Strain imaging using conventional and ultrafast ultrasound imaging: numerical analysis

In elasticity imaging, the ultrasound frames acquired during tissue deformation are analyzed to estimate the internal displacements and strains. If the deformation rate is high, high-frame-rate imaging techniques are required to avoid the severe decorrelation between the neighboring ultrasound images. In these high-frame-rate techniques, however, the broader and less focused ultrasound beam is transmitted and, hence, the image quality is degraded. We quantitatively compared strain images obtained using conventional and ultrafast ultrasound imaging methods. The performance of the elasticity imaging was evaluated using custom-designed, numerical simulations. Our results demonstrate that signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolutions in displacement and strain images acquired using conventional and ultrafast ultrasound imaging are comparable. This study suggests that the high-frame-rate ultrasound imaging can be reliably used in elasticity imaging if frame rate is critical     

Signal-to-noise based local decorrelation compensation for speckle interferometry applications

Speckle-based interferometric techniques allow assessing the whole-field deformation induced on a specimen due to the application of load. These high sensitivity optical techniques yield fringe images generated by subtracting speckle patterns captured while the specimen undergoes deformation. The quality of the fringes, and in turn the accuracy of the deformation measurements, strongly depends on the speckle correlation. Specimen rigid body motion leads to speckle decorrelation that, in general, cannot be effectively counteracted by applying a global translation to the involved speckle patterns. In this paper, we propose a recorrelation procedure based on the application of locally evaluated translations. The proposed procedure implies dividing the field into several regions, applying a local translation, and calculating, in every region, the signal-to-noise ratio (SNR). Since the latter is a correlation indicator (the noise increases with the decorrelation) we argue that the proper translation is that which maximizes the locally evaluated SNR. The search of the proper local translations is, of course, an interactive process that can be facilitated by using a SNR optimization algorithm. The performance of the proposed recorrelation procedure was tested on two examples. First, the SNR optimization algorithm was applied to fringe images obtained by subtracting simulated speckle patterns. Next, it was applied to fringe images obtained by using a shearography optical setup from a specimen subjected to mechanical deformation. Our results show that the proposed SNR optimization method can significantly improve the reliability of measurements performed by using speckle-based techniques.     

Internal displacement and strain imaging using ultrasonic speckletracking

Previous ultrasound speckle tracking methods have been extended, permitting measurement of internal displacement and strain fields over a wide dynamic range of tissue motion. The markedly increased dynamic range of this approach should lead to enhanced contrast resolution in strain and elasticity images. Results of experiments on gelatin-based, tissue equivalent phantoms show the capabilities of the method     

Elasticity imaging using conventional and high-frame rate ultrasound imaging: experimental study

High-frame rate ultrasound imaging is necessary to track fast deformation in ultrasound elasticity imaging, but the image quality may be degraded. Previously, we investigated the performance of strain imaging using numerical models of conventional and ultrafast ultrasound imaging techniques. In this paper, we performed experimental studies to quantitatively evaluate the strain images and elasticity maps obtained using conventional and high frame rate ultrasound imaging methods. The experiments were carried out using point target and tissue mimicking phantoms. The experimental results were compared with the results of numerical simulation. Our experimental studies confirm that the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and axial/lateral resolution of the displacement and strain images acquired using high-frame rate ultrasound imaging are slightly lower but comparable with those obtained using conventional imaging. Furthermore, the quality of elasticity images also exhibits similar trends. Thus, high-frame rate ultrasound imaging can be used reliably for static elasticity imaging to capture the internal tissue motion if the frame rate is critical.     

Control of phase of fringes in speckle interferometry for application of fringe scanning method

The speckle interferometry is an effective technique in the displacement measurement of a structure with a rough surface. However, when the fringe scanning technique is introduced to speckle interferometry for improving the measurement resolution, generally two speckle patterns before and after the deformation of the measurement object and another speckle pattern obtained under different conditions from these two speckle patterns are required at least. So, three speckle patterns are generally required for precise fringe analysis as a minimum condition. In this paper, a method for introducing the fringe scanning method is proposed by controlling the phase of the specklegram as a fringe image using filtering techniques. Then, the temporal fringe analysis method that uses only two speckle patterns are proposed for speckle interferometry. As the result of experiments, it is shown that high precise fringe analysis can be realized by the fringe scanning methods using only two speckle patterns for the displacement measurement with a large deformation.     

