特征空间和符号相干系数融合的最小方差超声波束形成

为了提高医学超声成像的空间分辨率,提出一种融合了特征空间最小方差与符号相干系数的波束形成方法。首先利用最小方差法计算回波数据的协方差矩阵和加权向量;然后对协方差矩阵进行特征分解得到信号子空间,并将加权向量投影到该空间上;最后计算符号相干系数,用于优化特征空间法得到的回波信号,最终获得超声成像数据。为验证算法的有效性,对医学超声成像中常用的点目标、斑目标进行仿真,对点目标仿体和人体颈动脉组织进行超声成像实验。结果表明:所提出的方法在分辨率、对比度以及稳健性等方面都优于传统的延时叠加算法、最小方差算法、特征空间最小方差法以及特征空间与相干系数融合的方法。      

改进的最小方差算法在超声成像中的应用

为了提高最小方差超声成像算法的分辨率、对比度以及对噪声的鲁棒性,提出一种改进的最小方差成像算法。该方法首先基于回波信号中期望信号与噪声信号的可分离性将信号划分为期望信号和噪声信号,然后根据最小方差原理,求出加权向量使期望信号功率最小,同时,为了增加算法对噪声的鲁棒性,对信号方向向量增加一对约束条件,进一步提高图像质量。在全发全收和合成孔径模式下对点目标和吸声斑进行仿真,结果表明所提算法在全发全收模式下,-6 dB处分辨率在最小方差基础上提高了1倍左右,在合成孔径模式下,对比度在特征空间最小方差算法基础上提高了8 dB,且远优于传统延时叠加算法。最后通过实验进一步表明改进的最小方差算法图像在分辨率、对比度及对噪声的鲁棒性等方面表现更优,可以有效的改善超声图像的质量。      

利用回波相干性的医学超声成像相位畸变校正方法

为了提高相位畸变条件下的医学超声成像的横向分辨率和对比度,提出了一种利用回波信号相干性的校正方法。首先给出平均相干系数的概念,并将其作为相位误差校正的度量,然后通过最大化平均相干系数逐步校正各个通道的延时误差,最后再利用经过校正的数据计算出一组新的相干系数并对回波信号进行加权优化,从而得到最终用于成像的数据。对点散射目标及斑散射目标的仿真结果分别显示,利用所提出的方法横向分辨率提高了约0.24 mm,对比度提高了约18 dB,且要优于邻近阵元互相关方法和相干系数直接加权的处理方法。利用回波相干性的相位畸变校正方法结合了相位误差校正和加权处理的优点,可以有效地改善医学超声成像的质量。      

A coherence-based phase aberration correction method in medical ultrasound

A coherence-based correction method was proposed in order to improve the lateral resolution and enhance the contrast of medical ultrasound imaging in the presence of phase aberration.The averaged coherence factor was proposed at first and used as a metric to evaluate phase aberration correction.By maximizing the averaged coherence factor,the time delay parameter of each channel was adjusted.A new set of coherence factors was calculated and the corrected data was optimized to form the final B-mode image.The simulations on point targets and a cyst phantom showed that the proposed method outperformed the nearest neighboring cross correlation method and conventional coherence-weighting method,and the lateral resolution and contrast ratio was improved by approximately 0.24mm and 18dB respectively.The proposed method combined the advantages of phase error correction and coherence-weighting,which could improve imaging qualities effectively in medical ultrasound.     

一种广义旁瓣相消的超声成像算法

针对最小方差和相干系数结合算法在超声成像对比度、分辨率和对噪声的鲁棒性方面存在的不足, 提出一种广义旁瓣相消的成像算法. 首先基于最小方差准则, 构造广义旁瓣相消器, 获得自适应与非自适应两部分加权向量, 然后根据接收信号的协方差矩阵构建特征阈值信号子空间, 并将获得的加权向量投影到此特征信号子空间的左奇异矢量空间中, 获得新的加权矢量. 通过Field II点目标和吸声斑仿真结果表明: 该方法获得的超声图像在对比度、分辨率和对噪声的鲁棒性上均优于传统延时叠加算法及最小方差和相干系数结合算法, 同时将广义旁瓣相消算法获得的加权向量与符号相干系数结合, 还能进一步提高超声图像质量. 最后通过geabr_0实验数据进行测试, 结果表明: 提出算法的分辨率和对比度及对噪声的鲁棒性上均优于传统延时叠加算法及最小方差和相干系数结合算法.     

