Spatial Compounding Research Articles

Artifact reduction in medical ultrasound

Interpretation of medical ultrasound images is confounded by the presence of significant acoustic imaging artifacts. These artifacts result from diffraction, propagation, and scattering effects and regional variations in these effects. Commercial diagnostic imaging systems have taken various design approaches to mitigate the artifacts. Anatomic features widely vary in size relative to the imaging wavelengths, resulting in angular scattering variations and speckle-limited resolution. Tissue attenuation varies greatly resulting in shadowing and enhancement. This review will look at the origins of certain acoustic imaging artifacts and discuss methods that have been applied to address their contribution to image quality. Included in the discussion will be acoustic methods (such as spatial compounding), and signal processing methods (such as attenuation compensation and frequency compounding). Design approaches and clinical image examples will be presented.

A finite element approach to dynamic modeling of flexible spatial compound bar–gear systems

A new approach is developed to model dynamic behaviors of spatial bar–gear systems with consideration of the stiffness of link, gear, and shaft. This approach is based on a specific finite element theory that has been very successful in dynamic modeling of flexible spatial link mechanisms over the last two decades. In particular, a new type of element in consistence with this specific finite element theory is developed in this paper, which completely describes the deformations of the spatial motions of a pair of spatial gears and their various types of gear errors induced by manufacturing and assembly processes. The approach is validated by comparison of the simulated results with the experimental results reported in the literature. Some new findings useful for designing bevel gear systems are also reported based on the simulation studies.

Real-time spatial compound imaging: Application to breast, vascular, and musculoskeletal ultrasound

Real-time spatial compound imaging (SonoCT) is an ultrasound technique that uses electronic beam steering of a transducer array to rapidly acquire several (three to nine) overlapping scans of an object from different view angles. These single-angle scans are averaged to form a multiangle compound image that is updated in real time with each subsequent scan. Compound imaging shows improved image quality compared with conventional ultrasound, primarily because of reduction of speckle, clutter, and other acoustic artifacts. Early clinical experience suggests that real-time spatial compound imaging can provide improved contrast resolution and tissue differentiation that is beneficial for imaging the breast, peripheral blood vessels, and musculoskeletal injuries. Future development of real-time spatial compound imaging will help address the bulk of general imaging applications by extending this technology to curved array transducers, tissue harmonics, panoramic imaging, and three-dimensional sonography.

3D spatial compounding of ultrasound images using image-based nonrigid registration

Medical ultrasound images are often distorted enough to significantly limit resolution during compounding (i.e., summation of images from multiple views). A new, volumetric image registration technique has been used successfully to enable high spatial resolution in three-dimensional (3D) spatial compounding of ultrasound images. Volumetric ultrasound data were acquired by scanning a linear matrix array probe in the elevational direction in a focal lesion phantom and in a breast in vivo. To obtain partly uncorrelated views, the volume of interest was scanned at five different transducer tilt angles separated by 4° to 6°. Pairs of separate views were registered by an automatic procedure based on a mutual information metric, using global full affine and thin-plate spline warping transformations. Registration accuracy was analyzed automatically in the phantom data, and manually in vivo, yielding average registration errors of 0.31 mm and 0.65 mm, respectively. In the vicinity of the warping control points, registrations obtained with warping transformations were significantly more accurate than full affine registrations. Compounded images displayed the expected reduction in speckle noise and increase in contrast-to-noise ratio (CNR), as well as better delineation of connective tissues and reduced shadowing. Compounding also revealed some apparent low contrast lobulations that were not visible in the single-sweep images. Given expected algorithmic and hardware enhancements, nonrigid, image-based registration shows great promise for reducing tissue motion and refraction artifacts in 3D spatial compounding.

In vitro spatial compound scanning for improved visualization of atherosclerosis

A new off-line multiangle ultrasound (US) compound scanner has been built with the purpose of investigating possible improvements in visualization of vascular structure. Images of two formalin-fixed human atherosclerotic plaques removed by carotid endarterectomy were recorded from seven insonification angles over a range of 42° and the individual images were combined (averaged) into a single image (spatial compounding). Compared to conventional B-mode imaging, this multiangle compound imaging (MACI) method features images with reduced angle-dependence, reduced random variation (speckle) and improved delineation of the plaque outline. With the MACI approach, it is, thus, easier to assess e.g., a possible residual lumen of an atherosclerotic artery as well as the level of echogenicity for the different plaque constituents.