Strain compounding: a new approach for speckle reduction

A new compounding technique for reducing speckle brightness variations is proposed. This method exploits the decorrelation between signals under different strain states. The different strain states can be created using externally applied forces such as the ones used in sonoelastography. Such forces produce three-dimensional tissue motion. By correcting only the in-plane (i.e., axial and lateral) motion, the images under different strain states have similar characteristics except for speckle appearance caused by the uncorrected out-of-plane (i.e., elevational) motion. Additional speckle decorrelation is also introduced through tissue motion correction caused by the change of effective in-plane sample volume geometry. Therefore, these images can be combined for speckle reduction with less degradation in in-plane spatial resolution than conventional approaches. In this paper, three-dimensional tissue motion under various strain conditions were simulated. It was found that significant speckle decorrelation existed at strains achievable in some clinical situations. Experiments were also conducted to test efficacy of this approach. Pulse-echo data from a gelatin-based phantom were acquired using a 5-MHz, single crystal transducer, and both conventional and compound B-mode images were formed. Results indicated that speckle brightness variations were reduced, and detectability of low contrast objects was enhanced. Performance limitations and fundamental differences between the proposed technique and existing techniques are discussed     

Nonlinear elasticity imaging: theory and phantom study

In tissue the Young's modulus cannot be assumed constant over a wide deformation range. For example, direct mechanical measurements on human prostate show up to a threefold increase in Young's modulus over a 10% deformation. In conventional elasticity imaging, these effects produce strain-dependent elastic contrast. Ignoring these effects generally leads to suboptimal contrast (stiffer tissues at lower strain are contrasted against softer tissues at higher strain), but measuring the nonlinear behavior results in enhanced tissue differentiation. To demonstrate the methods extracting nonlinear elastic properties, both simulations and measurements were performed on an agar-gelatin phantom. Multiple frames of phase-sensitive ultrasound data are acquired as the phantom is deformed by 12%. All interframe displacement data are brought back to the geometry of the first frame to form a three-dimensional (3-D) data set (depth, lateral, and preload dimensions). Data are fit to a 3-D second order polynomial model for each pixel that adjusts for deformation irregularities. For the phantom geometry and elastic properties considered in this paper, reconstructed frame-to-frame strain images using this model result in improved contrast to noise ratios (CNR) at all preload levels, without any sacrifice in spatial resolution. From the same model, strain hardening at all preload levels can be extracted. This is an independent contrast mechanism. Its maximum CNR occurs at 5.13% preload, and it is a 54% improvement over the best case (preload 10.6%) CNR for frame-to-frame strain reconstruction. Actual phantom measurements confirm the essential features of the simulation. Results show that modeling of the nonlinear elastic behavior has the potential to both increase detectability in elasticity imaging and provide a new independent mechanism for tissue differentiation.     

混凝土单轴压缩CT观测及三维变形场计算分析

利用微焦点工业CT对混凝土试件单轴压缩过程分阶段进行原位扫描,获得相应的系列CT图像。由数字体散斑法（Digital Volumetric Speckle Photography,DVSP）获得了试件内部三维位移场与应变场。混凝土内部非均匀细观结构可以作为散斑结构成为变形信息的载体用于试件内部变形的测量;通过位移场和应变场的分析,揭示了试件内部变形局部化区域的演化及破坏过程,局部化区域与试件最终破坏面位置相一致。根据试件的体积应变随轴向应力的变化曲线分别定义了小于CT尺度微裂隙和CT尺度裂隙的发育点,分别为峰值应力的54.7 %和82.0 %,这两点同时对应着试件扩容的起始点及塑性变形的起始点。结果表明,DVSP与CT技术相结合能够直观地揭示混凝土试件内部变形局部化产生及发展过程,为三维变形的精细测量及可视化提供了方法。     

Bayesian 2-D deconvolution: a model for diffuse ultrasound scattering

Observed medical ultrasound images are degraded representations of the true acoustic tissue reflectance. The degradation is due to blur and speckle and significantly reduces the diagnostic value of the images. To remove both blur and speckle, we have developed a new statistical model for diffuse scattering in 2-D ultrasound radio frequency images, incorporating both spatial smoothness constraints and a physical model for diffuse scattering. The modeling approach is Bayesian in nature, and we use Markov chain Monte Carlo methods to obtain the restorations. The results from restorations of some real and simulated radio frequency ultrasound images are presented and compared with results produced by Wiener filtering     

