SYNTHETIC-APERTURE TECHNIQUES WITH A VIRTUAL SOURCE ELEMENT

A new imaging technique has been proposed that combines conventional B -mode and synthetic aperture imaging techniques to overcome the limite d depth of field for a highly focused transducer. The new technique im proves lateral resolution beyond the focus of the transducer by consid ering the focus a virtual element and applying synthetic aperture focu sing techniques. In this paper, the use of the focus as a virtual elem ent is examined, considering the issues that are of concern when imagi ng with an array of actual elements: the tradeoff between lateral reso lution and sidelobe level, the tradeoff between system complexity (cha nnel count/amount of computation) and the appearance of grating lobes, and the issue of signal to noise ratio (SNR) of the processed image. To examine these issues, pulse-echo RF signals were collected for a tu ngsten wire in degassed water, monofilament nylon wires in a tissue-mi micking phantom, and cyst targets in the phantom. Results show apodiza tion lowers the sidelobes. but only at the expense of lateral resoluti on, as is the case for classical synthetic aperture imaging. Grating l obes are not significant until spatial sampling is more than one wavel ength, when the beam is not steered. Resolution comparable to the reso lution 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 mu m, and with syn thetic aperture processing the 6-dB beamwidths at 3, 5, and 7 mm beyon d the focus are 189 mu m, 184 mu m, and 215 mu m, respectively. The im age SNR is 38.6 dB when the wire is at the focus, and it is 32.8 dB, 3 5.3 dB, and 38.1 dB after synthetic aperture processing when the wire is 3, 5, and 7 mm beyond the focus, respectively. With these experimen ts, the virtual source has been shown to exhibit the same behavior as an actual transducer element in response to synthetic aperture process ing techniques.