Three-dimensional ultrasound imaging of the rotator cuff: spatial compounding and tendon thickness measurement

Three-dimensional (3-D) volume reconstructions of the shoulder rotator cuff were generated from freehand ultrasound (US) scans acquired with a magnetic tracking system. Image stacks acquired with lateral overlap from multiple acoustic windows were spatially compounded to provide an extended representation of the rotator cuff tendons. A semiautomated technique was developed for measuring rotator cuff thickness from the 3-D compound volumes. Scans of phantoms and volunteer subjects were used to evaluate the accuracy and repeatability of the thickness measurements. For an in vitro phantom with known thickness, the mean difference between the true value and the automatic measurements was 0.05 ± 0.28 mm. Thickness measurements made manually from 2-D images and automatically from 3-D volumes were different by 0.03 ± 0.44 mm in vitro and −0.06 ± 0.36 in vivo. Repeated thickness measurements in vivo differed by 0.06 ± 0.36 mm. The 3-D measurement technique offers a promising method for evaluating rotator cuff tendons.

Real Time Spatial Compound Imaging in breast ultrasound: Technology and early clinical experience

No abstract available.

Three-dimensional spatial compounding of ultrasound scans with weighting by incidence angle.

A three-dimensional (3D) ultrasound imaging system has been used to study spatial compounding of images acquired with different scanhead positions and orientations. A compounding algorithm has been developed that assigns regional weights depending on the local incidence angle of the ultrasound beam. Compound scans were performed of bones in vitro and the shoulder rotator cuff in volunteer subjects. Border measurements (peak value and width) were compiled as a function of ultrasound beam incidence angle and compared for single views and for maximum, mean and weighted mean compounding techniques. The weighted mean produces less variability than that of the maximum and mean for both intensity and border width. The weighted method also demonstrates less blurring of borders than the maximum and mean methods. Surfaces derived from the weighted reconstructions exhibited fewer gaps and fewer spurious connections between surfaces, which could be of particular importance for automated image analysis.

Speckle in Optical Coherence Tomography

Speckle arises as a natural consequence of the limited spatial-frequency bandwidth of the interference signals measured in optical coherence tomography (OCT). In images of highly scattering biological tissues, speckle has a dual role as a source of noise and as a carrier of information about tissue microstructure. The first half of this paper provides an overview of the origin, statistical properties, and classification of speckle in OCT. The concepts of signal-carrying and signal-degrading speckle are defined in terms of the phase and amplitude disturbances of the sample beam. In the remaining half of the paper, four speckle-reduction methods-polarization diversity, spatial compounding, frequency compounding, and digital signal processing-are discussed and the potential effectiveness of each method is analyzed briefly with the aid of examples. Finally, remaining problems that merit further research are suggested. © 1999 Society of Photo-Optical Instrumentation Engineers.

The application of k-space in pulse echo ultrasound

K-space is a frequency domain description of imaging systems and targets that can be used to gain insight into image formation. Although originally proposed in ultrasound for the analysis of experiments involving anisotropic scattering and for the design of acoustic tomography systems, it is particularly useful for the analysis of pulse echo ultrasonic imaging systems. This paper presents analytical and conceptual techniques for estimating the k-space representation of pulse echo imaging systems with arbitrary transmit and receive array geometries and apodizations. Simple graphical methods of estimating the first and second order statistics of speckle are presented. Examples are presented that utilize k-space to gain insight regarding the performance of spatial and frequency compounding and to describe the impact of synthetic receive aperture geometry on beamforming and speckle statistics. The Van Cittert-Zernike theorem is presented in k-space, and techniques for improving echo coherence are discussed.

3-D Kalman filter for image motion estimation

This paper presents a new three-dimensional (3-D) Markov model for motion vector fields. The three dimensions consist of the two space dimensions plus a scale dimension. We use a compound signal model to handle motion discontinuity in this 3-D Markov random field (MRF). For motion estimation, we use an extended Kalman filter as a pel-recursive estimator. Since a single observation can be sensitive to local image characteristics, especially when the model is not accurate, we employ windowed multiple observations at each pixel to increase accuracy. These multiple observations employ different weighting values for each observation, since the uncertainty in each observation is different. Finally, we compare this 3-D model with earlier proposed one-dimensional (1-D) (coarse-to-fine scale) and two-dimensional (2D) spatial compound models, in terms of motion estimation performance on a synthetic and a real image sequence.