特征空间最小方差波束形成与相关系数特征值加权相融合的超声成像算法

为了进一步提高超声成像的质量,提出一种信号特征空间的最小方差波束形成与相关系数特征值加权相融合的超声成像算法。利用超声回波信号具有一定的相关性,而相关系数空间最大特征值可以反映回波信号相关性较强的性质,将该特征值作为自适应加权系数对信号特征空间最小方差波束形成(EIBMV)的结果进行加权成像,得到高质量的成像结果。通过对散射点目标和斑目标的Field II仿真,结果表明该算法相对于EIBMV算法,亮斑对比度提高了4.22 dB,暗斑对比度提高了1.88 dB,并且进一步提高了横向分辨率。      

信噪比后滤波与特征空间融合的最小方差超声成像算法

为了提高超声成像空间分辨率和对比度,提出了一种信噪比后滤波与特征空间融合的最小方差波束形成算法。首先,利用信号子空间划分将最小方差算法得到的权矢量投影到信号子空间中提高成像对比度,然后基于信号相干性设计滤波系数,并引入基于信噪比的噪声加权系数,最终得到融合信噪比后滤波与特征空间的最小方差算法。为验证本算法的有效性,使用FieldⅡ对点目标和吸声斑目标进行了仿真实验验证,并采用密歇根大学geabr_0实验数据进行成像。实验结果表明:所提算法在对比度和分辨率上均有所提高,明显优于传统延时叠加算法,最小方差算法和ESBMV_wiener算法,且对噪声具有较强鲁棒性。      

A method of improving overall resolution in ultrasonic array imaging using spatio-temporal deconvolution

In this paper a beamforming method for ultrasonic array imaging is presented that performs both spatial and temporal deconvolution based on a minimum mean square error (MMSE) criteria. The presented MMSE receive mode beamformer performs a regularized inversion of the propagation operator for the ultrasonic array system at hand. The MMSE beamformer accounts for the transmit and receive processes, defined in terms of finite array element sizes, transmit focusing laws and electrical transducer characteristics. The MMSE beamformer is compared to the traditional delay-and-sum (DAS) beamformer with respect to both resolution and signal-to-noise ratio. The two algorithms are compared using both simulated and measured data. The simulated data was obtained using ultrasonic field simulations and the measured data was acquired using a linear phased array imaging wire targets in water. The results show that the MMSE beamformer has superior temporal and lateral resolution compared to DAS. It is also shown that the MMSE beamformer can be expressed as a filter bank, which enables parallel processing at high frame rates.     

Eigenspace-based minimum variance beamformer combined with Wiener postfilter for medical ultrasound imaging

In this paper, a new beamformer which combines the eigenspace-based minimum variance (ESBMV) beamformer with the Wiener postfilter is proposed for medical ultrasound imaging. The primary goal of this work is to further improve the medical ultrasound imaging quality on the basis of the ESBMV beamformer. In this method, we optimize the ESBMV weights with a Wiener postfilter. With the optimization of the Wiener postfilter, the output power of the new beamformer becomes closer to the actual signal power at the imaging point than the ESBMV beamformer. Different from the ordinary Wiener postfilter, the output signal and noise power needed in calculating the Wiener postfilter are estimated respectively by the orthogonal signal subspace and noise subspace constructed from the eigenstructure of the sample covariance matrix.We demonstrate the performance of the new beamformer when resolving point scatterers and cyst phantom using both simulated data and experimental data and compare it with the delay-and-sum (DAS), the minimum variance (MV) and the ESBMV beamformer. We use the full width at half maximum (FWHM) and the peak-side-lobe level (PSL) to quantify the performance of imaging resolution and the contrast ratio (CR) to quantify the performance of imaging contrast. The FWHM of the new beamformer is only 15%, 50% and 50% of those of the DAS, MV and ESBMV beamformer, while the PSL is 127.2 dB, 115 dB and 60 dB lower. What is more, an improvement of 239.8%, 232.5% and 32.9% in CR using simulated data and an improvement of 814%, 1410.7% and 86.7% in CR using experimental data are achieved compared to the DAS, MV and ESBMV beamformer respectively. In addition, the effect of the sound speed error is investigated by artificially overestimating the speed used in calculating the propagation delay and the results show that the new beamformer provides better robustness against the sound speed errors. Therefore, the proposed beamformer offers a better performance than the DAS, MV and ESBMV beamformer, showing its potential in medical ultrasound imaging.     

超声平面波同相正交信号频域波束形成算法

针对频域平面波波束形成算法尚不能直接应用于采用正交解调的超声成像系统中的实际问题,提出一种平面波同相正交信号频域波束形成方法.以平面波射频信号频域波束形成算法为基础,根据射频信号与同相正交信号波数空间变量之间的关系,推导了平面波同相正交信号与对应图像的频谱映射表达式,从而实现对正交解调后信号的频域超声图像重建.采用20...     

Watching ripples on crystals

We present a new method for imaging surface phonon focusing and dispersion at frequencies up to 1 GHz that makes use of ultrafast optical excitation and detection. Animations of coherent surface phonon wave packets emanating from a point source on isotropic and anisotropic solids are obtained with micron lateral resolution. We resolve rounded-square shaped wave fronts on the (100) plane of LiF and discover isolated pockets of pseudosurface wave propagation with exceptionally high group velocity in the (001) plane of TeO(2). Surface phonon refraction and concentration in a minute gold pyramid is also revealed.     

