基于K-wave的经颅超声聚焦仿真研究

由于颅骨形状不规则,声速、密度等声学参数分布不均匀,使用传统相控聚焦的方法进行经颅超声精确聚焦时会出现焦点偏移、焦点形状畸变、散焦的现象,时间反转法被认为是能够克服颅骨非均匀特性,实现经颅聚焦的有效手段。而利用传统有限元,有限差分法进行数值仿真时,往往需要划分十分精密的网格进行计算,耗费大量的时间。K空间伪谱法通过在空间域上进行傅里叶变换,时间域上进行有限差分的方法求解声波方程,能够在降低计算网格密度的同时保证计算精度。为了克服颅骨对聚焦超声造成的焦移影响,同时避免声学模型计算量过大的问题,本文使用Matlab K-wave开源工具箱,对食蟹猴颅骨CT数据进行三维声学建模,实现了基于时间反转法的经颅超声聚焦,讨论了时间反转法对于颅骨不规则形状及非均匀声速造成的焦移的补偿。仿真结果表明,K-wave在提高计算速度的同时能够服务于精准经颅超声聚焦,且相较于传统基于声时差法的相控聚焦方法,时间反转法在预设焦点位置,焦点强度更高,焦斑形状更为规则,聚焦效果更好。     

超声治疗中耦合液对声聚焦影响的研究

本文推导了凹球面聚焦换能器产生的声波经耦合液进入软组织后的声场计算公式，并对不同耦合液，耦合层厚度，温度对声聚焦的影响进行了研究，得到一一些对超声热疗具有指导意义的结果。     

Synchronous dynamic focusing for ultrasound imaging

An approach to dynamic focusing of ultrasound linear array scanners is presented, leading to the unique capability of implementing a focus that continuously tracks the return signal along the penetration depth. An electronically variable lens is obtained by a heterodyning process, in which the phases of echo signals at the array elements are equalized by mixing with suitable reference oscillations. These are generated by control of a single voltage-controlled oscillator, whose frequency is properly varied in synchronism with the delay of signal from different depths. The technique has been experimentally demonstrated by modifying the focusing processor of a conventional echographic linear scanner. Superior performances have been obtained with respect to fixed-focus operation mode. The image quality results are comparable with those of multizone-focus operation mode, in which the focus is varied over more transmit/receive cycles at the expense of lower frame rate.     

Unit-delay focusing architecture and integrated-circuit implementation for high-frequency ultrasound

High-frequency ultrasound (above 10 MHz) has been used successfully in many medical applications, including eye, skin, gastrointestinal, intravascular, and Doppler flow imaging. Most of these applications use single-element transducers, thereby imposing a tradeoff between resolution and depth of field. Fabrication difficulties and the need for high-speed electronic beamformers have prevented widespread use of arrays at high frequencies. In this paper, a unit-delay focusing architecture suitable for use with high-frequency ultrasound annular arrays is described. It uses a collection of identical, active delay cells that may be simultaneously varied to accomplish focusing. Results are presented for an analog integrated circuit intended for use with a five-element, 50-MHz planar annular array. Focusing is possible over an axial range for which the ratio of maximum to minimum f-number is 2.1. Unit-delay architectures also are described for curved annular arrays and linear arrays.     

The progressive focusing correction technique for ultrasound beamforming

This work presents a novel method for digital ultrasound beamforming based on programmable table look-ups, in which vectors containing coded focusing information are efficiently stored, achieving an information density of a fraction of bit per acquired sample. Timing errors at the foci are within half the period of a master clock of arbitrarily high frequency to improve imaging quality with low resource requirements. The technique is applicable with conventional as well as with deltasigma converters. The bit-width of the focusing code and the number of samples per focus can be defined to improve both memory size and F# with controlled timing errors. In the static mode, the number of samples per focus is fixed, and in the dynamic approach that figure grows progressively, taking advantage of the increasing depth of focus. Furthermore, the latter has the lowest memory requirements. The technique is well suited for research purposes as well as for real-world applications, offering a degree of freedom not available with other approaches. It allows, for example, modifying the sampling instants to phase aberration correction, beamforming in layered structures, etc. The described modular and scalable prototype has been built using low-cost field programmable gate arrays (FPGAs). Experimental measurements are in good agreement with the theoretically expected errors.     

