Superresolution imaging of scatterers in ultrasound B-scan imaging

A number of imaging systems exhibit speckle, which is caused by the interaction of a coherent pulse reflecting off of random reflectors. The limitations of these systems are quite serious because the speckle phenomenon creates a pattern of nulls and peaks from subresolvable scatterers or targets that are difficult to interpret. Another limitation of these pulse-echo imaging systems is that their resolution is dependent on the full spatial extent of the propagating pulse, usually several wavelengths in the axial or propagating dimension and typically longer in the transverse direction. This limits the spatial resolution to many multiples of the wavelength. This paper focuses on the particular case of ultrasound B-scan imaging and develops an inverse filter solution that eliminates both the speckle phenomenon and the poor resolution dependency on the pulse length and width to produce super-resolution ultrasound (SURUS) images. The key to the inverse filter is the creation of pulse shapes that have stable inverses. This is derived by use of the standard Z-transform and related properties. Although the focus of this paper is on examples from ultrasound imaging systems, the results are applicable to other pulse-echo imaging systems that also can exhibit speckle statistics.     

Speckle in ultrasonic imaging

The resolution of an ultrasonic pulse-echo imaging system is determined by the dimensions of the resolution cell. The image is characterized by a granular pattern, or ‘speckle’, which varies from place to place in the tissue; it can be explained in terms of the coherent formation of the echo from many small scatterers within the resolution cell. These scatterers are randomly distributed, and the reflected amplitude fluctuation becomes larger as the size of the resolution cell is reduced. Speckle can be smoothed either by small movements of the tissue during scanning, or by compound scanning.     

基于超声散射元间距的生物软组织内部应变与弹性估计

超声弹性成像有希望成为生物软组织病变诊断和力学特性评价的有效手段.提出利用小坡变换检测组织背向散射信号的不连续性,估计规则散射元空间分布及其变化,由此得到组织内部应变分布与弹性模量的新方法.结果表明,利用小波变换的散射元定位方法具有较强的抗干扰能力,基于散射元间距的运动估计方法避免了超声散斑的解相关性,计算量较小,无误差积累.组织内部应变和弹性估计能够区分杨氏弹性不同的组织,具有较高的空间分辨力.方法同时提供了组织弹性与微结构的信息.     

Speckle intensity mean value dependence on the number of scatterers

The number of scatterers contributing to the speckle amplitude can be determined by the spot size of the incident light or by the amplitude point spread function of the optical system. The speckle intensity mean value measurement results with two methods of choosing the number of scatterers applied are presented. The values of the rms phase deviation and correlation length obtained from two methods agree well. The properties of both methods are discussed. The possibilities for the roughness measurement are suggested.     

Spatial field distributions in the transition from ballistic to diffusive transport in randomly corrugated waveguides

A detailed analysis of the statistical properties of the transmitted field in corrugated waveguides in the transition from ballistic to diffusion transport is presented. The field distributions are found to be highly dependent on the incident mode excited. When the speckle contrast is one, our results agree with recent experiments where the intensity distribution follows the predictions of the random-phasor-sum model, even though the field phase is not uniformly distributed. Clear deviations from the Gaussian statistics are found which can be interpreted in terms of " K" distributions arising either from a small number of scatterers or from perturbations of the speckle pattern on propagation.     

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.     

Improvement of ultrasound speckle image velocimetry using image enhancement techniques

Ultrasound-based techniques have been developed and widely used in noninvasive measurement of blood velocity. Speckle image velocimetry (SIV), which applies a cross-correlation algorithm to consecutive B-mode images of blood flow has often been employed owing to its better spatial resolution compared with conventional Doppler-based measurement techniques. The SIV technique utilizes speckles backscattered from red blood cell (RBC) aggregates as flow tracers. Hence, the intensity and size of such speckles are highly dependent on hemodynamic conditions. The grayscale intensity of speckle images varies along the radial direction of blood vessels because of the shear rate dependence of RBC aggregation. This inhomogeneous distribution of echo speckles decreases the signal-to-noise ratio (SNR) of a cross-correlation analysis and produces spurious results. In the present study, image-enhancement techniques such as contrast-limited adaptive histogram equalization (CLAHE), min/max technique, and subtraction of background image (SB) method were applied to speckle images to achieve a more accurate SIV measurement. A mechanical sector ultrasound scanner was used to obtain ultrasound speckle images from rat blood under steady and pulsatile flows. The effects of the image-enhancement techniques on SIV analysis were evaluated by comparing image intensities, velocities, and cross-correlation maps. The velocity profiles and wall shear rate (WSR) obtained from RBC suspension images were compared with the analytical solution for validation. In addition, the image-enhancement techniques were applied to in vivo measurement of blood flow in human vein. The experimental results of both in vitro and in vivo SIV measurements show that the intensity gradient in heterogeneous speckles has substantial influence on the cross-correlation analysis. The image-enhancement techniques used in this study can minimize errors encountered in ultrasound SIV measurement in which RBCs are used as flow tracers instead of exogenous contrast agents.     

