Biomedical subjects

James Zagzebski

Publications and source records attributed to James Zagzebski.

8 recordsLinked to original sources

PubMed2006

Attenuation estimations using envelope echo data: analysis and simulations.

Previously we described a video signal analysis (VSA) method for measuring backscatter and attenuation from B-Mode image data. VSA computes depth-dependent ratios of the mean echo intensity from a sample to the mean echo intensity from a reference phantom imaged using identical scanner settings. The slope of a line-fit of this ratio (expressed in dB) versus depth is related to the attenuation of the sample. This paper investigates conditions for which the echo intensity ratio versus depth is independent of transducer pulsing characteristics and instrument settings, and depends only on the properties of the sample and the reference. A theoretical model is described for the echo signal power versus depth from a uniform medium containing scatterers. The model incorporates bandwidth, frequency and media attenuation. Results show that the sample-to-reference echo intensity ratio versus depth is a curve, the departure of which from a straight line is a function of the relative attenuation of the two media, the imaging system bandwidth and the initial frequency. The model also leads to a depth-dependent "effective frequency" determination in the VSA method. Model predictions are verified using RF signals computed by an acoustic pulse-echo simulation program.

Computer Simulation↗

PubMed2006

The Ultrasonix 500RP: a commercial ultrasound research interface.

Unlike researchers in magnetic resonance imaging who have considerable access to high level tools and to data at a very basic level on their scanners, those involved with ultrasound have found little in the way of meaningful and widespread access to even the most basic echo signals in their clinical systems. Interest has emerged, however, in ultrasound research interfaces on commercial scanners to provide access to raw ultrasound data and control of basic research functions. This paper describes initial experience gained on one such ultrasound system. The Ultrasonix 500RP system provides research access to the data at multiple points in the signal processing chain and allows control over most imaging parameters. The Ultrasonix system allows for three methods of research control. One is implemented along with the standard clinical imaging software using "mouseover" screens on the periphery of the application window. These screens are configured by the user to display various signal processing variables, which can be modified in real time. Second, the system can be controlled via a user-written remote control client application interacting through the clinical exam software. Lastly, the user can write a complete application which initializes the basic ultrasound module but need not use the Ultrasonix clinical exam software. All of the modes can be done locally on the scanner itself or via a network, and are based on software developed in C++ with libraries supplied with the scanner. Two examples are presented in this paper from the evaluation of the system in "real world" applications. Measurements of absolute backscatter coefficients and attenuation coefficients versus frequency are shown and elastograms utilizing spatial compounding are described.

Biomedical Research↗

PubMed2004

Errors in ultrasonic scatterer size estimates due to phase and amplitude aberration.

Current ultrasonic scatterer size estimation methods assume that acoustic propagation is free of distortion due to large-scale variations in medium attenuation and sound speed. However, it has been demonstrated that under certain conditions in medical applications, medium inhomogeneities can cause significant field aberrations that lead to B-mode image artifacts. These same aberrations may be responsible for errors in size estimates and parametric images of scatterer size. This work derives theoretical expressions for the error in backscatter coefficient and size estimates as a function of statistical parameters that quantify phase and amplitude aberration, assuming a Gaussian spatial autocorrelation function. Results exhibit agreement with simulations for the limited region of parameter space considered. For large values of aberration decorrelation lengths relative to aberration standard deviations, phase aberration errors appear to be minimal, while amplitude aberration errors remain significant. Implications of the results for accurate backscatter and size estimation are discussed. In particular, backscatter filters are suggested as a method for error correction. Limitations of the theory are also addressed. The approach, approximations, and assumptions used in the derivation are most appropriate when the aberrating structures are relatively large, and the region containing the inhomogeneities is offset from the insonifying transducer.

Acoustics↗

PubMed2004

Correlation of ultrasonic scatterer size estimates for the statistical analysis and optimization of angular compounding.

Ultrasonic scatterer size estimates generally have large variances due to the inherent noise of spectral estimates used to calculate size. Compounding partially correlated size estimates associated with the same tissue, but produced with data acquired from different angles of incidence, is an effective way to reduce the variance without making dramatic sacrifices in spatial resolution. This work derives theoretical approximations for the correlation between these size estimates, and the coherence between their associated spectral estimates, as functions of ultrasonic system parameters. A Gaussian spatial autocorrelation function is assumed to adequately model scatterer shape. Both approximations compare favorably with simulation results, which consider validation near the focus. Utilization of the correlation/coherence expressions for statistical analysis and optimization is discussed. Approximations, such as the invariance of phase and amplitude terms with angle, are made to obtain closed-form solutions to the derived spectral coherence near the focus and permit analytical optimization analysis. Results indicate that recommended parameter adjustments for performance improvement generally depend upon whether, for the system under consideration, the primary source of change in total coherence with rotation is phase term variation due to the change in the relative position of scattering sites, or field amplitude term variation due to beam movement.

