We present a novel, robust and accurate blood velocity estimation tech
nique that is implementable by elementary digital signal processing, I
n this technique, echoes from repeated firings of a transducer are res
ampled along a set of predetermined trajectories of constant velocitie
s, called ''butterfly lines'' because of their intersection at a refer
ence range. The slope of the trajectory on which the sampled signals s
atisfy a predetermined criterion appropriate for the type of signal in
question, provides an estimate of the velocity of the target, The sea
rch for this trajectory is called ''butterfly search,'' which can be c
arried out efficiently in a parallel processing scheme, The estimator
can be based on the RF echo, its envelope, or its quadrature component
s, We present the theory of the butterfly search and some preliminary
results, The butterfly search on quadrature components has shown super
ior noise immunity, with relatively few successive scan lines, and was
found to outperform all the common time domain and Doppler techniques
in simulations and phantom experiments with strong noise, The butterf
ly search can overcome many disadvantages faced by the present day tec
hniques, such as the stringent tradeoff criterion between imaging reso
lution and velocity resolution implicit in Doppler techniques, and the
need for computation-intensive operations.