Broadband optimal contrast resolution beamforming

This paper proposes a novel receive beamformer architecture for broadband imaging systems that uses unique finite impulse response (FIR) filters on each channel. The conventional delay-and-sum (DAS) beamformer applies receive apodization by weighting the signal on each receive channel prior to beam summation. Our proposed FIR beamformer passes the focused receive radio frequency (RF) signals through multi-tap FIR filters on each receive channel prior to summation. The receive FIR filters are constructed to maximize the contrast resolution of the system’s spatial response. The broadband FIR beamformer produces spatial point spread functions (PSFs) with narrower mainlobe widths and lower sidelobe levels than spatial PSFs produced by the conventional DAS beamformer.We present simulation results showing that FIR filters of modest tap lengths (3-7) can yield marked improvement in image contrast and point resolution. Specifically we show that 7-tap FIR filters can reduce sidelobe and grating lobe energy by 30 dB and improve contrast resolution by as much as 20 dB compared to conventional apodization profiles. This improvement in contrast resolution comes at the expense of a decrease in beamformer sensitivity. We investigate the effects of phase aberration and show in simulation results that the multi-tap FIR beamformer outperforms the unaberrated DAS beamformer by 8-12 dB even in the presence of moderate aberration characterized by a root-mean-square strength of 28 ns and a full-width at half-maximum correlation length of 3.6 mm. We show experimental results wherein multi-tap FIR filters decrease sidelobe energy in the resulting 2D spatial response while achieving a narrow mainlobe. We also show results where the FIR beamformer improves the contrast to noise ratio (CNR) in simulated B-mode cyst images by more than 4 dB. Our algorithm has the potential to significantly improve ultrasound beamforming in any application where the system response is reasonably well characterized. Furthermore, this algorithm can be used to increase contrast and resolution in one-way beamforming systems such as acousto-optic and opto-acoustic imaging.     

Beamforming using spatial matched filtering with annular arrays (L)

A linear array beamforming method for ultrasonic B-mode imaging using spatial matched filtering (SMF) and a rectangular aperture geometry was recently proposed Kim et al., [J. Acoust. Soc. Am. 120, 852-861 (2006)]. This letter extends those results to include circularly symmetric apertures. SMF applied to annular arrays can improve the lateral resolution and echo signal-to-noise ratio as compared with conventional dynamic-receive delay-sum beamforming. At high frequencies, where delay and sum beamforming is problematic, SMF showed greatly improved target contrast over an extended field of view.     

基于滤波成像的大视角数字全息技术

为提高数字全息再现像视角,提出一种基于滤波成像的数字全息技术来实现大视角的三维物体面型测量.利用离轴像面数字全息技术,通过在4F相干图像处理系统的空间频谱面处放置可移动的低通滤波器,使满足CCD分辨率的物光波与参考光波干涉形成全息图,并控制低通滤波及成像区域分别记录不同谱段的子全息图.再现时,首先对子全息图进行数字傅里叶变换,重构对应频谱段,并对频谱段进行拼接形成完整的物光频谱|而后通过数字再现获得大视角的数字全息再现像.利用该方法测量了圆柱形表面（光滑的缝纫针）的三维形貌,并取得了较好的实验结果.     

四光束剪切相干成像目标重构算法研究

针对传统剪切光束成像技术的准实时性问题,提出用口字形排布的四束光代替传统L形三束剪切光照射目标,研究了四光束剪切相干成像目标重构算法.只需单次测量就能同时重构出四幅目标图像,减少了用于降低散斑噪声、获取高质量图像所需的测量次数,同时大大减少了多组发射时的光束切换次数,提高了成像效率.在算法实现中,通过最小二乘法恢复出四组波前相位,利用散斑幅值的简单代数运算恢复波前幅值,从而重构出目标图像.仿真结果表明,与传统方法相比,在图像质量相同的前提下,本文方法所需的数据采集时间减少了至少1/2,不但提高了目标重构效率,还可为远程运动目标的成像识别提供更好的手段.     

