Barry H. Friemel

3-Dimensional compound ultrasound field of view

A compounded field of view ultrasound image is derived from correlated frames of ultrasound image data. An operator manually translates an ultrasound probe across a patient target area. Frames of sensed echo signals are processed to detect probe motion without the use of a dedicated position sensor or motion sensor. Motion is detected by correlating the frames for probe translation among as many as 6 degrees of freedom. Image registration then is performed for correlated portions to compound a large ultrasound image. Such image encompasses an area larger than a single field of view image frame for the given manually-scanned transducer probe.

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 systems 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 systems ability to display lateral motion with uniform coloration, in contrast to the two-color display of current ultrasonic Doppler instruments.

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.

Relative performance of two-dimensional speckle-tracking techniques: normalized correlation, non-normalized correlation and sum-absolute-difference

The authors present computer simulations that compare the performance of three two-dimensional block-matching algorithms as applied to ultrasonic speckle. The three algorithms, normalized correlation, non-normalized correlation and sum absolute difference (SAD), were applied to simulated speckle patterns with varying levels of noise and with different kernel sizes. The results show that SAD and normalized correlation have similar performance characteristics and are able to track speckle motion with a kernel size as small as one resolution cell. Non-normalized correlation performed poorly with small kernel sizes but improved as the kernel size increased.

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/spl deg/, 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.

Pulsed wave doppler processing using aliased spectral data

A method for producing Doppler ultrasound data at a user-requested pulse repetition frequency (PRF) utilizing undersampled echo signals. Echo signals are created in response to Doppler pulses that are transmitted into the patient at a rate less than a desired PRF. The echo signals are analyzed in the time domain to determine a velocity of scatterers in an area of tissue defined by a range gate. From the velocity, the Doppler shift of the scatterers is determined. The echo signals are interpolated to produce a number of samples equal in number to that which would have been produced had the Doppler pulses been transmitted at the user-requested PRF. The interpolated echo signals are then analyzed in the frequency domain which produces a number of spectra indicative of the velocity and direction of the moving scatterers. From the Doppler shift determined, the correct spectra is selected and displayed for a user. In addition, the present invention can be used to increase the amplitude of the pulses transmitted into a patient by lowering the transmit pulse frequency so that the total amount of ultrasonic energy delivered to the patient remains the same. The larger amplitude transmit pulses produce echo signals having a better signal-to-noise ratio.

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.

Lateral Velocity Profile and Volume Flow Measurements Via 2-D Speckle Tracking

Current Doppler-based techniques for measuring velocity profiles and volume flow rates are plagued by inaccuracies due to errors in estimating the direction of flow with respect to the transducer axis. In addition, such techniques are incapable of quantifying lateral flow. We describe a system which tracks 2-D ultrasonic speckle patterns in order to quantify both the axial and lateral components of motion, thereby obviating the need for angle estimation. The method involves tracking the motion of small regions of an image from one acoustic interrogation to the next using a time-domain pattern matching algorithm. We report on the ability of this system to measure laminar flow velocity profiles and volume flow rates in a laterally flowing phantom. Results indicate that the technique provides accurate velocity and volume flow estimates over a wide range of flow rates.

Initial results with the real-time SAD vector velocity estimation systems: constant velocity calibration

Doppler flow imaging systems are limited by angle-dependence and aliasing. A velocity estimator which uses the sum absolute difference (SAD) algorithm to track the motion of speckle in two dimensions between successive ultrasonic frames has been constructed. The systems performance is evaluated in quantifying velocities of a string phantom moving at various angles relative to the transducer axis. The results show that the system is capable of tracking velocities well beyond the Doppler aliasing limit along any direction within the imaging plane.<<ETX>>

A real-time system for angle-independent velocity detection

The first real-time angle-independent velocity detection system for ultrasound has been constructed. The system detects both the axial and lateral components of velocity using the sum absolute difference algorithm, a novel time-domain method for tracking speckle patterns. In addition to quantifying 2-D velocities, the system rejects echoes according to absolute brightness and vectors according to a best match criterion, filters the flow image and presents the information as a color map to the physician each 30-50 ms. Preliminary results are presented along with an introduction to the system architecture, a discussion of clinical applications and an outline of future directions for the project.<<ETX>>