Sub-nanometric precision displacement sensing using low mass and rough surfaces as movable targets

A novel speckle interferometric method has been developed to measure lengths and displacements over a distance of 100 μm with a resolution less than 1 nm using rough surfaces as movable targets instead of using conventional cube corners or mirrors. The speckle method is able to use Michelson and new configurations to combine the two scattered beams or fields from rough movable and stationary reference targets. The targets can be attached and can be produced directly onto movable actuator type of mechanisms. The phase information is extracted from the randomly modulated signals by employing an apertured disc system or an alternative optical fibre arrangement. This method exploits the concept of using narrow angle scatterers for economical use of laser light, enhanced reliability and modulation criteria. A prototype low-mass interferometric displacement sensor has been constructed to provide a compact and a flexible system. Apart from the measurement of changes in displacements in force and pressure measuring devices, it can also be used for the assessment of small range scanning probes, stylus profilers and in medical field the measurement of elastic properties of ear drums, etc. The sub-nanometre resolution and displacement values are repeatable to within ±1 nm can be demonstrated over distances of up to 2 mm by employing appropriate actuators and translation devices. The performance has been confirmed by comparison work against a conventional interferometric transducer.     

Locally adaptive Nakagami-based ultrasound similarity measures

The derivation of statistically optimal similarity measures for intensity-based registration is possible by modeling the underlying image noise distribution. The parameters of these distributions are, however, commonly set heuristically across all images. In this article, we show that the estimation of the parameters on the present images largely improves the registration, which is a consequence of the more accurate characterization of the image noise. More precisely, instead of having constant parameters over the entire image domain, we estimate them on patches, leading to a local adaptation of the similarity measure. While this basic idea of creating locally adaptive metrics is interesting for various fields of application, we present the derivation for ultrasound imaging. The domain of ultrasound is particularly appealing for this approach, due to the inherent contamination with speckle noise. Furthermore, there exist detailed analyses of suitable noise distributions in the literature. We present experiments for applying a bivariate Nakagami distribution that facilitates modeling of several scattering scenarios prominent in medical ultrasound. Depending on the number of scatterers per resolution cell and the presence of coherent structures, different Nakagami parameters are required to obtain a valid approximation of the intensity statistics and to account for distributional locality. Our registration results on radio-frequency ultrasound data confirm the theoretical necessity for a spatial adaptation of similarity metrics.     

Quantitative characterization of holographic displays based on liquid-crystal-on-silicon spatial light modulator

This report discusses the effect of speckle size on the quality of holographic images based on a liquid-crystal-on-silicon (LCoS) spatial light modulator (SLM). Further, it proposes methods of quantifying the average speckle size and holographic image resolution. These methods enable both characteristics to be compared using the same unit (the number of pixels in the holographic image), providing an intuitive and effective comparative analysis method. In particular, by varying the LCoS resolution ratio, the change in the resolvable minimum pixels of the holographic image is interpreted in conjunction with the average speckle size; moreover, an analysis of the correlation between the latter two is presented. This approach, based on LCoS resolution division, could provide useful insights into single-SLM-based, full-color holographic displays using space division. Furthermore, it could be extended to other components, including more advanced LCoS SLMs, and used to identify the relative effects on image quality with speckles.     

三层膜管壁结构的颈动脉超生仿真

颈动脉超声仿真 有助于血管壁超声检测及信号处理方法的性能评价。提出三层膜管壁结构的颈动脉超声仿真方法。根据组织散射点分布的形状和密度与超声回声斑点分布的变化规律,对临床图像统计分析,确定对应组织散射点分布、密度及强度等特征参数,利用FIELD II产生超声射频回波信号并得到B超图像。30 例仿真试验结果表明,血管壁内、中、外膜及血流回声的统计分布与临床B超图像的一致;与中心频率为8 MHz的结果相比,12 MHz仿真的内、中、外膜厚度及管腔直径测量均值与预设值相等,最大相对误差分别为4.01%, 1.25%, 0.04%及0.15%,仿真效果更为逼真。     

