Figure 4

Benchtop ultrasound flow experiments demonstrated monotonic relationship between flow velocity and area-adjusted velocity index. The theoretical relationship between area-adjusted velocity index and flow velocity was probed by passing Doppler fluid through polydimethylsiloxane microfluidic flow phantoms at various flow rates. (A) Low scale (high sensitivity) showed susceptibility to noise, especially at lower flow velocity, which interfered with the monotonic relationship. At higher flow rates, this improved somewhat, but Spearman’s correlation remained low. This susceptibility to noise is undesirable, and thus we have avoided low scale recordings. (B) At scale of 0.9 (intermediate sensitivity) and (C) at maximum scale (minimum sensitivity), the monotonic relationship between velocity and flow is clear. Both scales resulted in highly statistically significant Spearman’s rank correlation, confirming monotonicity of the velocity-to-velocity-index relationship.