Ultrafast 3D Ultrasound Localization Microscopy Using a 32 $\times$ 32 Matrix Array

Abstract

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.

Super-Resolution Imaging with Ultrasound for Visualization of the Renal Microvasculature in Rats Before and After Renal Ischemia: A Pilot Study

“…The 3D reconstructions give a more complete morphological examination, but do not allow MB tracking in the elevational plane or motion compensation in all three directions. This could become feasible with other types of probes, e.g., row-column and matrix ultrasound probes that are currently being developed for in vivo SR imaging [ 54 , 55 , 56 ].…”

Super-resolution Ultrasound Imaging

“…The lack of information in the elevation direction is particularly limiting in ultrasound localization microscopy, making velocimetry inaccurate and motion correction incomplete and providing only a single plane per acquisition. Fortunately, new results (Heiles et al 2019) on 3-D reconstruction capabilities are encouraging (Fig. 11).…”

“…However, matrix arrays are costly and present some technological hurdles at higher frequencies, making the technology less available. Heiles et al (2019) described in vitro a 3-D super-resolution imaging device in which each of the 1024 channels (32 £ 32) was connected to a programmable ultrafast scanner (Provost et al 2014). In vivo, this apparatus was able to reconstruct entire super-resolved volumes of the rat brain, alleviating the previously described drawbacks with 3-D tracking and motion correction.…”

“…However, motion that occurs orthogonal to the image plane remains a challenge. In this context, advancing CEUS imaging from 2-D to true 3-D using matrix transducers, which has been done in vitro (Harput et al 2019a;Heiles et al 2019), may not only allow display of the vascular network in much greater comprehensiveness and detail, but also holds promise for a precise motion correction in all directions (Harput et al 2019b).…”

“…11. Volumetric ultrasound localization microscopy implemented on an anesthetized rat brain using a 2-D matrix array with center frequency of 9 MHz (Heiles et al 2019). The voxel size is 10 mm; the region of imaging is 13.28 mm deep, 10.5 mm wide and 11.4 mm long.…”