A new time domain narrowband velocity estimation technique for Doppler ultrasound flow imaging

The spatial and temporal resolution of Doppler ultrasound flow imaging systems are limited by the velocity estimation strategy. In order to achieve improved performance a new time-domain narrowband estimation method is developed and incorporated in a novel receiver structure. This system is evaluated using simulated and experimentally measured RF Doppler data.;A significant reduction in velocity estimation variance can be achieved by simultaneously processing both temporal and spatial information contained within the sample volume. This is accomplished by the formation of a Doppler correlation matrix using a two dimensional sequence of spatially sampled Doppler signal 'snapshots', which closely model the range-velocity spread nature of the distributed red blood cell target contained within the sample volume. By using the analytic form of the backscattered echo signal to produce the Doppler correlation matrix, the quadrature demodulation stage of a traditional narrowband receiver can be replaced by a single Hilbert transform operation as well as serve as the basis for a method to correct for estimation bias introduced by frequency dependent attenuation and scattering by tissue and blood.;Principal component spectral analysis of the Doppler signal 'snapshot' correlation matrix leads to a new and robust mode Doppler frequency estimator as well as a novel clutter detection/suppression strategy. Processing only the dominant subspace of the Doppler correlation matrix is shown to significantly reduce the Cramer-Rao bounds on the estimation error of target velocity as compared to traditional narrowband blood velocity estimation methods. To circumvent frequency domain analysis, a time-domain solution is given for the velocity estimate using the root-MUSIC algorithm, making the new estimator highly amenable for real-time implementation.;Based on both the simulated and measured RF Doppler data, the proposed receiver structure demonstrates significant improvements over current narrow and wideband methods in terms of the mean-square estimation error, especially under low signal to noise conditions and small numbers of transmit bursts. This makes it potentially attractive for high frame rate color flow imaging.