A fast method for speckle tracking

Speckle tracking method has been studied for velocity vector imaging in medical ultrasound for many years. However, the method is slow as compared to the conventional color Doppler imaging due to a larger amount of computation is needed. In this study, a method is developed to reduce the amount of computation. Instead of using an entire rectangular block in a 2D kernel (full kernel) to calculate the sum of absolute difference between image frames, only data in two diagonal lines, two bisectors, or a combination of the diagonal and bisector lines (see Fig. 1(b), (c), and (d)), are used. Both simulation and experiment were conducted to verify the proposed method. 11 frames of images are reconstructed with the delay-and-sum (D&S) and the high-frame-rate (HFR) imaging methods of a single plane wave transmission. In the simulation, two point scatterers, located at depths of 10 and 70 mm with velocities of 0.5 and 1.0 m/s respectively, are placed in the imaging area. Ten velocities are estimated by speckle tracking using both the full kernel and the crosses. The average errors of velocities measured are 7.3% for D&S and 5.5% for HFR imaging methods with diagonal lines, 2.5% for D&S and 4.0% for HFR imaging methods with bisectors, 3.6% for D&S and 3.2% for HFR imaging methods when both diagonal and bisector lines are used, and 2.6% for D&S and 2.5% for HFR imaging methods based on the full kernel. In the experiment, two small glass beads (point scatterers), located at depths of around 10 and 70 mm with velocities of 0 and a fixed value respectively, were placed in a water tank. The average measurement errors are 15.5% for D&S and 11.4% for HFR imaging methods with diagonal lines, 10.7% for D&S and 12.5% for HFR imaging methods with bisectors, 13.2% for D&S and 9.3% for HFR imaging methods when both diagonal and bisector lines are used, and 10.3% for D&S and 8.7% for HFR imaging methods with the full kernel. These results were o- tained with a kernel area of 1.8 × 1.8 mm (or 100 × 100=10000 pixels for the full kernel and 4 × 100=400 pixels for a combination of diagonal and bisector lines). In this condition, the computation time is 18.5 times smaller with the combined diagonal and bisector lines than that with the full kernel (excluding the fixed image reading time). As the size of the kernel is increased, there will be more reduction in computation time with the new method. The new speckle tracking method can achieve similar accuracies in velocity estimations while significantly reducing the computation time.