The ultrasound brain helmet: new transducers and volume registration for in vivo simultaneous multi-transducer 3-D transcranial imaging

Because stroke remains an important and time-sensitive health concern in developed nations, we present a system capable of fusing 3-D transcranial ultrasound volumes acquired from two sides of the head. This system uses custom sparse array transducers built on flexible multilayer circuits that can be positioned for simultaneous imaging through both temporal acoustic windows, allowing for potential registration of multiple real-time 3-D scans of cerebral vasculature. We examine hardware considerations for new matrix arrays-transducer design and interconnects-in this application. Specifically, it is proposed that SNR may be increased by reducing the length of probe cables. This claim is evaluated as part of the presented system through simulation, experimental data, and in vivo imaging. Ultimately, gains in SNR of 7 dB are realized by replacing a standard probe cable with a much shorter flex interconnect; higher gains may be possible using ribbon-based probe cables. In vivo images are presented, showing cerebral arteries with and without the use of microbubble contrast agent; they have been registered and fused using a simple algorithm which maximizes normalized cross-correlation.     

Orthogonal quadratic chirp signals for simultaneous multi-zone focusing in medical ultrasound imaging

In medical ultrasound imaging, multi-zone focusing on transmission is used to enhance the lateral resolution at the expense of frame rate. As an alternative, this paper proposes a simultaneous multi-zone focusing method using orthogonal quadratic chirp signals to improve lateral resolution without sacrificing frame rate. In the proposed method, two weighted quadratic chirp signals with different spectra are simultaneously transmitted with different transmit time delays for multi-zone focusing. Because the two weighted quadratic chirps can be designed to have a desired level of cross-correlation after compression, the degradation of axial resolution resulting from the division of a spectrum is minimized. Through simulation, the performances of the proposed method were evaluated and compared with those of two-cycle pulsed excitation as a gold standard and two sub-band weighted linear chirps. In the simulation, the proposed method improved -6-dB and -20-dB lateral beam widths by factors of 1.67 and 1.84, respectively, compared with the pulsed excitation. The degradation of axial resolution in the proposed method was maximally 43% less than that in the linear chirp case. The results demonstrate that the proposed method is useful in the improvement of overall ultrasound image quality because the axial resolution of conventional ultrasound images is generally a few times higher than the lateral resolution.     

Description of phase singularities and their application to focusing design

We describe the focusing region associated with transmittances, analyzing its associated phase function. We show that generic features can be studied from the differential equation for focusing geometry, which is obtained through angular representation for diffraction fields. With the treatment, we recover the results for circular zone plates, and by introducing a linear transformation into the transmittance function we generate structures that keep the ability to generate focusing. According to the choice of the parameters involved, the diffraction field presents new focusing regions, whose three-dimensional geometry and spatial evolution can be described in a selective fashion with analysis of only the phase singularities associated with the diffraction field and avoidance of the integral representation. The treatment is also applied to a simple lens. We recover the theoretical predictions obtained by Berry and Upstill [M. V. Berry and C. Upstill, in Progress in Optics, E. Wolf, ed. (North-Holland, Amsterdam, 1980), Vol. XVIII, p. 259], and these predictions are corroborated experimentally. The results obtained are shown.     

Effects of non-optimal focusing on dual-frequency ultrasound measurements of bone

In pulse-echo (PE) ultrasound measurements, the use of focused transducers is desirable for quantitative assessment of bone characteristics because of the attenuation in the overlying soft tissues. However, the variable thickness and composition of the soft tissue overlying bone affect the focal depth of the ultrasound beam and induce errors into the measurements. To compensate for the attenuation-related effects caused by the interfering soft tissue (i.e., fat and lean tissue), a dual-frequency ultrasound (DFUS) technique was recently introduced. The aim of this study was to investigate the effect of non-optimal focal depth of the ultrasound beam on the determination of the integrated reflection coefficient (IRC) of bone when overlaid by an interfering layer composed of oil and water. The feasibility of the DFUS-based correction of the IRC was evaluated through numerical simulations and experimental measurements. Even when the interfering layer-bone interface was out of focus, the total thickness of the interfering layer could be accurately determined with the technique. However, based on the simulations, the errors in the determination of the composition of the interfering layer increased (0.004 to 113.8%) with the increase in distance between the interfering layer-bone interface and the focus of the ultrasound beam. Attenuation compensation, based on the true composition of the interfering layer, resulted in an average relative error of 22.3% in the IRC values calculated from the simulations. Interestingly, the attenuation compensation with the interfering layer composition estimated using the DFUS technique resulted in a smaller average relative error of 14.9% in the IRC values. The simulations suggest that DFUS can reduce the errors induced by soft tissue in bone PE ultrasound measurements. The experimental measurements indicate that the accuracy of the IRC measurements is rather similar when using DFUS correction or correction based on the true composition of the interfering layer. However, the results suggest that accurate determination of soft tissue composition may be difficult without optimal focusing of the ultrasound beam on the soft tissue-bone interface.     

