  Ultrasound Localization Microscopy (ULM) offers a cost-effective modality for
microvascular imaging by using intravascular contrast agents (microbubbles).
However, ULM has a fundamental trade-off between acquisition time and spatial
resolution, which makes clinical translation challenging. In this paper, in
order to circumvent the trade-off, we introduce a spatiotemporal filtering
operation dubbed velocity filtering, which is capable of separating contrast
agents into different groups based on their vector velocities thus reducing
interference in the localization step, while simultaneously offering blood
velocity mapping at super resolution, without tracking individual microbubbles.
As side benefit, the velocity filter provides noise suppression before
microbubble localization that could enable substantially increased penetration
depth in tissue typically by 4cm or more. We provide a theoretical analysis of
the performance of velocity filter. Numerical experiments confirm that the
proposed velocity filter is able to separate the microbubbles with respect to
the speed and direction of their motion. In combination with subsequent
localization of microbubble centers, e.g. by matched filtering, the velocity
filter improves the quality of the reconstructed vasculature significantly and
provides blood flow information. Overall, the proposed imaging pipeline in this
paper enables the use of higher concentrations of microbubbles while preserving
spatial resolution, thus helping circumvent the trade-off between acquisition
time and spatial resolution. Conveniently, because the velocity filtering
operation can be implemented by fast Fourier transforms(FFTs) it admits fast,
and potentially real-time realization. We believe that the proposed velocity
filtering method has the potential to pave the way to clinical translation of
ULM.
