Yuriy Tasinkevych

Electromagnetic Scattering by Periodic Grating of Pec Bars

Electromagnetic wave scattering by a periodic array of perfectly conducting bars is studied in this paper. In the proposed approach the amplitudes of the spatial harmonics in free space above and below the array are expanded in a Fourier series with coefficients being properly chosen Legendre polynomials. As a result, the boundary and edge conditions are satisfied directly by field representation. The method results in a small system of linear equations for unknown expansion coefficients to be solved numerically. Some numerical examples are given, presenting a comparison to the mode matching technique.

Modified multi-element synthetic transmit aperture method for ultrasound imaging: A tissue phantom study.

The paper presents the modified multi-element synthetic transmit aperture (MSTA) method for ultrasound imaging. It is based on coherent summation of RF echo signals with apodization weights taking into account the finite size of the transmit subaperture and of the receive element. The work presents extension of the previous study where the modified synthetic transmit aperture (STA) method was considered and verified [1]. In the case of MSTA algorithm the apodization weights were calculated for each imaging point and all combinations of the transmit subaperture and receive element using their angular directivity functions (ADFs). The ADFs were obtained from the exact solution of the corresponding mixed boundary-value problem for periodic baffle system modeling the transducer array. Performance of the developed method was tested using Field II simulated synthetic aperture data of point reflectors for 4MHz 128-element transducer array with 0.3mm pitch and 0.02mm kerf to estimate the visualization depth and lateral resolution. Also experimentally determined data of the tissue-mimicking phantom (Dansk Fantom Service, model 571) obtained using 128 elements, 4MHz, linear transducer array (model L14-5/38) and Ultrasonix SonixTOUCH Research platform were used for qualitative assessment of imaging contrast improvement. Comparison of the results obtained by the modified and conventional MSTA algorithms indicated 15dB improvement of the noise reduction in the vicinity of transducers surface (1mm depth), and concurrent increase in the visualization depth (86% augment of the scattered amplitude at the depth of 90mm). However, this increase was achieved at the expense of minor degradation of the lateral resolution of approximately 8% at the depth of 50mm and 5% at the depth of 90mm.

Synthetic Aperture Method in Ultrasound Imaging

Medical ultrasound imaging is a technique that has become much more prevalent than other medical imaging techniques since it is more accessible, less expensive, safe, simpler to use and produces images in the real time. However, images produced by an ultrasound imaging system, must be of sufficient quality to provide accurate clinical interpretation. The most commonly used image quality measures are spatial resolution and image contrast which can be determined in terms of beam characteristics of an imaging system: beam width and sidelobe level. In the design of an imaging system, the optimal set of system parameters is usually found as a trade-off between the lowest side-lobe peak and the narrowest beam of an imaging system. In conventional ultrasound imaging system, when one transducer (in mechanical wobble) or linear array are used, the quality of images directly depends on the transducer acoustic field. Also in conventional ultrasound imaging the image is acquired sequentially one image line at a time that puts a strict limit on the frame rate that is important in real-time imaging system. Low frame rate means that moving structures (e.g. heart valves) are not easily imaged and diagnosis may be impaired. This limitation can be reduced by employing synthetic aperture (SA) imaging. The basic idea of the SA method is to combine information from emissions close to each other. The synthetic aperture method has previously not been used in medical imaging. This method is a contrast to the conventional beamforming, where only imaging along one line in receiving is used. This means that every image line is visualized as many times as the number of elements used. This will create an equal amount of low resolution images, which are summed up to create one high resolution image. Problems with medical ultrasound include low imaging depth, and high resolution is achieved only in the region where the transducer is focused. Another problem is decreasing SNR with depth. The basic idea with synthetic aperture is to combine information from emissions close to each other. This is a contrast to the conventional beamforming, were only imaging along one line in receiving is used. This means that every image line is visualized as many times as the number of elements used. This will create an equal amount of low resolution images which are summed up to create one high resolution image. One of the important processes in ultrasound imaging systems is beamforming. There are many different beamforming methods. In this work both the synthetic transmit aperture (STA) (Trots, et al. 2009) and the multi-element STA (Trots, et al. 2010) methods for medical ultrasound imaging system are discussed. In the case of the multi-element STA imaging

Optimization in the Multi-element Synthetic Transmit Aperture Method for Ultrasound Imaging

The paper deals with optimization of the transmit aperture in the multi-element synthetic transmit aperture method (MSTA) for ultrasound imaging. In contrast to the conventional synthetic transmit aperture (STA) method, MSTA allows to increase the system frame rate and provides the best compromise between penetration depth and lateral resolution.

Beam-Forming Electrostrictive Matrix

In this paper a two-dimensional ultrasound transducer comprising crossed periodic metal electrodes placed on both sides of electrostrictive layer and representing the matrix rows and columns is described. Such a system is capable of electronic beam-steering of generated wave both in elevation and azimuth. The wave-beam control is achieved by addressable driving of two-dimensional matrix transducer through proper voltage supply of electrodes on opposite surfaces of the layer. In this paper a semi-analytical method of analysis of the considered transducer is proposed, which is a generalization of the well-known BIS-expansion method. It was earlier exploited with great success in the theory of interdigital transducers of surface acoustic waves, theory of elastic wave scattering by cracks and certain advanced electrostatic problems. The paper presents evaluation of stress in the electrostrictive layer excited by potentials applied to electrodes. The corresponding nontrivial electrostatic problem is formulated. Some numerical examples showing the resulting generated electrostrictive stress in the dielectric layer are presented for a simple case of one upper strip excited by a uniform voltage and all bottom strips grounded.