Spatial compounding in 3D imaging of limbs.

Effects of spatial compounding on image resolution and speckle noise are studied. Using computer simulation, it is shown that spatial compounding using averaged reconstruction can significantly improve lateral resolution while slightly deteriorate axial resolution. The amount of net resolution improvement depends mainly on the compound angle, but is insensitive to the number of component images used in compounding. While the fact that spatial compounding can effectively reduce speckle noise is well known, the analysis in this paper indicates that to maximize speckle reduction, the component echo amplitudes must meet two conditions: to be mutually independent and to have the same mean power. These findings provide useful guidelines for the analysis and optimization of the performance of an ultrasound scanning system that has been specially developed for imaging residual limbs.

Three-dimensional spatial compounding of ultrasound images

One of the most promising applications of 3-D ultrasound lies in the visualization and volume estimation of internal 3-D structures. Unfortunately, the quality of the ultrasound data can be severely degraded by artefacts and speckle, making automatic analysis of the 3-D data sets very difficult. In this paper we investigate the use of 3-D spatial compounding to reduce speckle. We develop a new statistical theory to predict the improvement in signal-to-noise ratio with increased levels of compounding, and verify the predictions empirically. We also investigate how registration errors can affect automatic volume estimation of structures within the compounded 3-D data set. Having established the need to correct these errors, we present a novel reconstruction algorithm which uses landmarks to register each B-scan accurately as it is inserted into the voxel array. In a series of in vitro and in vivo trials, we demonstrate that 3-D spatial compounding is very effective for improving the signal-to-noise ratio, but correction of registration errors is essential.

Ultrasonic diagnostic transducer array with elevation focus

An ultrasonic diagnostic transducer array is provided for providing electronic focusing in the longitudinal plane and elevational focusing. Elements of the array are subdiced in the elevational direction to provide subelements with aspect ratios varying in proportion to their distance to the central longitudinal axis of the array. Such variation affords varying electro-mechanical coupling coefficients to the subelements such that the intensity of the transmitted energy is centered about the central longitudinal axis. In a second embodiment elements exhibit extensions in the elevational direction which vary in proportion to their displacement from the longitudinal center of the array. The extended elements are acoustically separated into subelements in the elevational direction to provide elevational focusing or spatial compounding of the transmitted acoustic energy.

The detection of breast microcalcifications with medical ultrasound.

Microcalcifications are small crystals of calcium apatites which form in human tissue through a number of mechanisms. The size, morphology, and distribution of microcalcifications are important indicators in the mammographic screening for and diagnosis of various carcinomas in the breast. Although x-ray mammography is currently the only accepted method for detecting microcalcifications, its efficacy in this regard can be reduced in the presence of dense parenchyma. Current ultrasound scanners do not reliably detect microcalcifications in the size range of clinical interest. The results of theoretical, simulation, and experimental studies focused on the improvement of the ultrasonic visualization of microcalcifications are presented. Methods for estimating the changes in microcalcification detection performance which result from changes in aperture geometry or the presence of an aberrator are presented. An analysis of the relative efficacy of spatial compounding and synthetic receive aperture geometries in the detection of microcalcifications is described. The impact of log compression of the detected image on visualization is discussed. Registered high resolution ultrasound and digital spot mammography images of microcalcifications in excised breast carcinoma tissue and results from the imaging of suspected microcalcifications in vivo are presented.

Spatial compounding in ultrasonic imaging using an articulated scan arm

A spatial compounding system has been designed to improve the quality of B-mode echographic images. It consists of constructing an improved image from the combination of several different images of the same cross-sectional plane. The “final” image is constructed by the registration and the superposition of the “original” images. For this, the relative position in the space of the original images has to be known. The use of a localization articulated arm, on which the ultrasonic probe is fixed, makes this possible. The main advantages of the technique are, on one hand, the elimination of the acoustic shadows following a strong reflector structure and, on the other hand, the reduction of the speckle generated in echographic images. The method of reconstruction has been validated on agar gel phantoms and provides good accuracy. In vivo experiments on human beings have also been performed. Acoustic shadows caused by bones in cross-sectional images of the thigh and the arm are eliminated. All the contours of the femur and humerus can be observed in the final images. The reduction of speckle is shown in kidney images and the signal-to-noise ratio improvement is quantified as a function of the number of images involved in the reconstruction.

