Velocity cannot be measured unambiguously when it exceeds the Nyquist limit in pulsed Doppler Ultrasonography. We
propose the use of a velocity direct difference (VDD) method, which integrates acceleration directly, as a way to improve the
depth and velocity product limit. The limit associated with the VDD method is approached when the difference between the two
phase differences (acceleration) approximates the value of ±π. The VDD method has two problems, however. First, any error
in the initial value remains until it has been corrected. Second, false acceleration aliasing may occur when the signal-to-noise
ratio (SNR) is low. The SNR may deteriorate, for example, around the null frequency of the moving-target indication (MTI)
filter. We developed three kinds of algorithms to correct errors introduced in this way. We were able to experimentally
determine the velocity of the blood flow in the carotid artery when blood velocity exceeded the Nyquist limit.