Ultrasound Localization Microscopy of the Brain: The Missing Micro Vasculature

Ultrasound Localization Microscopy (ULM) is a powerful technique able to realize the brain microvasculature down to the micron level. Despite recent developments into retrieving more functional information (i.e., pulsatility measurements, blood flow velocity, and neurovascular coupling), there is still a need for biomarkers with high clinical significance. Capillaries, the microvascular unit at the interface between neurons and the circulatory system, may be important biomarkers for neural health. However, ULM in its current iteration cannot readily, with confidence, proclaim to capture capillary dynamics. Thus, this paper aims to explore the question, where are capillaries in ULM? We propose a solution involving long ensemble lengths for input into spatiotemporal singular value decomposition clutter filters to recover velocity profiles that exhibit capillary flow behavior. We show that with longer ensembles, we can retrieve more capillary units than standard implementations.