Displacement and strain imaging of coronary arteries withintraluminal ultrasound

Tissue elasticity can be estimated from displacement and strain images acquired under controlled deformation. We extend this approach for coronary arteries, deformed and imaged by an integrated angioplasty balloon and ultrasonic imaging probe. Because the lumen cross section of a severely occluded artery is not circular, we have also developed a technique to perform all motion computations in the reference frame of the lumen's geometric center. This coordinate system is independent of the imaging catheter and consequently referencing to this frame removes artifacts associated with probe motion within the balloon during deformation. Displacements and strains estimated by phase-sensitive correlation-based speckle tracking were used to distinguish arterial plaques in simulated coronary arteries of differing elastic moduli: hard, soft, and homogenous. We have also applied these methods to images of a homogeneous gelatin phantom collected with the integrated probe. The maximum phantom displacement was about 40 pm, and the maximum radial normal strain was about 4% (absolute value). The spatial dependence of these quantities shows good agreement with theoretically predicted values     

Reduced peak-hopping artifacts in ultrasonic strain estimation using the Viterbi algorithm

Internal strain resulting from tissue deformation can be estimated by correlation processing of speckle patterns within complex (i.e., radio frequency) ultrasound images acquired during deformation. At large deformations, the magnitude of the correlation coefficient peak can be significantly lower than unity, so that random speckle correlations will exceed the true peak. This effect is called ?peak hopping? and causes significant errors in displacement and deformation estimates. Here we investigate the Viterbi algorithm, a dynamic programming procedure, to overcome peak-hopping artifacts by finding the most likely sequence of hidden states in a sequence of observed events. It is well suited to motion estimation in elasticity- imaging studies because adjacent tissue elements remain adjacent following deformation. Particularly, tissue elements along an ultrasonic beam in one image lie along a 3-D continuous curve in the next image instant. The observed event in this case is the correlation coefficient of a pixel at a certain displacement. Radio-frequency data were generated before and after deformation with an average strain of 6%. Simulations were performed for a homogenous medium and for a medium with a stiffer inclusion. Results show that Viterbi processing of speckle-tracking outputs can significantly reduce peak-hopping artifacts.     

High-resolution elasticity imaging for tissue engineering

An elasticity microscope provides high resolution images of tissue elasticity. With this instrument, it may be possible to monitor cell growth and tissue development in tissue engineering. To test this hypothesis, elasticity micrographs were obtained in two model systems commonly used for tissue engineering. In the first, strain images of a tissue-engineered smooth muscle sample clearly identified a several hundred micron thick cell layer from its supporting matrix. Because a one-dimensional mechanical model was appropriate for this system, strain images alone were sufficient to image the elastic properties. In contrast, a second system was investigated in which a simple one-dimensional mechanical model was inadequate. Uncultured collagen microspheres embedded in an otherwise homogeneous gel were imaged with the elasticity microscope. Strain images alone did not clearly depict the elastic properties of the hard spherical cell carriers. However, reconstructed elasticity images could differentiate the hard inclusion from the background gel. These results strongly suggest that the elasticity microscope may be a valuable tool for tissue engineering and other applications requiring the elastic properties of soft tissue at high spatial resolution (75 mum or less).     

Correlation between optical coherence tomography images and histology of pigskin

Noninvasive imaging techniques such as optical coherence tomography (OCT) are being widely used for early diagnostics of a variety of pathologies. Traditional tissue preparation for histological evaluation alters the dimensions of histological images such that differences between the dimensions of histologic and OCT images can be as great as 2.5-fold. This discrepancy complicates quantitative interpretation of OCT images. Here we used a steel ring sewn to the pigskin to avoid tissue deformation due to excision and fixation and used a surgical suture as a fixed tissue marker. With these techniques, we achieved good correlation between OCT and histological images with differences of less than 10%. The resulting method significantly enhances the diagnostic capabilities of the OCT technique.     

High resolution speckle interferometry using virtual speckle pattern produced by information of deformation process

A high resolution new fringe analysis method for ESPI with only one camera is proposed by using features of speckle interferometry in a deformation process of a measured object. The profile of intensity of each speckle of the speckle patterns in the deformation process is analyzed by the Hilbert transform. A virtual speckle pattern for creating a carrier fringe image is produced artificially by using the information of profiles of intensities of speckles. The deformation map of the measured object can be detected by the virtual speckle pattern in an operation based on the spatial fringe analysis method. Experimental results show that the difference between the results by the new and the ordinary methods is 0.1 rad as standard deviation. From the results, it is confirmed that the high resolution measurement can be performed by this method the same as compared to the ordinary measurement method which needs to employ three speckle patterns.     

