Biomedical subjects

William F Walker

Publications and source records attributed to William F Walker.

11 recordsLinked to original sources

PubMed2006

A comparison between spline-based and phase-domain time-delay estimators.

We previously presented a novel time-delay estimator that uses cubic splines to estimate continuous time delays from sampled data. Previous simulation results showed that the spline-based algorithm yields superior performance when compared to the normalized correlation and the sum squared differences (SSD) algorithms implementing a variety of subsample interpolators. In this short communication, we present additional simulation results that compare the spline-based time-delay estimator to well-known phase-domain estimators. Under comparable conditions, spline-based methods exhibit biases approximately one tenth and standard deviation only one hundredth of those observed for the phase-based approaches.

Letter↗

PubMed2005

A spline-based algorithm for continuous time-delay estimation using sampled data.

Time delay estimation (TDE) lies at the heart of signal processing algorithms in a broad range of application areas, including communications, coherent imaging, speech processing, and acoustics. In medical ultrasound for example, TDE is used in blood flow estimation, tissue motion measurement, tissue elasticity estimation, phase aberration correction, and a number of other algorithms. Because of its central significance, TDE accuracy, precision, and computational cost are of critical importance. Furthermore, because TDE is typically performed on sampled signals-and delay estimates are usually desired over a continuous domain-time delay estimator performance should be considered in conjunction with associated interpolation. In this paper we present a new time-delay estimator that directly determines continuous time-delay estimates from sampled data. The technique forms a spline-based, piecewise continuous representation of the reference signal then solves for the minimum of the sum squared error between the reference and the delayed signals to determine their relative time delay. Computer simulation results clearly show that the proposed algorithm significantly outperforms other algorithms in terms of jitter and bias over a broad range of conditions. We also describe a modified version of the algorithm that includes companding with only a minor increase in computational cost.

Algorithms↗

PubMed2005

Determination of an optimal image frame interval for frame-to-frame ultrasound image motion tracking.

Several important clinical applications depend on accurate ultrasound image frame-to-frame motion estimation. Assuming that there is a degree of finite noise in the image frames and that speckle partially decorrelates between successive frames during freehand scanning, we hypothesize that an optimal inter-frame interval (step size) must exist that provides the smallest relative dimensional error over a set of accumulated motion estimates. Smaller frame increments suffer from less decorrelation-related inaccuracy but present greater potential for cumulative error because more estimates are used over any specific dimensional interval. We studied these effects using a combination of theoretical modeling, numerical simulation, and experiments. Components of diagonal motion due to the limitations of manual transducer movement were considered as the cause of decorrelation. The results were examined for four different angles of the diagonal motion and two different signal-to-noise ratio (SNR) values. These indicate that an optimal step size does exist and that this is dependent on many variables including SNR, angle of the diagonal motion, transducer geometry, lens focusing parameters, transducer operating frequency, and beamforming parameters. In practical experiments, we found that the optimal step size generally required using every available image frame rather than 'skipping' any intermediate frames.

Algorithms↗

PubMed2004

Sonorheometry: a noncontact method for the dynamic assessment of thrombosis.

Inappropriate blood coagulation plays a central role in the onset of myocardial infarction, stroke, pulmonary embolism, and other thrombotic disorders. The ability to screen for an increased propensity to clot could prevent the onset of such events by appropriately identifying those at risk and enabling prophylactic treatment. Similarly, the ability to characterize the mechanical properties of clots in vivo might improve patient outcomes by better informing treatment strategies. We have developed a technique called sonorheometry. Unlike existing methods, sonorheometry is able to assess mechanical properties of coagulation with minimal disturbance to the delicate structure of a forming thrombus. Sonorheometry uses acoustic radiation force to produce small, localized displacements within the sample. Time delay estimation is performed on returned ultrasound echoes to determine sample deformation. Mechanical modeling and parametric fitting to experimental data yield maps of mechanical properties. Sonorheometry is well suited to both in vitro and in vivo applications. A control experiment was performed to verify that sonorheometry provides mechanical characterization in agreement with that from a conventional rheometer. We also examined thrombosis in blood samples taken from four subjects. This data suggests that sonorheometry may offer a novel and valuable method for assessing the thrombogenicity of blood samples.

Blood↗

PubMed2004

Direct sampled I/Q beamforming for compact and very low-cost ultrasound imaging.