Direct focusing modifications of elliptical gaussian beam by non-circular apertures

Abstract

For focusing the elliptical Gaussian beam directly, the effects of a non-circular aperture on the focusing properties are studied. The focusing properties for different shapes of apertures, which include a circle, an ellipse and a rectangle, are calculated and compared. Moreover, for different elliptical Gaussian beams, an empirical aperture selection rule that can be used to circularize the focusing spot is proposed. The energy transmission ratios are also considered in this paper.     

Design and fabrication of a diffractive phase element for wavelength demultiplexing and spatial focusing simultaneously

The design of a diffractive phase element (DPE) that simultaneously implements wavelength demultiplexing and focusing is carried out on the basis of the general theory of amplitude-phase retrieval. The designed DPE is fabricated with optical contact lithography. Three masks are needed to produce the surface-relief structure of the DPE with eight quantized levels in depths. Experiments demonstrate that the designed DPE can successfully implement both the functions of demultiplexing three different-wavelength beams and focusing each component at a predesignated position simultaneously. Experimental measurements are in good agreement with the results of numerical simulations.     

A phase aberration correction method for ultrasound imaging

A computationally efficient method for phase aberration correction in ultrasound imaging is presented. The method is based on time delay estimation via minimization of the sum of absolute differences between radio frequency samples of adjacent array elements. Effects of averaging estimated aberration patterns over scan angle and truncation to a single bit wordlength are examined. Phase distortions due to near-field inhomogeneities are simulated using silicone rubber aberrators. Performance of the method is tested using experimental data. Simulation studies addressing different factors affecting efficiency of the method, such as the number of iterations, window length, and the number of scan angles used for averaging, are presented. Images of a standard resolution phantom are reconstructed and used for qualitative testing.     

Parallel integral projection transform for straight electrode localization in 3-D ultrasound images

In surgical practice, small metallic instruments are frequently used to perform various tasks inside the human body. We address the problem of their accurate localization in the tissue. Recent experiments using medical ultrasound have shown that this modality is suitable for real-time visualization of anatomical structures as well as the position of surgical instruments. We propose an image-processing algorithm that permits automatic estimation of the position of a line-segment-shaped object. This method was applied to the localization of a thin metallic electrode in biological tissue. We show that the electrode axis can be found through maximizing the parallel integral projection transform that is a form of the Radon transform. To accelerate this step, hierarchical mesh-grid algorithm is implemented. Once the axis position is known, localization of the electrode tip is performed. The method was tested on simulated images, on ultrasound images of a tissue mimicking phantom containing a metallic electrode, and on real ultrasound images from breast biopsy. The results indicate that the algorithm is robust with respect to variations in electrode position and speckle noise. Localization accuracy is of the order of hundreds of micrometers and is comparable to the ultrasound system axial resolution.     

Shift-invariant,DWT-based "projection" method for estimation of ultrasound pulse power spectrum

An approach to computing estimates of the ultrasound pulse spectrum from echo-ultrasound RF sequences, measured from biological tissues, is proposed. It is computed by a "projection" algorithm based on the Discrete Wavelet Transform (DWT) using averaging over a range of linear shifts. It is shown that the robust, shift invariant estimate of the ultrasound pulse power spectrum can be obtained by the projection of RF line log spectrum on an appropriately chosen subspace of L2(R) (i.e., the space of square-integrable functions) that is spanned by a redundant collection of compactly supported, scaling functions. This redundant set is formed from the traditional (in Wavelet analysis) orthogonal set of scaling functions and also by all its linear (discrete) shifts. A proof is given that the estimate, so obtained, could be viewed as the average of the orthogonal projections of the RF line log spectrum, computed for all significant linear shifts of the RF line log spectrum in frequency domain. It implies that the estimate is shift-invariant. A computationally efficient scheme is presented for calculating the estimate. Proof is given that the averaged, shift-invariant estimate can be obtained simply by a convolution with a kernel, which can be viewed as the discretized auto-correlation function of the scaling function, appropriate to the particular subspace being considered. It implies that the computational burden is at most O(n log2 n), where n is the problem size, making the estimate quite suitable for real-time processing. Because of the property of the wavelet transform to suppress polynomials of orders lower than the number of the vanishing moments of the wavelet used, the presented approach can be considered as a local polynomial fitting. This locality plays a crucial role in the performance of the algorithm, improving the robustness of the estimation. Moreover, it is shown that the "averaging" nature of the proposed estimation allows using (relatively) poorly regular wavelets (i.e., short filters), without affecting the estimation quality. The latter is of importance whenever the number of calculations is crucial.     