Evaluational Spatial Compounding

Spatial compounding has long been explored to reduce coherent speckle noise in medical ultrasound. By laterally translating a one-dimensional array, partially correlated measurements made at different look directions can be obtained and incoherently averaged. The lateral resolution, however, is limited by the sub-array length used for each independent measurement. To reduce speckle contrast without compromising lateral resolution, a new spatial compounding technique using two-dimensional, anisotropic arrays is proposed. This technique obtains partially correlated measurements by steering the image plane elevationally with small inclinations. Incoherent averaging is then performed by adding image magnitudes. Therefore, contrast resolution is improved only at the price of a slightly wider elevational beam. Note that although anisotropic arrays have limited steering capability in elevation, grating lobes are not considered influential since only small inclinations are needed between measurements. Simulations have been performed to show both the change in spatial resolution and the improvement in contrast resolution. Results indicated minimal increase in the correlation length both laterally and axially. It was also shown that detectability can be significantly enhanced by increasing the number of measurements or increasing the differential inclination between measurements. This technique is therefore effective for reducing speckle noise while maintaining in-plane spatial resolution. Furthermore, it demonstrates a new application of two-dimensional anisotropic arrays in spite of their limited elevational steering capability.

Elevational Spatial Compounding

Spatial compounding has long been explored to reduce coherent speckle noise in medical ultrasound. By laterally translating a one-dimensional array, partially correlated measurements made at different look directions can be obtained and incoherently averaged. The lateral resolution, however, is limited by the sub-array length used for each independent measurement. To reduce speckle contrast without compromising lateral resolution, a new spatial compounding technique using two-dimensional, anisotropic arrays is proposed. This technique obtains partially correlated measurements by steering the image plane elevationally with small inclinations. Incoherent averaging is then performed by adding image magnitudes. Therefore, contrast resolution is improved only at the price of a slightly wider elevational beam. Note that although anisotropic arrays have limited steering capability in elevation, grating lobes are not considered influential since only small inclinations are needed between measurements. Simulations have been performed to show both the change in spatial resolution and the improvement in contrast resolution. Results indicated minimal increase in the correlation length both laterally and axially. It was also shown that detectability can be significantly enhanced by increasing the number of measurements or increasing the differential inclination between measurements. This technique is therefore effective for reducing speckle noise while maintaining in-plane spatial resolution. Furthermore, it demonstrates a new application of two-dimensional anisotropic arrays in spite of their limited elevational steering capability.

Partially Coherent Transducers: The Random Phase Transducer Approach

Ultrasound speckle is a consequence of the stochastic nature of the reflectivity of scattering media (e.g., biological tissue) and of the coherent nature of piezoelectric transducers. This speckle noise can be reduced by the use of incoherent processing techniques (e.g., spatial compounding, incoherent summation, random phase and phase insensitive transducers). We present a unified framework that explains the limitations of incoherent processing in terms of the information grain theory. This theory predicts the gains in SNR as well as the losses in directivity. We also present the random phase transducer approach to incoherence to total coherence. We present applications to speckle reduction, detection of specular reflectors, attenuation estimation and ultrasound imaging. We show that totally incoherent transducers completely remove diffraction effects. They might be used in attenuation estimation, in which case, correction for diffraction is no longer required, in order to obtain unbiased estimates. Partially coherent transducers might also be useful in imaging to reduce speckle noise.

Partially coherent transducers: The random phase transducer approach

Ultrasound speckle is a consequence of the stochastic nature of the reflectivity of scattering media (e.g., biological tissue) and of the coherent nature of piezoelectric transducers. This speckle noise can be reduced by the use of incoherent processing techniques (e.g., spatial compounding, incoherent summation, random phase and phase insensitive transducers.). We present a unified framework that explains the limitations of incoherent processing in terms of the information grain theory. This theory predicts the gains in SNR as well as the losses in directivity. We also present the random phase transducer approach to incoherent processing. This approach allows continuous control of the coherence from total incoherence to total coherence. We present applications to speckle reduction, detection of specular reflectors, attenuation estimation and ultrasound imaging. We show that totally incoherent transducers completely remove diffraction effects. They might be used in attenuation estimation, in which case, correction for diffraction is no longer required, in order to obtain unbiased estimates. Partially coherent transducers might also be useful in imaging to reduce speckle noise.