A wide variety of beamforming approaches are applied in modern ultrasound scanners, ranging from optimal time domain beamforming strategies at one end to rudimentary narrowband schemes at the other. Although significant research has been devoted to improving image quality, usually at the expense of beamformer complexity, we are interested in investigating strategies that sacrifice some image quality in exchange for reduced cost and ease in implementation. This paper describes the direct sampled in-phase/quadrature (DSIQ) beamformer, which is one such low-cost, extremely simple, and compact approach. DSIQ beamforming relies on phase rotation of I/Q data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional demodulation. We describe an efficient hardware implementation of the beamformer, which results in significant reductions in beamformer size and cost. We present the results of simulations and experiments that compare the DSIQ beamformer to more conventional approaches, namely, time delay beamforming and traditional complex demodulated I/Q beamforming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are also presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for low end scanners. We also describe implementation of the DSIQ beamformer in an inexpensive hand-held ultrasound system being developed in our laboratory.

Algorithms↗

PubMed2003

A novel beamformer design method for medical ultrasound. Part I: Theory.

The design of transmit and receive aperture weightings is a critical step in the development of ultrasound imaging systems. Current design methods are generally iterative, and consequently time consuming and inexact. We describe a new and general ultrasound beamformer design method, the minimum sum squared error (MSSE) technique. The MSSE technique enables aperture design for arbitrary beam patterns (within fundamental limitations imposed by diffraction). It uses a linear algebra formulation to describe the system point spread function (psf) as a function of the aperture weightings. The sum squared error (SSE) between the system psf and the desired or goal psf is minimized, yielding the optimal aperture weightings. We present detailed analysis for continuous wave (CW) and broadband systems. We also discuss several possible applications of the technique, such as the design of aperture weightings that improve the system depth of field, generate limited diffraction transmit beams, and improve the correlation depth of field in translated aperture system geometries. Simulation results are presented in an accompanying paper.

Algorithms↗

PubMed2003

A novel beamformer design method for medical ultrasound. Part II: Simulation results.

In the first part of this work, we introduced the minimum sum squared error (MSSE) technique of ultrasound beamformer design. This technique enables the optimal design of apertures to achieve arbitrary system responses. In the MSSE technique, aperture weights are calculated and applied to minimize the sum squared error (SSE) between the desired and actual system responses. In this paper, we present the results of simulations performed to illustrate the implementation and validity of the MSSE technique. Continuous wave (CW) and broadband simulations are presented to demonstrate the application of the MSSE method to obtain arbitrary system responses (within fundamental physical limitations of the system). We also describe CW and broadband simulations that implement the MSSE method for improved conventional depth of field (DOF) and for improved correlation DOF in translated aperture geometries. Using the MSSE technique, we improved the conventional DOF by more than 200% in CW simulations and more than 100% in broadband simulations. The correlation DOF in translated aperture geometries was improved by more than 700% in both CW and broadband simulations.

Algorithms↗

PubMed2003

A comparison of the performance of time-delay estimators in medical ultrasound.

Time-delay estimation (TDE) is a common operation in ultrasound signal processing. In applications such as blood flow estimation, elastography, phase aberration correction, and many more, the quality of final results is heavily dependent upon the performance of the time-delay estimator implemented. In the past years, several algorithms have been developed and applied in medical ultrasound, sonar, radar, and other fields. In this paper we analyze the performances of the widely used normalized and non-normalized correlations, along with normalized covariance, sum absolute differences (SAD), sum squared differences (SSD), hybrid-sign correlation, polarity-coincidence correlation, and the Meyr-Spies method. These techniques have been applied to simulated ultrasound radio frequency (RF) data under a variety of conditions. We show how parameters, which include center frequency, fractional bandwidth, kernel window size, signal decorrelation, and signal-to-noise ratio (SNR) affect the quality of the delay estimate. Simulation results also are compared with a theoretical performance limit set by the Cramér-Rao lower bound (CRLB). Results show that, for high SNR, high signal correlation, and large kernel size, all of the algorithms closely match the theoretical bound, with relative performances that vary by as much as 20%. As conditions degrade, the performances of various algorithms differ more significantly. For signals with a correlation level of 0.98, SNR of 30 dB, center frequency of 5 MHz with a fractional bandwidth of 0.5, and kernel size of 2 micros, the standard deviation of the jitter error is on the order of few nanoseconds. Normalized correlation, normalized covariance, and SSD have an approximately equal jitter error of 2.23 ns (the value predicted by the CRLB is 2.073 ns), whereas the polarity-coincidence correlation performs less well with a jitter error of 2.74 ns.

Algorithms↗

PubMed2003

Radiation force imaging of viscoelastic properties with reduced artifacts.

