2-D motion estimation using two parallel receive beams.

We describe a method for estimating 2-D target motion using ultrasound. The method is based on previous ensemble tracking techniques, which required at least four parallel receive beams and 2-D pattern matching. In contrast, the method described requires only two parallel receive beams and 1-D pattern matching. Two 1-D searches are performed, one in each lateral direction. The direction yielding the best match indicates the lateral direction of motion. Interpolation provides sub-pixel magnitude resolution. We compared the two beam method with the four beam method using a translating speckle target at three different parallel beam steering angles and transducer angles of 0, 45, and 90 degrees. The largest differences were found at 90 degrees, where the two beam method was generally more accurate and precise than the four beam method and also less prone to directional errors at small translations. We also examined the performance of both methods in a laminar flow phantom. Results indicated that the two beam method was more accurate in measuring the flow angle when the flow velocity was small. Computer simulations supported the experimental findings. The poorer performance of the four beam method was attributed to differences in correlation among the parallel beams. Specifically, center beams 2 and 3 correlated better with each other than with the outer beams. Because the four beam method used a comparison of a kernel region in beam pair 2-3 with two different beam pairs 1-2 and 3-4, the 2-to-1 and 3-to-4 components of this comparison increased the incidence of directional errors, especially at small translations. The two beam method used a comparison between only two beams and so was not subject to this source of error. Finally, the two beam method did not require amplitude normalization, as was necessary for the four beam method, when the two beams were chosen symmetric to the transmit axis. We conclude that two beam ensemble tracking can accurately estimate motion using only two parallel receive beams.

Speckle tracking for multi-dimensional flow estimation.

Speckle tracking methods overcome the major limitations of current Doppler methods for flow imaging and quantification: angle dependence and aliasing. In this paper, we review the development of speckle tracking, with particular attention to the advantages and limitations of two-dimensional algorithms that use a single transducer aperture. Ensemble tracking, a recent speckle tracking method based upon parallel receive processing, is described. Experimental results with ensemble tracking indicate the ability to measure laminar flow in a phantom at a beam-vessel angle of 60 degrees, which had not been possible with previous 2D speckle tracking methods. Finally, important areas for future research in speckle tracking are briefly summarized.

A novel interpolation strategy for estimating subsample speckle motion.

Multidimensional, high-resolution ultrasonic imaging of rapidly moving tissue is primarily limited by sparse sampling in the lateral dimension. In order to achieve acceptable spatial resolution and velocity quantization, interpolation of laterally sampled data is necessary. We present a novel method for estimating lateral subsample speckle motion and compare it with traditional interpolation methods. This method, called grid slopes, requires no a priori knowledge and can be applied to data with as few as two samples in the lateral dimension. Computer simulations were performed to compare grid slopes with two conventional interpolation schemes, parabolic fit and cubic spline. Results of computer simulations show that parabolic fit and cubic spline performed poorly at translations greater than 0.5 samples, and translations less than 0.5 samples were subject to an estimation bias. Grid slopes accurately estimated translations between 0 and 1 samples without estimation bias at high signal-to-noise ratios. Given that the grid slopes interpolation technique performs well at high signal-to-noise ratios, one pertinent clinical application might be tissue motion tracking.

Speckle decorrelation due to two-dimensional flow gradients.

The performance of ultrasonic velocity estimation methods is degraded by speckle decorrelation, the change in received echoes over time. Because ultrasonic speckle is formed by the complex sum of echoes from subresolution scatterers, it is sensitive to the relative motion of those scatterers. Velocity gradients in flowing blood result in relative scatterer motion and can be a significant source of speckle decorrelation. Computer simulations were performed to evaluate speckle decorrelation due to two-dimensional flow gradients. Results indicate that decorrelation due to flow gradients is sensitive to the angle of flow and has a maximum at a beam-vessel angle of 0 degrees , i.e., purely axial flow. A quantitative summary of the major factors causing speckle decorrelation indicates that flow gradients are the most significant contributors under the conditions modeled.

Ensemble tracking for 2D vector velocity measurement: Experimental and initial clinical results.

We describe a new method, called ensemble tracking, for estimating two-dimensional velocities with ultrasound. Compared to previous speckle tracking techniques, ensemble tracking measures motion over smaller times and distances, increasing maximum velocities and reducing errors due to echo decorrelation. Ensemble tracking uses parallel receive processing, 2D pattern matching, and interpolation of the resulting tracking grid to estimate sub-pixel speckle translations between successive ultrasonic acquisitions. In this study, small translations of a tissue mimicking phantom were quantified at transducer angles of 0 degrees , 45 degrees , and 90 degrees . Measurements over three parallel beam spacings and all transducer angles had mean errors from -4% to +11%, when parallel beam amplitudes were normalized. Such amplitude normalization substantially improved results at 45 degrees and 90 degrees . The amplitude, spacing, and correlation between the parallel beams were quantified, and their effects on the accuracy and precision of estimates are discussed. Finally, initial clinical results demonstrate the ability to track and display blood flow in the carotid artery.

Three-dimensional flow images by reconstruction from two-dimensional vector velocity maps.

