezoelectric subdiced section. In addition, some modes have strong normal displacements, which may influence the bandwidth and the pressure field in front of the transducer. In addition, the dispersion curves reveal strong coupling between waveguide modes due to the anisotropy of the piezoelectric crystal. The effect of the matching layers was to cluster extensional and flexural modes within a certain frequency range. Finally, inversion layer is found to have a minor effect on the dispersion curves. This analysis may provide a means to analyze and understand the dynamic response of 1D-array ultrasound transducers.     

Observation of the optoacoustic effect in an optical fibre

The rediationless absorption of light propagating in optical fibres induces both stress waves and thermal waves in the fibre medium. Detection of such waves is termed the optoacoustic effect. Successful detection in optical fibres is easily obscured by scattered light impinging on the detector. We have obtained a signal of 2 mV on a piezoceramic transducer attached to a commercial step-index optical fibre after passage of a 1 J ruby laser light pulse along the fibre axis. No signal was, however, produced when the transducer was not in contact with the fibre. Similar results were obtained using chopped cw laser radiation but then more care had to be taken since the transducers used had a measureable sensitivity to the low frequency chopped radiation emitted perpendicular to the optical fibre axis.