A simple method to reduce aliasing artifacts in color flow mode imaging.

It is a well known limitation in conventional blood velocity estimation using a phase estimation approach, that aliasing artifacts are present, when the blood velocities exceed a value determined by half the pulse repetition frequency (the Nyquist frequency). This paper proposes a simple anti-aliasing discriminator (AAD) method based on using two different pulse repetition frequencies to increase the aliasing limit to twice the Nyquist frequency. The method is evaluated in simulations using the Field II program. The axial velocity in a virtual blood vessel is found along one axial line, where N=10 emissions are used for each velocity estimate. Mean standard deviation and mean absolute bias are 4.9% and 23.8%, respectively, when no attempt is done to suppress the aliasing. When the AAD is used the corresponding values are 3.4% and 1.0%. When median filtering is applied, the values are 1.1% and 0.6%. The method is evaluated experimentally using the ultrasound scanner RASMUS and a circulating flowrig with parabolic flow. The mean standard deviation and mean absolute bias are 4.0% and 20.3%, respectively, when no attempt is done to suppress the aliasing. When the anti-aliasing discriminator is used the corresponding values are 3.6% and 1.2%. The values are 1.0% and 0.8%, when median filtering is performed.