Super harmonic ultrasound for motion-independent localization microscopy: applications to microvascular imaging from low to high flow rates

Recent advances in high frame rate biomedical ultrasound have led to the development of ultrasound localization microscopy, a method of imaging microbubble contrast agents beyond the diffraction limit of conventional coherent imaging techniques. By localizing and tracking the positions of thousands of individual microbubbles, ultra-high resolution vascular maps are generated which can be further analyzed to study disease. Isolating bubble echoes from tissue signal is a key requirement for super-resolution imaging which relies on the spatiotemporal separability and localization of the bubble signals. To date, this has been accomplished either during acquisition using contrast imaging sequences or post-beamforming by applying a spatiotemporal filter to the B-mode images. Super harmonic imaging is another contrast imaging method that separates bubbles from tissue based on their strongly nonlinear acoustic properties. This approach is highly sensitive, and, unlike spatiotemporal filters, it does not require decorrelation of contrast agent signals. Since this super harmonic method does not rely on bubble velocity, it can detect completely stationary and moving bubbles alike. In this work, we apply super harmonic imaging to ultrasound localization microscopy and demonstrate an average improvement in SNR of 10.3 dB in vitro when compared to the standard singular value decomposition filter approach and an increase in SNR at low flow (0.27 mum/frame) from 5dB to 16.5 dB. Additionally, we apply this method to imaging a rodent kidney in vivo and measure vessels as small as 20 mum in diameter after motion correction.