Session: P2D

The synthetic aperture focusing technique (SAFT) has been previously employed to increase the penetration and to extend the depth of focus. This is particularly important in high frequency ultrasonic imaging because dynamic focusing using arrays is not yet possible. However, performance of the SAFT is limited due to its high sidelobes. In this paper, an adaptive weighting technique based on a focusing-quality index is introduced to suppress the sidelobes. The focusingquality index is derived from the Fourier spectrum of the scan-line data along the mechanical scan direction after the SAFT focusing delays have been applied. The index is defined as the ratio of the energy in the pre-specified low-frequency region around dc to the total energy, and is also known as the energy ratio (ER). The spectrum can be viewed as the approximation of the two-way radiation pattern centered on the synthesized beam direction. The low-frequency region corresponds to the mainlobe and the high-frequency components correspond to the sidelobes. Thus, a high ER indicates that the image intensity should be maintained and a low ER should be used to reduce the sidelobe contributions from the outside objects. It then becomes clear that ER can be used as a weighting factor to the SAFT image to improve the image quality. Experimental ultrasound data from a 50-MHz imaging system with a single-crystal transducer (6 mm diameter and geometric focus at 12 mm) is used to demonstrate the efficacy on both wire targets and speckle-generating objects. Both 50-MHz fundamental imaging and 50-MHz tissue harmonic imaging are tested. Results show that the sidelobe level of the SAFT image is suppressed by more than 10 dB after weighting. The noise floor of the weighted image is about 8 dB lower than that of the SAFT image. The weighted image has 32% contrast-tonoise ratio improvement over the SAFT image when imaging an anechoic cyst. The adaptive weighting technique is also effective at sidelobe reduction for both fundamental and tissue harmonic imaging. In vivo imaging of mouse kidney, liver and embryo is also performed to further demonstrate the effectiveness. The principles, experimental results, and implementation issues of the new technique will be presented.