High-frequency power Doppler imaging of angiogenesis can be challenging given the presence of small blood
vessels and slow flow velocities. In the presence of substantial Doppler artifacts such as false-positive color
pixels or undetected vessels, color pixel density (CPD) and related vascularity metrics do not provide accurate
estimates of vascular volume fraction. As a step towards improved microvascular quantification, flow-phantom
experiments were performed to establish relationships between CPD and wall filter cut-off velocity for various
combinations of vessel size (160, 200, 250, 300, and 360 μm), flow velocity (4, 3, 2, 1, and 0.5 mm/s), and
transducer frequency (30 and 40 MHz). Three distinct regions were observed in plots of CPD versus wall filter
cut-off velocity: overestimation of CPD at low cut-offs, underestimation of CPD at high cut-offs, and a plateau
at intermediate cut-offs. The CPD at the plateau closely matched the phantom's actual vascular volume fraction.
The length of the plateau corresponded with the flow-detection performance of the Doppler system, which was assessed using receiver operating characteristic analysis. Color pixel density versus wall filter cut-off curves from analogous in vivo experiments exhibited the same shape, including a distinct CPD plateau. The similar shape of the flow-phantom and in vivo curves suggests that the presence of a plateau can be used to identify the best-estimate CPD value in an in vivo experiment. The ability to identify the best CPD estimate is expected to improve quantification of angiogenesis and anti-angiogenic treatment responses with power Doppler.