Sidelobe reduction using null subtraction imaging

Abstract

In this work, we propose a novel anodization scheme where three different apodizations are introduced to create three separate images that are then combined nonlinearly to produce better image quality, i.e., null subtraction imaging (NSI). All apodizations were on receive only. The first apodization consisted of a zero mean weighting with the left half of aperture a weight of + 1 and the right half of aperture a weight of −1. The second apodization was a DC offset version of the first apodization and the third apodization was a flipped version of the second. Images were created by combining the envelope signals of three apodizations with different weights. The images were subtracted from one another resulting in a reduction in sidelobe levels, a narrowing of the main lobe and improvement in image quality. An L14-5 array transducer and a Verasonics system were used to capture the RF channel data using seven plane waves spanning angles between −12° and 12°. Both coherent compounding and filtered incoherent compounding were used as a tradeoff between lateral resolution and contrast to noise ratio (CNR). Images were constructed from wire targets, a tissue-mimicking phantom, and rat tumors in vivo. Image quality was assessed through the CNR and intensity of sidelobes visible in the images. Images created using NSI were compared to images created using Hanning apodization with delay and sum (DAS) and a generalized coherence factor (GCF) approach. Improvements in image quality were found when using NSI compared to the other methods.