Improving broadband ultrasound attenuation assessment in cancellous bone by mitigating the influence of cortical bone: Phantom and in-vitro study

HighlightsTrabecular bone attenuation is determined using reflected echoes.Impact of cortical bone on cancellous bone attenuation is corrected.Two‐fold reduction in the relative error of BUA assessment is found in experiments. ABSTRACT The purpose of this work was to present a new approach that allows the influence of cortical bone on noninvasive measurement of broadband ultrasound attenuation (BUA) to be corrected. The method, implemented here at 1 MHz makes use of backscattered signal and once refined and clinically confirmed, it would offer an alternative to ionizing radiation based methods, such as DEXA (Dual‐energy X‐ray absorptiometry), quantitative computed tomography (QCT), radiographic absorptiometry (RA) or single X‐ray absorptiometry (SXA), which are clinically approved for assessment of progress of osteoporosis. In addition, as the method employs reflected waves, it might substantially enhance the applicability of BUA ‐ from being suitable to peripheral bones only it would extend this applicability to include such embedded bones as hip and femoral neck. The proposed approach allows the cortical layer parameters used for correction and the corrected value and parameter of the cancellous bone (BUA) to be determined simultaneously from the single (pulse‐echo) bone backscattered wave; to the best of the authors’ knowledge such approach was not previously reported. The validity of the method was tested using acoustic data obtained from a custom‐designed bone‐mimicking phantom and a calf femur. The relative error of the attenuation coefficient assessment was determined to be 3.9% and 4.7% for the bone phantom and calf bone specimens, respectively. When the cortical shell influence was not taken into account the corresponding errors were considerably higher 8.3% (artificial bone) and 9.2% (calf femur). As indicated above, once clinically proven, the use of this BUA measurement technique in reflection mode would augment diagnostic power of the attending physician by permitting to include bones, which are not accessible for transmission mode evaluation, e.g. hip, spine, humerus and femoral neck.

Synthetic Aperture Cardiac Imaging with Reduced Number of Acquisition Channels. A Feasibility Study

Commercially available cardiac scanners use 64–128 elements phased-array (PA) probes and classical delay-and-sum beamforming to reconstruct a sector B-mode image. For portable and hand-held scanners, which are the fastest growing market, channel count reduction can greatly decrease the total power and cost of devices. The introduction of ultra-fast imaging methods based on plane waves and diverging waves provides new insight into heart’s moving structures and enables the implementation of new myocardial assessment and advanced flow estimation methods, thanks to much higher frame rates. The goal of this study was to show the feasibility of reducing the channel count in the diverging wave synthetic aperture image reconstruction method for phased-arrays. The application of ultra-fast 32-channel subaperture imaging combined with spatial compounding allowed the frame rate of approximately 400 fps for 120 mm visualization to be achieved with image quality obtained on par with the classical 64-channel beamformer. Specifically, it was shown that the proposed method resulted in image quality metrics (lateral resolution, contrast and contrast-to-noise ratio), for a visualization depth not exceeding 50 mm, that were comparable with the classical PA beamforming. For larger visualization depths (80–100 mm) a slight degradation of the above parameters was observed. In conclusion, diverging wave phased-array imaging with reduced number of channels is a promising technology for low-cost, energy efficient hand-held cardiac scanners.

Orthogonal Golay Codes With Local Beam Pattern Correction in Ultrasonic Imaging

The goal of this study is to improve the synthetic transmit aperture (STA) imaging method by employing the transducer array element beam pattern correction combined with emission of mutually orthogonal complementary Golay sequences. The transmit-receive scheme based on simultaneous emission of different Golay pairs by adjacent transmit subapertures is implemented to decrease the image reconstruction time. A brief discussion on the fundamentals of the orthogonal Golay complementary sequences is provided and their advantages for the STA imaging method are demonstrated. The performance of the developed approach was tested using FIELD II simulated synthetic aperture data from the point reflectors, which allowed to estimate both; the penetration depth and the lateral resolution. In the work the 128 element, 5 MHz, linear array transducer was used. The obtained results showed that the applying the beam pattern correction leads to the image quality improvement in the vicinity of the transducer face. Specifically, the noise level evaluated between the point reflectors at the depth of 4 mm decreased from - 14.1 dB for the case of omnidirectional source to - 38.7 dB when the element beam pattern correction was implemented. The simulation proved that the overall imaging quality was improved considerably.

Correspondence: Interdigitated interdigital transducer for surface elastometry of soft damping tissue

Measurement of the shear elastic constant of soft and highly damping tissue of high Poisson ratio is quite a challenging task. It is proposed to evaluate shear wave velocity and damping of tissue by measuring the shear skimming bulk waves using one interdigitated interdigital transducer on a piezoelectric layer, such as polyvinylidene fluoride, applied to the surface of the small tissue sample.

Electrostatics of Planar Multielectrode Sensors with Application to Surface Elastometry

Systems of planar electrodes arranged on dielectric or piezoelectric layers are applied in numerous sensors and transducers. In this paper electrostatics of such electrode systems is presented and exploited in the analysis of distributed piezoelectric transducer dedicated to surface elastometry of biological tissues characterized by large Poisson modulus. The fundamental Matlab® code for analyzing complex planar multiperiodic electrode systems is also presented.