It is well-known that changes in the mechanical properties of tissues are correlated with the presence of disease. In the eye, for example, the vitreous body undergoes dramatic changes in mechanical properties during age-related degradation. These changes may play a significant role in the formation of retinal detachment or other vitreoretinal diseases. We previously presented a noninvasive method called kinetic acoustic vitreoretial examination (KAVE), which may be used to detect these mechanical changes. KAVE uses acoustic radiation force as a means to produce small, localized displacements within the tissues. Returning echoes are processed using ultrasonic motion tracking so that the response of the tissue to the induced force can be evaluated. By repeating this process at a number of locations, images depicting viscoelastic properties of tissues can be formed. Through the combination of appropriate mechanical modeling and signal processing, we are able to generate images of parameters such as relative mass, relative elasticity, and relative viscosity. These parameters are called relative because they depend on the force applied, which is typically unknown. In this paper, we present new force-free images depicting the time constant tau, the damping ratio xi, and the natural frequency omega of the phantom material. These images are significant in that they lack the artifacts common in the relative property images. Experiments were conducted on a set of three acrylamide-based phantoms with varying gel concentrations. We present images depicting B-mode echogenicity, maximum radiation force-induced displacement, relative material parameters, and force-free characteristics of the series of phantoms. The presented force-free images depict mechanical properties without artifacts from local force variation due to acoustic reflection, refraction, and attenuation. Force-free images should prove particularly useful for in vivo imaging through inhomogeneous tissues.

Artifacts↗

PubMed2002

Comparison of PSGL-1 microbead and neutrophil rolling: microvillus elongation stabilizes P-selectin bond clusters.

A cell-scaled microbead system was used to analyze the force-dependent kinetics of P-selectin adhesive bonds independent of micromechanical properties of the neutrophil's surface microvilli, an elastic structure on which P-selectin ligand glycoprotein-1 (PSGL-1) is localized. Microvillus extension has been hypothesized in contributing to the dynamic range of leukocyte rolling observed in vivo during inflammatory processes. To evaluate PSGL-1/P-selectin bond kinetics of microbeads and neutrophils, rolling and tethering on P-selectin-coated substrates were compared in a parallel-plate flow chamber. The dissociation rates for PSGL-1 microbeads on P-selectin were briefer than those of neutrophils for any wall shear stress, and increased more rapidly with increasing flow. The microvillus length necessary to reconcile dissociation constants of PSGL-1 microbeads and neutrophils on P-selectin was 0.21 microm at 0.4 dyn/cm2, and increased to 1.58 microm at 2 dyn/cm2. The apparent elastic spring constant of the microvillus ranged from 1340 to 152 pN/microm at 0.4 and 2.0 dyn/cm2 wall shear stress. Scanning electron micrographs of neutrophils rolling on P-selectin confirmed the existence of micrometer-scaled tethers. Fixation of neutrophils to abrogate microvillus elasticity resulted in rolling behavior similar to PSGL-1 microbeads. Our results suggest that microvillus extension during transient PSGL-1/P-selectin bonding may enhance the robustness of neutrophil rolling interactions.

Antibodies, Monoclonal↗

PubMed2002

Development and characterization of a vitreous mimicking material for radiation force imaging.

In many medical ultrasound applications tissue-mimicking phantoms are of fundamental importance for the performance of controlled experiments. Traditionally, such phantoms have been constructed using gelatin and agar gels. Although the use of these materials has become standard, few alternative materials have not been fully explored. In this paper, we present a protocol developed in our laboratory that reliably produces very soft, acrylamide-based phantoms that can mimic both acoustical and mechanical characteristics of the vitreous body of the eye. Following the described protocol, a series of phantoms were constructed ranging in acrylamide concentration from 1.60% to 1.70%. Measurements across the series yielded attenuation coefficients of 0.067-0.140 dB/cm/MHz, depending on acrylamide concentration. Speed of sound ranged between 1499 and 1510 m/s, also depending on acrylamide concentration. Published values for the vitreous gel indicate an attenuation of 0.10 dB/cm/MHz and a speed of sound of 1510 m/s, making our phantoms an excellent analog of this tissue. One application of these acrylamide phantoms is to test the efficacy of the Kinetic Acoustic Vitreoretinal Examination (KAVE), a tool developed in our laboratory with the potential to aid in the diagnosis of vitreoretinal disorders. KAVE utilizes acoustic radiation force to generate small, localized displacements within the vitreous-mimicking gel. These localized displacements are quantified to yield maximum displacement, relative elasticity, and relative viscosity images. We present KAVE images of a set of four phantoms with different gel concentrations. Although B-mode and relative viscosity images exhibit no significant differences, maximum displacement, and relative elasticity images clearly differentiate gels of different concentrations. Maximum displacements ranged between 30 and 5 microns, depending on acrylamide concentration. The results presented in this paper show that soft gel phantoms can be produced in a range of elasticities not previously reported, and that these phantoms are useful for testing ultrasound instruments designed for evaluation of the vitreous gel. Furthermore, the use of acrylamide-based gels may also offer a valuable and attractive alternative for many other ultrasound applications.

Acrylic Resins↗