Ultrasound Localization Microscopy (ULM) provides images of the microcirculation in-depth in living tissue. Micrometric bubbles are injected in the bloodstream with a concentration sufficiently small to appear isolated in the ultrasound images. By localizing each microbubble successively, the resolution becomes sub-wavelength. In addition, the high framerate of the ultrasound scanner offers the ability to track individual microbubbles and provide blood flow velocimetry.
Its implementation in two-dimensions is limited by the elevation projection and tedious plane-by-plane acquisition. Volumetric ULM alleviates these issues and can map the vasculature of entire organs in one acquisition with isotropic resolution. However, its optimal implementation requires many independent acquisition channels, leading to complex custom hardware.
In this work, we implemented volumetric ultrasound imaging with a multiplexed matrix probe driven by a single commercial ultrasound scanner. The multiplexed solution preserves the imaging quality but requires multiple emission-reception with 4 sub-apertures to reconstruct a full volume. The maximal volume rate is therefore reduced. We proposed and compared three different sub-aperture multiplexing combinations for localization microscopy in silico and in vitro. We evaluated the approach for micro-angiography of the rat brain in vivo with an intact skull. In the rat brain, 100,000 volumes were acquired within 7 min with the optimized ultrasound sequence and revealed vessels down to 31 µm in diameter with flows from 4.3 mm/s to 28.4 mm/s.
This article demonstrates the ability to perform a complete angiography with unprecedented resolution in the living rat’s brain with a simple and light setup through the intact skull. We foresee that it might contribute to democratize 3D ULM for both preclinical and clinical studies.