动态白光散斑法的灵敏度及其提高

本文用非相干成象系统频谱分析的概念,分析了动态白光散斑法的灵敏度问题。提出了提高灵敏度的方法,使之不仅能测量大的动态变形,而且也能测量小的动态变形,如果用光弹性材料作试件,能同时得到动态等差线,进行主应力分离。     

Effects of red blood cell aggregates dissociation on the estimation of ultrasound speckle image velocimetry

Ultrasound speckle image of blood is mainly attributed by red blood cells (RBCs) which tend to form RBC aggregates. RBC aggregates are separated into individual cells when the shear force is over a certain value. The dissociation of RBC aggregates has an influence on the performance of ultrasound speckle image velocimetry (SIV) technique in which a cross-correlation algorithm is applied to the speckle images to get the velocity field information. The present study aims to investigate the effect of the dissociation of RBC aggregates on the estimation quality of SIV technique. Ultrasound B-mode images were captured from the porcine blood circulating in a mock-up flow loop with varying flow rate. To verify the measurement performance of SIV technique, the centerline velocity measured by the SIV technique was compared with that measured by Doppler spectrograms. The dissociation of RBC aggregates was estimated by using decorrelation of speckle patterns in which the subsequent window was shifted as much as the speckle displacement to compensate decorrelation caused by in-plane loss of speckle patterns. The decorrelation of speckles is considerably increased according to shear rate. Its variations are different along the radial direction. Because the dissociation of RBC aggregates changes ultrasound speckles, the estimation quality of SIV technique is significantly correlated with the decorrelation of speckles. This degradation of measurement quality may be improved by increasing the data acquisition rate. This study would be useful for simultaneous measurement of hemodynamic and hemorheological information of blood flows using only speckle images.     

An ultrasound simulation method for carotid arteries with a wall structure of three membranes

Ultrasound simulation for carotid arteries is helpful to the performance assessments of vessel wall detection and signal processing methods by using ultrasound techniques.An ultrasound simulation method of carotid artery wall with a three-membrane structure is proposed in present study.According to the ultrasound speckle distributions varying with the shapes and densities of scatterer distributions,as well as the statistic results of the clinical images,the parameters of distributions,densities and intensities of scatterers for different kinds of tissues in the carotid artery phantoms are determined.Each region is acoustically characterized using FIELD II software to produce the radio frequency echo signals,from which ultrasound images are derived.The results based on 30 simulations show that the echo distributions of the intimae,mediae,adventitias and blood are consistent with the clinical ones.Moreover,compared with the results from the central frequency of 8 MHz,the mean measurements for thicknesses of the intima,media and adventitia membranes,as well as the lumen diameter from the simulation images based on 12 MHz are the same as the preset ones,and the maximum relative errors are the 4.01%,1.25%,0.04%and 0.15%,respectively.The simulation under this condition is more realistic.     

A two-dimensional CVIB imaging system with a speckle tracking algorithm

Quantitative ultrasound tissue characterization based on integrated backscatter (IB) has shown great potential in detecting myocardial ischemia. The magnitude of the cyclic variation in IB (CVIB) has been considered one promising parameter in assessing regional myocardial contractile performance. This lab has previously developed a novel ultrasonic fusion imaging method based on CVIB. However, the major problem for clinical applications of this technique is that the myocardial tissue could not be tracked effectively without cardiologist’s intervention. This paper introduced a speckle tracking method into the CVIB-weighted imaging system, called speckle tracking algorithm with adaptive window size (STAWAWS), to track myocardial tissue particle automatically. This method provides a way to obtain the particle’s positions frame by frame in a series of B-mode images. Then using the RF signals according to the particle’s positions the IB curve can be calculated to produce CVIB value. The method was applied on the experimental and clinical data cases’s analysis. The results of dog’s data processing showed that this method could eliminate the misunderstanding of myocardial ischemia especially near the endocardium. The results of clinical data suggested that this method had clinical significance in detecting ischemic myocardium. Though the CVIB-weighted images obtained by the use of this auto-tracking method can improve the accuracy of detecting myocardial ischemia, it is not real-time analysis and the clinical data cases are not sufficient. Further clinical validation is still needed in the future’ work.     

