Power Doppler and color flow imaging with null subtraction imaging

Null subtraction imaging (NSI) is a nonlinear beamforming technique that uses nulls in a beam pattern instead of the mainlobe to beat the diffraction limit. In this study, we used a 256-element linear array with 55-μm pitch to image the microvasculature of a mouse brain using NSI and delay and sum (DAS) with plane wave compounding. We also explored combining pulse inversion (PI) with NSI, i.e., we transmitted pulses at center frequency of 15.6 MHz to obtain the second harmonic at 31.2 MHz. Color flow imaging (CFI) at the fundamental frequency was also generated using traditional methods and NSI and compared. Higher spatial resolution and contrast in power Doppler images were observed in the NSI images compared to DAS (spatial resolution of 1/4–1/6 of a wavelength). The CFI images constructed using NSI resolved small vessels with flows in opposite directions that were not observed in the CFI using DAS, where vessels of opposite flows appeared averaged together as one vessel due to lower spatial resolution. The computation time was increased by only 40% for NSI compared to DAS. No contrast agents were used in these images.