A comparison of flow tracking techniques: spatial quadrature with phase-sensitive axial demodulation versus speckle tracking

Spatial quadrature and speckle tracking are both techniques which extend the estimation of flow velocity or tissue motion velocity to two or three dimensions. The authors applied both under similar experimental conditions to assess and compare their performance. They also describe a new spatial quadrature processor that obviates the need for axial alignment under conditions of mixed axial-lateral flow. This processor avoids the estimate bias produced by jitter in the axial alignment step previously described, and thus allows the non-axial component(s) of flow to be tracked without sacrificing axial resolution. In the authors´ flow experiment, summed radiofrequency echo data were captured in an M-mode scan geometry using 2:1 parallel receive processing in both spatial quadrature and two-beam speckle tracking configurations using a 7.5 MHz linear array and a modified commercial ultrasound scanner. Laminar flow was established in a wall-less vessel phantom using a computer-controlled pump. The authors captured echo data from this phantom at beam-vessel angles of 60° and 90° (i.e. pure lateral flow). These data were processed off-line to estimate flow velocity profiles. Volume flow rates were calculated from these profiles by integration. The mean volume flow rates estimated with 2-D speckle tracking agreed with the known flow rates with a maximum relative error of 7% for true lateral flow and 10% for mixed axial-lateral flow. The equivalent values for spatial quadrature were 5% and 15%