Improving lateral resolution in ultrasonic Imaging by utilizing nulls in the beam pattern

Reduction of lateral sidelobes results in improved ultrasonic imaging. In general, apodization is used to lower sidelobes in exchange for increasing the width of the mainlobe and thus decreasing lateral resolution. Null Subtraction Imaging (NSI) is a technique that uses different on-receive apodizations on copies of the same image to to reduce sidelobe levels while also improving lateral resolution. In the NSI technique, several apodization functions are applied on receive. The first apodization weight is applied having a zero mean value when summed across the aperture. This places a null at the broadside of the receive pattern. The second apodization is the same as the first apodization except that a constant offset in weight is applied (non-zero mean). The third apodization is the transpose of the second. The images created with the different weighting schemes are then added to form a new image with improved sidelobe performance and dramatically better lateral resolution compared to conventional apodization. To evaluate the performance of this technique, experiments were performed with an ATS539 phantom containing wire targets to assess lateral resolution and cylinder targets to assess contrast. A 9L4 array was used in the measurements connected to an Ultrasonix RP system with a SonixDAQ. Plane waves were emitted from the array and ultrasound echoes were received by each array element. Image reconstruction involved using Delay and Sum beamforming with apodization. Images were constructed using NSI and compared with rectangular weighted apertures. In experiments, the lateral resolution was observed to improve by a factor of five or better when compared to rectangular apodization. Image quality was assessed by estimation of lateral resolution (-3-dB receive beamwidth), the mainlobe to sidelobe ratio (MSR) in dB and estimation of the contrast-to-noise ratio (CNR). At f/# = 1 focusing with NSI, the -6-dB beamwidth on receive as measured from a small wire target in the ATS phantom was 0.414λ. Sidelobes were observed to decrease at each f/# by an average of 17 dB with NSI compared to rectangular apodization. However, contrast of hyperechoic targets were lost when utilizing the NSI scheme.