Adaptive imaging and spatial compounding in the presence of aberration

Spatial compounding reduces speckle and increases image contrast by incoherently averaging images acquired at different viewing angles. Adaptive imaging improves contrast and resolution by compensating for tissue-induced phase errors. Aberrator strength and spatial frequency content markedly impact the desirable operating characteristics and performance of these methods for improving image quality. Adaptive imaging, receive-spatial compounding, and a combination of these two methods are presented in contrast and resolution tasks under various aberration characteristics. All three imaging methods yield increases in the contrast-to-noise ratio (CNR) of anechoic cysts; however, the improvements vary depending on the properties of the aberrating layer. Phase correction restores image spatial frequencies, and the addition of compounding opposes the restoration of image spatial frequencies. Lesion signal-to-noise ratio (SNR), an image quality metric for predicting lesion detectability, shows that combining spatial compounding with phase correction yields the maximum detectability when the aberrator strength or spatial frequency content is high. Examples of these modes are presented in thyroid tissue.     

Multiwindowed defocused electronic speckle photographic system for tilt measurement

Defocused speckle photography has long been used to measure rotations of rough surfaces. By addition of a suitably perforated mask, some measurement properties, such as range and lateral resolution, may be changed at will. In particular, the maximum measurable tilt can be significantly increased, although at the expense of poorer lateral resolution. Advantages of this compared with previously described techniques include independent tuning of speckle size and optical system aperture and greater adaptability to various measuring needs. The benefits and disadvantages of the new and old techniques are thoroughly compared.     

Despeckling of medical ultrasound images

Speckle noise is an inherent property of medical ultrasound imaging, and it generally tends to reduce the image resolution and contrast, thereby reducing the diagnostic value of this imaging modality. As a result, speckle noise reduction is an important prerequisite, whenever ultrasound imaging is used for tissue characterization. Among the many methods that have been proposed to perform this task, there exists a class of approaches that use a multiplicative model of speckled image formation and take advantage of the logarithmical transformation in order to convert multiplicative speckle noise into additive noise. The common assumption made in a dominant number of such studies is that the samples of the additive noise are mutually uncorrelated and obey a Gaussian distribution. The present study shows conceptually and experimentally that this assumption is oversimplified and unnatural. Moreover, it may lead to inadequate performance of the speckle reduction methods. The study introduces a simple preprocessing procedure, which modifies the acquired radio-frequency images (without affecting the anatomical information they contain), so that the noise in the log-transformation domain becomes very close in its behavior to a white Gaussian noise. As a result, the preprocessing allows filtering methods based on assuming the noise to be white and Gaussian, to perform in nearly optimal conditions. The study evaluates performances of three different, nonlinear filters--wavelet denoising, total variation filtering, and anisotropic diffusion--and demonstrates that, in all these cases, the proposed preprocessing significantly improves the quality of resultant images. Our numerical tests include a series of computer-simulated and in vivo experiments.     

An elasticity microscope. Part II: Experimental results

For pt.I see ibid., vol.44, no.6, pp.1304-19 (1997). Initial experimental results from a 50 MHz elasticity microscope are shown. Using methods discussed previously, we present measured displacement and normal axial strain fields from a tissue mimicking phantom. Results from this homogenous gel are compared to a finite element simulation of the deformation experiment. The spatial resolution is estimated to be approximately 52 μm for axial displacements, and 71 μm for normal axial strains. These estimates were further tested by imaging a phantom containing a hard cylindrical inclusion with cross-sectional diameter of 265 μm. By examining the strain transition between regions in this image, the spatial resolution of the normal axial strain was verified to be at most 88 μm. A typical experiment produces peak normal axial strain around 3%. These experiments demonstrate the potential of high frequency ultrasound as a means for elasticity microscopy. Preliminary deformation experiments are presented on porcine epidermis     

Fringe analysis method for electronic speckle pattern interferometry using only speckle patterns before and after deformation

Electronic speckle pattern interferometry is employed in many industrial fields as a useful deformation measurement method. However, two speckle patterns obtained before and after the deformation are necessary for measurement. Furthermore, at least three speckle patterns are required for high resolution measurement using ordinary fringe scanning technologies. In this paper, a novel method that can measure high speed deformations using a limited number of speckle patterns without using high speed cameras is proposed. The method enables application to dynamic deformation analysis because the method involves analysis using only two speckle patterns obtained before and after the deformation. A novel optical system that can record some spatial information into each speckle is set up for the method. In experimental results, it is confirmed that the out-of-plane deformation measurement can be precisely performed by the method and that the resolution power is almost equivalent to that of the ordinary method.