Dr. William F. Walker Profile

Dr. William F. Walker

Associate Professor at Univ of Virginia

SPIE Involvement:

Author

Proceedings Article | 10 March 2008 Paper

Beamformer enhancement by post-processing for improved spatial resolution and signal-to-noise ratio

Kevin Owen, Drake Guenther, William Walker

Proceedings Volume 6920, 692002 (2008) https://doi.org/10.1117/12.772942

KEYWORDS: Signal to noise ratio, Phased arrays, Spatial resolution, Point spread functions, Radon, Ultrasonography, Spatial filters, Optical simulations, Electronic filtering, Systems modeling

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Proceedings Article | 12 March 2007 Paper

Receive channel FIR filters for enhanced contrast in medical ultrasound

Drake Guenther, William Walker

Proceedings Volume 6513, 651303 (2007) https://doi.org/10.1117/12.709702

KEYWORDS: Point spread functions, Apodization, Finite impulse response filters, Ultrasonography, Quantum cascade lasers, Phased arrays, Image resolution, Optical filters, Signal to noise ratio, Imaging systems

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Proceedings Article | 12 March 2007 Paper

Multi-dimensional spline-based non-rigid image registration

Francesco Viola, Drake Guenther, Ryan Coe, William Walker

Proceedings Volume 6513, 65130U (2007) https://doi.org/10.1117/12.710574

KEYWORDS: Motion estimation, Statistical analysis, Error analysis, Computer simulations, Image registration, Transducers, Neodymium, Speckle pattern, Ultrasonography, Point spread functions

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Image registration, or equivalently motion estimation, plays a central role in a broad range of ultrasound applications including elastography, estimation of blood or tissue motion, radiation force imaging, and extended field of view imaging. Because of its central significance, motion estimation accuracy, precision, and computational cost are of critical importance. Furthermore, since motion estimation is typically performed on sampled signals, while estimates are usually desired over a continuous domain, performance should be considered in conjunction with associated interpolation. We have previously presented a highly accurate, spline-based time delay estimator that directly determines sub-sample time delay estimates from sampled data. The algorithm uses cubic splines to produce a continuous time representation of a reference signal and then computes an analytical matching function between this reference and a delayed signal. The location of the minima of this function yields estimates of the time delay. In this paper we describe a MUlti-dimensional Spline-based Estimator (MUSE) that allows accurate and precise estimation of multi-dimensional displacements/strain components from multi-dimensional data sets. In this paper we describe the mathematical formulation for three-dimensional (3D) motion/strain estimation and present simulation results to assess the intrinsic bias and standard deviation of this algorithm and compare it to currently available multi-dimensional estimators. In 1,000 noise-free simulations we found that 2D MUSE exhibits maximum bias errors of 4.8nm and 297nm in range and azimuth respectively. The maximum simulated standard deviation of estimates in both dimensions was comparable at 0.0026 samples (corresponding to 54nm axially and 378nm laterally). These results are two to three orders of magnitude lower than currently used 2D tracking methods. Simulation of performance in 3D yielded similar results to those observed in 2D. We also performed experiments using 2D MUSE on an Ultrasonix Sonix RP imaging system with an L14-5/38 linear array transducer operating at 6.6MHz. With this experimental data we found that bias errors were significantly smaller than geometric errors induced by machining of the transducer mount.

Proceedings Article | 16 March 2006 Paper

The sonic window: second generation results

William Walker, Michael Fuller, Edward Brush, Matthew D. C. Eames, Kevin Owen, Karthik Ranganathan, Travis Blalock, John Hossack

Proceedings Volume 6147, 61470B (2006) https://doi.org/10.1117/12.661507

KEYWORDS: Digital signal processing, Ultrasonography, Transducers, Prototyping, Analog electronics, LCDs, Signal processing, Phased arrays, System integration, Signal to noise ratio

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Medical Ultrasound Imaging is widely used clinically because of its relatively low cost, portability, lack of ionizing radiation, and real-time nature. However, even with these advantages ultrasound has failed to permeate the broad array of clinical applications where its use could be of value. A prime example of this untapped potential is the routine use of ultrasound to guide intravenous access. In this particular application existing systems lack the required portability, low cost, and ease-of-use required for widespread acceptance. Our team has been working for a number of years to develop an extremely low-cost, pocket-sized, and intuitive ultrasound imaging system that we refer to as the "Sonic Window." We have previously described the first generation Sonic Window prototype that was a bench-top device using a 1024 element, fully populated array operating at a center frequency of 3.3 MHz. Through a high degree of custom front-end integration combined with multiplexing down to a 2 channel PC based digitizer this system acquired a full set of RF data over a course of 512 transmit events. While initial results were encouraging, this system exhibited limitations resulting from low SNR, relatively coarse array sampling, and relatively slow data acquisition. We have recently begun assembling a second-generation Sonic Window system. This system uses a 3600 element fully sampled array operating at 5.0 MHz with a 300 micron element pitch. This system extends the integration of the first generation system to include front-end protection, pre-amplification, a programmable bandpass filter, four sample and holds, and four A/D converters for all 3600 channels in a set of custom integrated circuits with a combined area smaller than the 1.8 x 1.8 cm footprint of the transducer array. We present initial results from this front-end and present benchmark results from a software beamformer implemented on the Analog Devices BF-561 DSP. We discuss our immediate plans for further integration and testing. This second prototype represents a major reduction in size and forms the foundation of a fully functional, fully integrated, pocket sized prototype.

