Ultrafast 3D Ultrasound Localization Microscopy Using a 32x32 Matrix Array

Ultrasound Localization Microscopy can map blood vessels with a resolution much smaller than the wavelength by localizing microbubbles. Current implementations of the technique are limited to 2-D planes or small fields of view in 3D. These suffer from minute-long acquisitions, out-of-plane microbubbles and tissue motion. In this study, we exploit the recent development of 4D ultrafast ultrasound imaging to insonify an isotropic volume up to 20000 times per second and perform localization microscopy in the three dimensions. Specifically, a 32x32 elements, 9-MHz matrix-array probe connected to a 1024-channel programmable ultrasound scanner was used to achieve sub-wavelength volumetric imaging of both the structure and vector flow of a complex 3D structure (a main canal branching out into two side canals). To cope with the large volumes and the need to localize the bubbles in the three dimensions, novel algorithms were developed based on deconvolution of the beamformed microbubble signal. For tracking, individual particles were paired following a Munkres assignment method and velocimetry was done following a Lagrangian approach. ULM was able to clearly represent the 3D shape of the structure with a sharp delineation of canal edges (as small as 230 µm) and separate them with a spacing as low as 52µm. The compounded volume rate of 500Hz was sufficient to describe velocities in the [2.5-150] mm/s range and to reduce the maximum acquisition time to 12s. This study demonstrates the feasibility of in vitro 3D ultrafast ultrasound localization microscopy and opens up the way towards in vivo volumetric ULM.