A method for constructing three-dimensional images of flow is described. The technique involves the acquisition of numerous closely spaced planes, each comprised of a map of the two-dimensional velocities measured in that plane. Each such vector velocity map is formed by tracking the motion of small regions of ultrasonic speckle between two ultrasonic acquisitions separated by a short time interval. In contrast to current Doppler velocity methods, this technique measures both the axial and lateral components of flow and is not subject to aliasing. The resulting series of two-dimensional vector velocity maps is then combined into a three-dimensional data set, which can be manipulated with appropriate software to yield quantitative three-dimensional displays of the flow within the interrogated volume. In this article we present such images obtained from measurements of in vitro laminar flow in a vessel, as well as a free jet phantom. The results allow comprehensive visualization of the three-dimensional flow characteristics, indicating promise for more complete and quantitative clinical assessment of blood flow.

Experimental velocity profiles and volumetric flow via two-dimensional speckle tracking.

The performance of a two-dimensional speckle tracking system in measuring in vitro laminar flow is evaluated. The system uses a pattern matching algorithm to track subresolution-sized speckle regions between successive ultrasonic 2D pulse-echo acquisitions in order to determine both the axial and the lateral components of velocity. In this study, multiple 2D vector velocity maps were acquired in real time using a calibrated laminar flow phantom, and then statistically analyzed off-line. At a 90 degrees transducer angle, volumetric flow rates computed from measured velocity profiles exhibited excellent linearity (R2 > 0.99), with a mean error of -6.1%, over the range 5-30 mL/s. At 105 degrees and 120 degrees, experimental volume flow rates also agreed well with actual rates, although measured velocity profiles appeared more irregular with decreasing Doppler angles. Velocity profiles estimated using sampled radio-frequency data rather than envelope-detected data were inconsistent due to an insufficient sampling rate and the quantization of the velocity grid. Results indicate that excellent flow velocity and volume rate estimates can be obtained from vector velocity measurements along a single line of sight, without a priori knowledge of the flow direction, at transducer angles near 90 degrees where Doppler instruments are prone to large errors.

A real time system for quantifying and displaying two-dimensional velocities using ultrasound.

This paper describes a system that has been developed for measuring two-dimensional velocities in real time using ultrasound. The instrument tracks interframe speckle pattern motion using a Sum-Absolute-Difference (SAD) algorithm in order to produce a vector map of 2D velocities. The system's parallel architecture allows calculation of approximately 20,000 vectors per second using the current tracking geometry. A programmable graphics processor encodes individual velocity vectors with color and displays them superimposed on the B-mode image in real time. In vitro tests indicate that the system can track velocities well over the Doppler aliasing limit in any direction in the scan plane with greater than 94% accuracy. A color encoded image obtained from a flow phantom highlights the system's ability to display lateral motion with uniform coloration, in contrast to the two-color display of current ultrasonic Doppler instruments.

Real-time system for angle-independent US of blood flow in two dimensions: initial results.

The authors developed an ultrasound system that enables the speckle patterns produced by echoes from moving blood to be tracked in real time. Unlike current color Doppler flow imagers, this system allows the measurement of blood velocities in any direction within the imaging plane. The authors used this device to image flow in the human jugular vein and contrasted the image with one obtained under similar circumstances with color Doppler flow imaging. The authors demonstrated that this system can display in vivo lateral blood flow in real time. Further development of the system, including the incorporation of wall filters to enhance weak blood echoes and parallel techniques to reduce data acquisition time, will allow clinical imaging of flow with velocities of several meters per second in any direction without aliasing or dependence on the Doppler angle.

Phase aberration correction using echo signals from moving targets. II: Experimental system and results.

A method for correcting errors due to near-field tissue inhomogeneities in phased array ultrasound images is evaluated experimentally. The method uses the brightness of a moving speckle-generating target, such as blood, as a quality factor to correct for unknown phase aberrations. A real time experimental system utilizing the technique has been constructed and is described. Initial results from in vitro studies using a flow phantom are compared to theoretical predictions. The results indicate that the technique can provide significant improvements in image quality when imaging through aberrating media, and may find application in clinical imaging through skull and fatty layers.

Phase aberration correction using echo signals from moving targets. I: Description and theory.

Inhomogeneous acoustic velocity in human tissue introduces phase aberration in ultrasonic imaging systems and degrades image quality. A novel technique that employs echo signals from moving diffuse targets, such as flowing blood, as an image quality factor to compensate for phase aberration is described. Such signals can be obtained by subtracting the images of two consecutive target interrogations. The fundamental statistics of the quality factor and other related parameters are developed to provide a theoretical basis for the technique.

A novel method for angle independent ultrasonic imaging of blood flow and tissue motion.

Tracking the speckle patterns produced by moving targets has been shown effective for angle independent imaging of blood flow and tissue motion. While speckle tracking overcomes major limitations of Doppler-based flow imaging, the computational complexity of commonly used cross correlation algorithms currently limits it to off-line studies. A much simpler algorithm for angle independent motion imaging is described in this paper. This method requires only one absolute difference operation per pixel, compared to eight operations for normalized cross correlation. Quantitative studies using speckle-generating targets translated by fixed amounts both axially and laterally indicate that the technique tracks moving speckle as accurately as correlation. Color flow images generated from clinical blood and liver data highlight the success of the technique for tracking both large and small motions in two dimensions. The algorithm's suitability for implementation in digital hardware makes possible the development of clinical instruments for angle independent ultrasonic imaging of blood flow and tissue motion in real time.