Proceedings Article | 28 April 2004 Paper

A prototype low-cost handheld ultrasound imaging system

Karthik Ranganathan, Mary Santy, Michael Fuller, Shiwei Zhou, Travis Blalock, John Hossack, William Walker

Proceedings Volume 5373, (2004) https://doi.org/10.1117/12.537724

KEYWORDS: Phased arrays, Ultrasonography, Digital signal processing, Data centers, Transducers, Apodization, Linear filtering, Image quality, Prototyping, Optical simulations

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Proceedings Article | 28 April 2004 Paper

Synthetic aperture methods for angular scatter imaging

Drake Guenther, Karthik Ranganathan, Michael McAllister, K. Rigby, William Walker

Proceedings Volume 5373, (2004) https://doi.org/10.1117/12.540671

KEYWORDS: Scattering, Spatial resolution, Backscatter, Spatial frequencies, Tissues, Acoustics, Data acquisition, Transducers, Ultrasonography, Multiple scattering

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Angular scatter offers a new source of tissue contrast and an opportunity for tissue characterization in ultrasound imaging. We have previously described the application of the translating apertures algorithm (TAA) to coherently acquire angular scatter data over a range of scattering angles. While this approach works well at the focus, it suffers from poor depth of field (DOF) due to a finite aperture size. Furthermore, application of the TAA with large focused apertures entails a tradeoff between spatial resolution and scattering angle resolution. While large multielement apertures improve spatial resolution, they encompass many permutations of transmit/receive element pairs. This results in the simultaneous interrogation of multiple scattering angles, limiting angular resolution. We propose a synthetic aperture imaging scheme that achieves both high spatial resolution and high angular resolution. In backscatter acquisition mode, we transmit successively from single transducer elements, while receiving on the same element. Other scattering angles are interrogated by successively transmitting and receiving on different single elements chosen with the appropriate spatial separation between them. Thus any given image is formed using only transmit/receive element pairs at a single separation. This synthetic aperture approach minimizes averaging across scattering angles, and yields excellent angular resolution. Likewise, synthetic aperture methods allow us to build large effective apertures to maintain a high spatial resolution. Synthetic dynamic focusing and dynamic apodization are applied to further improve spatial resolution and DOF. We present simulation results and experimental results obtained using a GE Logiq 700MR system modified to obtain synthetic aperture TAA data. Images of wire targets exhibit high DOF and spatial resolution. We also present a novel approach for combining angular scatter data to effectively reduce grating lobes. With this approach we have been able to push the grating lobes below -50 dB in simulation and effectively eliminate their presence in the experimental wire target images.

Proceedings Article | 23 May 2003 Paper

Constrained adaptive beamforming: point and contrast resolution

Jake Mann, William Walker

Proceedings Volume 5035, (2003) https://doi.org/10.1117/12.479959

KEYWORDS: Phased arrays, Ultrasonography, Image processing, Finite impulse response filters, Transducers, Optical filters, Image resolution, Imaging systems, Fourier transforms, Tissues

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Proceedings Article | 11 April 2002 Paper

Novel aperture design method in ultrasound imaging

Karthik Ranganathan, William Walker

Proceedings Volume 4687, (2002) https://doi.org/10.1117/12.462159

KEYWORDS: Point spread functions, Ultrasonography, Chemical elements, Apodization, Computer simulations, Transducers, Linear algebra, Imaging systems, Continuous wave operation

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Proceedings Article | 11 April 2002 Paper

Analysis of clot formation with acoustic radiation force

Francesco Viola, Diane Longo, Michael Lawrence, William Walker

Proceedings Volume 4687, (2002) https://doi.org/10.1117/12.462160

KEYWORDS: Blood, Acoustics, Veins, Data modeling, Signal processing, Heart, Blood circulation, Arteries, Lung, Ultrasonography

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Proceedings Article | 30 May 2001 Paper

Development of a parallel acquisition system for ultrasound research

Christopher Fabian, Kailash Ballu, John Hossack, Travis Blalock, William Walker

Proceedings Volume 4325, (2001) https://doi.org/10.1117/12.428238

KEYWORDS: Data acquisition, Phased arrays, Ultrasonography, Tissues, Imaging systems, Signal processing, Transducers, Algorithm development, Data archive systems, Blood circulation

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Proceedings Article | 30 May 2001 Paper

Significance of correlation in ultrasound signal processing

William Walker

Proceedings Volume 4325, (2001) https://doi.org/10.1117/12.428192

KEYWORDS: Signal to noise ratio, Speckle pattern, Ultrasonography, Signal processing, Point spread functions, Imaging systems, Error analysis, Interference (communication), Speckle, Scattering

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Proceedings Article | 12 April 2000 Paper

Novel approaches to angular scatter imaging using the translating apertures algorithm

Michael McAllister, William Walker

Proceedings Volume 3982, (2000) https://doi.org/10.1117/12.382246

KEYWORDS: Point spread functions, Scattering, Breast, Ultrasonography, Detection and tracking algorithms, Scatter measurement, Target acquisition, Tissues, Speckle, Coherence (optics)

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Proceedings Article | 12 April 2000 Paper

Tissue elasticity using acoustic radiation force

Laura Negron, William Walker, Francisco Fernandez

Proceedings Volume 3982, (2000) https://doi.org/10.1117/12.382239

KEYWORDS: Tissues, Scattering, Point spread functions, Acoustics, Imaging systems, Signal to noise ratio, Rayleigh scattering, Data acquisition, Ultrasonography, Breast

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