Synthetic aperture techniques with a virtual source element

摘要

A new imaging technique has been proposed that combinesnconventional B-mode and synthetic aperture imaging techniques tonovercome the limited depth of field for a highly focused transducer. Thennew technique improves lateral resolution beyond the focus of thentransducer by considering the focus a virtual element and applyingnsynthetic aperture focusing techniques. In this paper, the use of thenfocus as a virtual element is examined, considering the issues that arenof concern when imaging with an array of actual elements: the tradeoffnbetween lateral resolution and sidelobe level, the tradeoff betweennsystem complexity (channel count/amount of computation) and thenappearance of grating lobes, and the issue of signal to noise ration(SNR) of the processed image. To examine these issues, pulse-echo RFnsignals were collected for a tungsten wire in degassed water,nmonofilament nylon wires in a tissue-mimicking phantom, and cyst targetsnin the phantom. Results show apodization lowers the sidelobes, but onlynat the expense of lateral resolution, as is the case for classicalnsynthetic aperture imaging. Grating lobes are not significant untilnspatial sampling is more than one wavelength, when the beam is notnsteered. Resolution comparable to the resolution at the transducer focusncan be achieved beyond the focal region while obtaining an acceptablenSNR. Specifically, for a 15-MHz focused transducer, the 6-dB beamwidthnat the focus is 157 Μm, and with synthetic aperture processing then6-dB beamwidths at 3, 5, and 7 mm beyond the focus are 189 Μm, 184nΜm, and 215 Μm, respectively. The image SNR is 38.6 dB when thenwire is at the focus, and it is 32.8 dB, 35.3 dB, and 38.1 dB afternsynthetic aperture processing when the wire is 3, 5, and 7 mm beyond thenfocus, respectively. With these experiments, the virtual source has beennshown to exhibit the same behavior as an actual transducer element innresponse to synthetic aperture processing techniques