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University of Illinois at Urbana-Champaign | Department of Electrical and Computer Engineering | Department of Bioengineering
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William D. O'Brien, Jr. publications:

Michael L. Oelze publications:

Synthetic Aperture Imaging

Synthetic aperture techniques with a virtual source element.

Raw data collected in a degassed waterbath using the 15-MHz transducer with a 25-micron
tungsten wire positioned 5 mm beyond the focus. Axes are labelled in mm.

A new imaging technique has been proposed1 that combines conventional B-mode and synthetic aperture imaging techniques to overcome the limited depth of field for a highly focused transducer. The new technique improves lateral resolution beyond the focus of the transducer by considering the focus a virtual element and applying synthetic aperture focusing techniques. In this paper, the use of the focus as a virtual element is examined, considering the issues that are of concern when imaging with an array of actual elements: the tradeoff between lateral resolution and sidelobe level, the tradeoff between system complexity (channel count/amount of computation) and the appearance of grating lobes, and the issue of signal to noise ratio (SNR) of the processed image.

Previous data processed using rectangular apodization weights.

To examine these issues, pulse-echo RF signals are collected for a tungsten wire in degassed water, monofilament nylon wires in a tissue-mimicking phantom, and cyst targets in the phantom. Results show apodization lowers the sidelobes, but only at the expense of lateral resolution, as is the case for classical synthetic aperture imaging. Grating lobes are not significant until spatial sampling is more than one wavelength, when the beam is not steered. Resolution comparable to the resolution at the transducer focus can be achieved beyond the focal region while obtaining an acceptable SNR. Specifically, for a 15-MHz focused transducer, the 6-dB beamwidth at the focus is 157 microns and with synthetic aperture processing the 6-dB beamwidths at 3, 5, and 7 mm beyond the focus are 189 microns, 184 microns, and 215 microns, respectively. The image SNR is 38.6 dB when the wire is at the focus, and it is 32.8 dB, 35.3 dB, and 38.1 dB after synthetic aperture processing when the wire is 3, 5, and 7 mm beyond the focus, respectively. With these experiments, the virtual source has been shown to exhibit the same behavior as an actual transducer element in response to synthetic aperture processing techniques. More detail on this study appears in the Frazier and O'Brien study of synthetic aperture technique(2).

Comparison of -6 dB beamwidth vs. depth for a tungsten wire in a degassed waterbath before and after SA
processing for the 15-MHz (solid lines) and 20-MHz (dashed lines) transducers.


1 C. Passman and H. Ermert, "A 100 MHz ultrasound imaging system for dermatologic and ophthalmologic diagnostics," IEEE Trans. Ultrason., Ferroelect., Freq. Cont., vol. 43, no. 4, pp.545-552, 1996.

2 C. Frazier and W. D. O'Brien, Jr., "Synthetic aperture techniques with a virtual source element," IEEE Trans. Ultrason., Ferroelect., Freq. Cont., 45, 196-207, 1998.
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