Example of the determination of the cut-off velocity and direction estimation error in a typical acquisition from the phantom. (A) The blue curve indicates the true phantom velocity at the sample position as determined from the tachometer signal of the phantom (please see Figure 1). The yellow dots indicate velocity estimates from the tracking software. Note that the almost ideal tracking up to a certain velocity (yellow shaded areas) and the complete failure of tracking above this velocity (“cut-off velocity”). (B) Statistical approach for the objective determination of the cut-off velocity. Same tracking data as in (A) are now displayed vs. the true velocity. The failure of tracking is clearly visible as marked deviation from the line of agreement. Data pairs were included stepwise with increasing true velocities and the correlation coefficient was calculated (orange line). This coefficient reaches its maximum at the cut-off velocity and drops dramatically afterwards. This maximum was automatically detected and used as definition of the cut-off velocity (orange dashed line). (C) The red line represents the flow direction estimate of the tracking software from the same sample position as in (A). The true motion direction is indicated by the blue line. Note the relatively stable direction estimates even when velocity estimation is failing due to too high velocities. (D) Same data as in (C) shown in relation to the true velocity of the phantom. Note the clear, but mild deterioration of tracking above the cut-off velocity (orange dashed line).