Analysis of Variance↗

PubMed2004

Harmonic ultrasound fields through layered liquid media.

Harmonic field generation through a layered liquid media is studied experimentally and theoretically. Lateral and axial beam profiles of the fundamental to the 4th harmonic component of the field from a focused, 19-mm diameter transducer were measured using a calibrated hydrophone in a water tank. Measurements were performed before and after the insertion of a cylindrical phantom containing vegetable oil. A frequency domain numerical solution to the "KZK" equation was used to calculate the beam profile, taking into account the acoustic properties of the medium and phantom. Effects of nonlinear propagation, diffraction, attenuation, and reflection are include in the calculation. Agreement within 5% was obtained between measurements and theoretical predictions throughout the mid- and far-field of the transducer for both the uniform path and the layered media. Measurements also were carried out using an unfocused transducer as a receiver. The shape of the axial beam profile using this receiver agreed very well with the theoretical prediction using the "KZK" equation, after accounting for phase variations over the finite-sized detector in the calculated field.

Algorithms↗

PubMed2003

Statistics of ultrasonic scatterer size estimation with a reference phantom.

A theoretical expression for the variance of scatterer size estimates is derived for a modified least squares size estimator used in conjunction with a reference phantom method for backscatter coefficient measurement. A Gaussian spatial autocorrelation function is assumed. Simulations and phantom experiments were performed to verify the results for backscatter and size variances. The dependence of size estimate errors upon free experimental parameters is explored. Implications of the findings for the optimization of scatterer size estimation are discussed. The utility of scatterer size parametric imaging is examined through the signal to noise ratio comparison with standard ultrasonic B-mode imaging.

Analysis of Variance↗

PubMed2002

Pressure-dependent attenuation in ultrasound contrast agents.

Although microbubble contrast agents are believed to respond differently under different driving-pressure amplitudes, few studies have been performed to extensively study the pressure-dependence of their properties. In this paper, attenuation coefficients of two contrast agents (Optison and Definity) were measured under different driving-pressure amplitudes using a narrowband incident pulse. The attenuation of both contrast agents was found to increase with increasing driving pressure. Simulations using the Rayleigh, Plesset, Noltingk, Neppiras, Poritsky (RPNNP) equation were performed to study this behavior. Simulation results show that significant harmonic generation at high driving-pressure amplitudes contributes to the higher attenuation. Other possible explanations for this behavior were also examined. Attenuation coefficients of two contrast agents were also measured using a broadband method. The results showed great inconsistency when the center frequency of the incident broadband pulse was changed, indicating that broadband techniques may not be suitable for contrast-agent attenuation measurements.

Albumins↗

PubMed2002

A test phantom for estimating changes in the effective frequency of an ultrasonic scanner.

OBJECTIVE: Ultrasonic frequency is an important performance feature of B-mode scanners. It is particularly relevant when comparing instruments from different manufacturers and reporting clinical results. We investigated a test phantom to independently measure an effective imaging frequency, including effects of depth-dependent attenuation and frequency filtering during echo reception. METHODS: The approach capitalizes on variations of the frequency dependence of backscatter with scatterer size. A tissue-mimicking phantom containing 48-microm-diameter scatterers was constructed. Embedded at depths of 1, 3, 7, and 9 cm were sets of cylindrical inclusions, each containing tissue-mimicking material with a different scatterer size and number density. Computer simulations helped establish scatterer parameters for the cylinder bodies that resulted in image contrast versus the background that varied with frequency, with each cylinder transitioning from negative to positive contrast at a different frequency. Acoustic properties of the phantom were verified by a laboratory apparatus. Initial tests of the effectiveness of the phantom were done by imaging with several scanners using various frequency and imaging settings on transducers. RESULTS: Images were obtained with 2 clinical scanners in which modest changes in the image acquisition parameters were adjusted. Image contrast between test cylinders and background corresponded to operating frequency with a multihertz transducer. Changes in observable contrast consistent with a shift in operating frequency were not always accompanied by visual indicators that such changes in the scanning protocol had occurred. CONCLUSIONS: The test phantom performs as predicted by computer simulations and theoretical calculations of backscatter versus frequency. Contrast on images of the test phantom produced by clinical systems correlates with scanner frequency settings, showing feasibility. Relative shifts in effective frequency and operating bandwidth can be assessed from these contrast differences between settings with this test phantom.

Humans↗