基于Rician分布的散斑噪声的超声图像模拟算法*

为了衡量各种去噪算法的性能,在干净无噪声的图像上添加接近真实且可控的散斑噪声是非常重要的,提出一种基于Rician分布的不完全发育的斑点噪声的超声图像模拟算法。该算法考虑到了声波区域中包含孤立的强散射体的情况,同时结合了超声成像的扫描过程。以合成图像和肾脏图像为体模进行了模拟实验,并对最终生成的伪超声图像进行了噪声分布统计及拟合检验。实验结果表明该算法生成的伪超声图像在视觉上和理论上都接近真实的超声图像。     

FLAKE STRUCTURE IN THE SPECKLE FIELD PRODUCED BY A FEW SCATTERERS

This paper reports a new phenomenon, namely, flake structure in the speckle field produced by a few scatterers. This structure appears in the off-axis region and its size is much larger than that of the normal speckles. The one-dimensional simulations show that as the roughness of the random surface sample increases or its lateral correlation length decreases, the flakes go farther away from the axis, their relative intensities increase, their sizes become bigger and the fluctuations in them decrease. A sub-scatterer model that we call isoclinic element is proposed to explain the formation and properties of flakes. Experimental evidence shows the existence of the flake structure.     

内窥式散斑类共聚焦系统层析能力分析

本文研究了内窥式散斑类共聚焦系统(Endoscope-based speckle quasi-confocal system,EBSQCS)的层析能力。从散斑类共聚焦显微镜(Speckle quasi-confocal microscope,SQCM)成像原理出发,详细分析了内窥镜光学结构对散斑类共聚焦显微镜散斑场波动的影响规律,推导了内窥镜光学结构与内窥式散斑类共聚焦系统轴向分辨力的关系。实验测得了基于光纤束的内窥式散斑类共聚焦系统的轴向分辨力曲线。选用放大倍率4倍,光纤直径5μm的内窥镜系统的内窥式散斑类共聚焦系统轴向分辨力曲线的全峰半高是散斑类共聚焦显微镜的2.3倍,与理论计算值相符。实验结果表明内窥式散斑类共聚焦系统具有很好的轴向层析能力。     

基于多分辨分析的合成孔径声纳相干斑抑制研究

相干斑噪声的存在使合成孔径声纳图像不能正确反映目标的反射特性,严重影响了图像的质量,降低了对图像的分析和理解性能。一个良好的合成孔径声纳图像相干斑抑制算法,必须在有效抑制相干斑噪声的同时,尽量保持图像中的边缘、点目标等细节信息。小波变换具有多分辨特性,可以利用其多分辨特性进行图像噪声消除。由于合成孔径声纳图像相干斑具有乘性噪声的性质,这就需要对其合成孔径声纳图像进行对数变换,把乘性噪声转换成加性噪声。然后对加性噪声进行小波多分辨分析和阈值处理,剔除小于阈值的小波系数后进行逆小波变换和指数变换,从而获得抑制相干斑噪声后的新的合成孔径声纳图像。从相干斑抑制的结果可以看出,基于多分辨分析的相干斑噪声抑制算法能够有效降低合成孔径声纳图像中的相干斑噪声。     

Semi-empirical bone model for determination of trabecular structure properties from backscattered ultrasound

A novel semi-empirical scattering model of trabecular bone facilitating its characterization and allowing optimization of the interrogating pulse-echo transducer performance was developed. The model accounts for spatial density distribution of the trabeculae and includes measurement conditions such as pressure–time waveform of the probing ultrasound wave, the emitted field structure, and the transfer function and limited bandwidth of the acoustic source operating in pulse-echo mode. These measurement conditions are of importance as they modify the scattered echoes, which in turn are linked to the micro-architecture of the bone. The bone was modeled by a random distribution of long and thin cylindrical scatterers having randomly varying diameters and mechanical properties, and oriented perpendicularly to the ultrasound beam axis. To mimic clinically encountered conditions the relevant empirical data obtained at 1 MHz were input to the model. The data included pulse-echo source pressure field distribution in the focal zone and the above mentioned transfer function. With these data the model allowed frequency dependent backscattering coefficient of the simulated bone structure and its statistical properties to be determined. The results obtained indicated that the computer simulation is of particular relevance in studying scattering properties of the cancellous bone and holds promise as a tool to determine the relationship between the physical dimensions and shape of the scatterers and for monitoring of osteoporosis. The results of simulations also indicated that the new bone model proposed is well suited to mimic clinically relevant conditions. In contrast to the existing bone models, which usually assume scatterers to be randomly distributed as infinitely long identical cylinders with a cross-section much smaller than the probing ultrasound wave, the new model includes two populations of scatterers having different physical dimensions and also allows the mechanical properties of the scatterers to be varied.