Multigate transcranial Doppler ultrasound system with real-time embolic signal identification and archival

An integrated system for acquisition and processing of intracranial and extracranial Doppler signals and automatic embolic signal detection has been developed. The hardware basis of the system is a purpose-built acquisition/processing board that includes a multigate Doppler unit controlled through a computer. The signal-processing engine of the system contains a fast Fourier transform (FFT)-based, spectral-analysis unit and an embolic signal-detection unit using expert system reasoning theory. The system is designed so that up to four receive gates from a single transducer can be used to provide useful reasoning information to the embolic signal-detection unit. Alternatively, two transducers can be used simultaneously, either for bilateral transcranial Doppler (TCD) investigations or for simultaneous intra- and extracranial investigation of different arteries. The structure of the software will allow the future implementation of embolus detection algorithms that use the information from all four channels when a single transducer is used, or of independent embolus detection in two sets of two channels when two transducers are used. The user-friendly system has been tested in-vitro, and it has demonstrated a 93.6% sensitivity for micro-embolic signal (MES) identification. Preliminary in-vivo results also are encouraging.     

Precise and fast phase wraps reduction in fringe projection profilometry

Phase wraps in a 2D wrapped phase map can be completely eliminated or greatly reduced by frequency shifting. But it usually cannot be optimally reduced using conventional fast Fourier transform (FFT) because the spectrum can be shifted only by a integer number in the frequency domain. In order to achieve a significant phase wrap reduction, we propose a fast and precise two-step method for phase wraps reduction in this paper, which is based on the iterative local discrete Fourier transform (DFT). Firstly, initial estimate of the frequency peak is obtained by FFT. Then sub-pixel spectral peak with high resolution is determined by iteratively upsampling the local DFT around the initial peak location. Finally, frequency shifting algorithm that operates in the spatial domain is used to eliminate phase wraps. Simulations and experiments are conducted to demonstrate the superb computing efficiency and overall performance of the proposed method.     

经颅聚焦超声治疗脑血栓的数值仿真研究

脑卒中是致残和致死的首因,经颅脑卒中治疗具有无创和颅内出血风险低等优势,目前经颅聚焦超声治疗血栓性缺血脑卒中时使用参数尚不明确。基于志愿者头颅CT图像和82阵元相控换能器建立三维数值仿真模型,利用时域有限差分法数值解析Westervelt声波非线性传播方程,对0.5～1.0 MHz超声激励频率和输入声功率等参数进行数值仿真筛选。结果表明：频率相同时焦点处形成的负压越大所需输入声功率越大,经颅所需输入声功率约为开颅的1.5倍;频率越高焦域面积越小但焦域处的旁瓣增多;频率相同时经颅和开颅模型的焦域形状和大小相近但经颅时的旁瓣较强;焦点处负压达到具有溶栓效果的-6 MPa和具有显著溶栓效果的-8 MPa时所需声功率随频率的提高先减少后增加且频率为0.8 MHz时最小;辐照时间和占空比对焦点位置和焦域面积没有影响。     

Cascaded free-flow isoelectric focusing for improved focusing speed and resolution

This work presents the first implementation of cascaded stages for a microfabricated free-flow isoelectric focusing (FF-IEF) device. Both analytical and computational models for IEF suggest device performance will be improved by utilizing multiple stages to reduce device residence time. These models are shown to be applicable by using focusing of small IEF markers as a demonstration. We also show focusing of fluorescently tagged proteins under different channel geometries, with the most efficient focusing occurring in the cascaded design, as predicted by theory. An additional aim of this work is to demonstrate the compatibility of cascaded FF-IEF with common bioanalytical tools. As an example, outlet fractions from cascaded FF-IEF were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Processing of whole cell lysate followed by immunoblotting for cell signaling markers demonstrates the reduction of albumin from samples, as well as the enrichment of apoptotic markers.