  Plane wave imaging (PWI) with angular compounding has gained in popularity
over recent years because it provides high frame rates and good image
properties. However, most linear arrays used in clinical practice have a pitch
that is equal to than the wavelength of ultrasound. Hence, the presence of
grating lobes is a concern for PWI using multiple transmit angles. The presence
of grating lobes produces clutter in images and reduces the ability to observe
tissue contrast. Techniques to reduce or eliminate the presence of grating
lobes for PWI using multiple angles will result in improved image quality. Null
subtraction imaging (NSI) is a nonlinear beamforming technique that has been
explored for improving the lateral resolution of ultrasonic imaging. However,
the apodization scheme used in NSI also eliminates or greatly reduces the
presence of grating lobes. Imaging tasks using NSI were evaluated in
simulations and physical experiments involving tissue-mimicking phantoms and
rat tumors in vivo. Images created with NSI were compared with images created
using traditional delay and sum (DAS) with Hann apodization and images created
using a generalized coherence factor (GCF). NSI was observed to greatly reduce
the presence of grating lobes in ultrasonic images, compared to DAS with Hann
and GCF, while maintaining spatial resolution and contrast in the images.
Therefore, NSI can provide a novel means of creating images using PWI with
multiple steering angles on clinically available linear arrays while reducing
the adverse effects associated with grating lobes.
