Estimation of High Velocities in Synthetic-Aperture Imaging—Part I: Theory

This paper describes a new pulse sequence design and estimation approach, which can increase the maximum detectable velocity in synthetic-aperture (SA) velocity imaging. In SA, N spherical or plane waves are emitted, and the sequence is repeated continuously. The N emissions are combined to form a high-resolution image (HRI). Correlation of HRIs is employed to estimate velocity, and the combination of N emissions lowers the effective pulse repetition frequency by N. Interleaving emission sequences can increase the effective pulse repetition frequency to the actual pulse repetition frequency, thereby increasing the maximum detectable velocity by a factor of N. This makes it possible to use longer sequences with better focusing properties. It can also increase the possible interrogation depth for vessels with large velocities. A new cross-correlation vector flow estimator is also presented, which can further increase the maximum detectable velocity by a factor of 3. It is based on transverse oscillation (TO), a preprocessing stage, and cross-correlation of signals beamformed orthogonal to the ultrasound propagation direction. The estimator is self-calibrating without estimating the lateral TO wavelength. This paper develops the theory behind the two methods. The performance is demonstrated in the accompanying paper for convex and phased array probes connected to the synthetic aperture real-time ultrasound system scanner for parabolic flow for both conventional and SA imaging.