  Ultrafast ultrasound localization microscopy (ULM), which has shown promising
results in microvascular imaging, overcomes the typical trade-off between
resolution and penetration depth. Combining ultrasound contrast agents and high
frame rate imaging enables ULM to visualize microvasculature and quantify flow.
However, the quality of the microvascular maps obtained depends on the
signal-to-noise ratio of the received signals, image reconstruction techniques,
and the microbubble (MB) localization and tracking algorithms used. Most
reported research in ULM employs the conventional delay and sum (DAS)
beamforming technique for image reconstruction despite its limited contrast and
resolution. In this work, a filtered delay multiply and sum (F-DMAS)
beamforming approach with non-steered plane wave transmit was employed for ULM,
and its performance was compared with the conventional DAS-based approach for
the different localization algorithms available in the Localization and
Tracking Toolbox for Ultrasound Localization Microscopy. We also introduce two
novel image quality measures that can overcome the limitations of conventional
quality metrics that require suitable targets for evaluation. We also report
the preliminary in-vitro investigation of F-DMAS with B-mode and power Doppler
maps for microvascular imaging. The results are promising with enhanced
contrast and lateral resolution, and suggest that further experimental studies
are warranted.