预放大数字全息显微系统的特点

为了提高预放大数字全息显微系统的成像质量,采用理论分析与实验验证相结合的方法,分别对采用平面及球面参考光记录的预放大数字全息显微系统进行了研究和比较。结果表明:在通常的实验条件下,系统的横向分辨力主要取决于显微物镜的成像分辨力;记录距离较小时,两种系统的横向分辨力均随着记录距离的增大略有降低;但当记录距离较大时,球面参考光预放大数字全息系统的横向分辨力降低得更为明显,即平面参考光预放大数字全息显微系统较为优越;在记录距离为0的情况下,即像面数字全息成像情况下,两种系统的再现像均具有最高的分辨力,在利用普通工业用传感器条件下,横向分辨力远超过了2.19 m,且像质较好。因此,尽可能减小全息图的记录距离,或者采用像面数字全息系统,可以有效提高数字全息系统的成像分辨力。     

利用小尺寸电荷耦合器件实现数字全息高分辨成像

为了在降低数字全息显微成像系统成本的同时实现高分辨成像,对像面数字全息显微术的记录与再现过程进行了理论分析,结合系统的点扩散函数,对该系统的横向分辨率进行了分析. 得到了如下结论：像面数字全息显微系统的横向分辨率对电荷耦合器件（CCD）光敏面尺寸变化不敏感;对于常见的CCD 器件,其像元尺寸的变化对该系统的横向分辨率影响甚微. 此外,对像面数字全息显微系统的成像特点进行了分析,结果表明：利用像面数字全息系统可以实现物体信息的完整记录与再现,其成像分辨率及像质优于预放大数字全息系统. 利用搭建的数字全息实验记录系统,从强度及位相两方面对理论分析结果进行了验证,实验结果表明了理论分析的正确性. 关键词： 像面数字全息术 横向分辨率 点扩散函数 小尺寸电荷耦合器件     

Transillumination Laser Computed Tomography System with Fiber-Optic-Based Coherent Detection Imaging Method - High Spatial-Resolution and Quantitative Tomographic Imaging of Highly Scattering Objects -

We recently proposed and developed a novel transillumination laser computed tomography (CT) imaging system using a fiber-optic method based on coherent detection imaging (CDI) for biomedical use. Use of optical fibers enables portability and robustness against environmental changes in a room, such as variable temperature, air-flow shifts, and unexpected vibrations. In addition, motion-artifact-free images can be obtained because measurements can be performed with the object fixed. In the present paper, we experimentally investigate in detail the fundamental imaging properties of the system, which has a spatial resolution of 500 μm, a dynamic range of approximately 120 dB, and a minimum-detectable-optical power of 10⁻¹⁴W as a result of the excellent properties of the heterodyne detection. Based on experimental observations, the proposed system can reconstruct tomographic images of highly scattering objects in the transillumination mode, similar to X-ray CT, at sub-millimeter spatial resolution and with quantitativeness. Finally, we demonstrate with experiments using a physical phantom that the imaging system possesses high resolution and quantitative imaging abilities for highly scattering objects.     

Development of a 64 channel ultrasonic high frequency linear array imaging system

In order to improve the lateral resolution and extend the field of view of a previously reported 48 element 30 MHz ultrasound linear array and 16-channel digital imaging system, the development of a 256 element 30 MHz linear array and an ultrasound imaging system with increased channel count has been undertaken. This paper reports the design and testing of a 64 channel digital imaging system which consists of an analog front-end pulser/receiver, 64 channels of Time-Gain Compensation (TGC), 64 channels of high-speed digitizer as well as a beamformer. A Personal Computer (PC) is used as the user interface to display real-time images. This system is designed as a platform for the purpose of testing the performance of high frequency linear arrays that have been developed in house. Therefore conventional approaches were taken it its implementation. Flexibility and ease of use are of primary concern whereas consideration of cost-effectiveness and novelty in design are only secondary. Even so, there are many issues at higher frequencies but do not exist at lower frequencies need to be solved. The system provides 64 channels of excitation pulsers while receiving simultaneously at a 20–120 MHz sampling rate to 12-bits. The digitized data from all channels are first fed through Field Programmable Gate Arrays (FPGAs), and then stored in memories. These raw data are accessed by the beamforming processor to re-build the image or to be downloaded to the PC for further processing. The beamformer that applies delays to the echoes of each channel is implemented with the strategy that combines coarse (8.3 ns) and fine delays (2 ns). The coarse delays are integer multiples of the sampling clock rate and are achieved by controlling the write enable pin of the First-In-First-Out (FIFO) memory to obtain valid beamforming data. The fine delays are accomplished with interpolation filters. This system is capable of achieving a maximum frame rate of 50 frames per second. Wire phantom images acquired with this system show a spatial resolution of 146 μm (lateral) and 54 μm (axial). Images with excised rabbit and pig eyeball as well as mouse embryo were also acquired to demonstrate its imaging capability.     

子阵列差分系数加权的超声成像算法

传统的自适应加权系数可以有效改善超声图像的分辨率和对比度,但会影响背景图像的散斑特性,因此提出子阵列差分加权系数(SDF)以提升图像质量.该系数适用于传统线扫描超声成像,首先对阵列接收到的回波信号采用子阵列平滑的方式计算均值,得到信号中高相干性成份的强度,再用差分的方式反映相邻阵元回波信号之间的差异性,最后对延时叠加(...