Improving Spatial Resolution Using Incoherent Subtraction of Receive Beams Having Different Apodizations

In ultrasonic imaging, reduction of lateral sidelobes can result in an improved image with less distortion and fewer artifacts. In general, apodization is used to lower sidelobes in exchange for increasing the width of the main lobe, and thus decreasing lateral resolution. Null subtraction imaging (NSI) is a nonlinear image processing technique that uses different receive apodizations on copies of the same RF data to maintain low sidelobe levels while simultaneously improving lateral resolution. The images created with three different apodization functions are combined to form an image with low sidelobe levels and apparent improvements in lateral resolution compared to conventional rectangular apodization. To evaluate the performance of this technique for different imaging tasks, experiments were performed on an ATS539 phantom containing wire targets to assess lateral resolution and cylindrical anechoic and hyperechoic targets to assess contrast. NSI images were compared against rectangular apodized images and minimum variance beamformed images. In experiments, the apparent lateral resolution was observed to improve by a factor of more than 35× when compared to rectangular apodization. Image quality was assessed by the estimation of lateral resolution (-6-dB receive beamwidth), main-lobe-to-sidelobe ratio, and contrast-to-noise ratio (CNR). Imaging with NSI using a focal number of 2 (f/2), the -6-dB beamwidth on receive as measured from a small wire target in the ATS phantom was 0.03λ compared to 2.79λ for rectangular apodization. Sidelobes were observed to decrease by 32.9 dB with NSI compared to rectangular apodization. However, the ability to observe the contrast of anechoic and hyperechoic targets reduced when utilizing the NSI scheme, i.e., the CNR decreased from -3.05 to -1.01 for anechoic targets and 1.65 to 0.45 for the hyperechoic targets.