4 Discussion

In our proposed method for measurement of small tissue motion, the interval of frames between which the phase shift of echoes is estimated can be increased as described in [47] if the number of computations are desired to be reduced. However, aliasing does not occur because the large global motion is tracked (compensated) by the conventional method at a high frame rate of 286 Hz. The global motion between 2 key frames is the distance corresponding to an interval of sampled points, 19.25 μm in this study (speed of sound: 1,540 m/s). Therefore, the residual displacements at other radial positions after global motion compensation are less than 19.25 μm after global motion compensation. When the center frequency of the received RF echo is exactly the same as the nominal center frequency (10 MHz) of the employed ultrasonic probe, a displacement of 19.25 μm produces a phase shift of RF echoes of π/2. The residual displacement at each radial position after global motion compensation is smaller than 19.25 μm, which is smaller than the aliasing limit of 38.5 μm at 10 MHz.