Multi-Frequency Encoding for Rapid Color Flow and Quadroplex Imaging

Ultrasonic color flow maps are made by estimating the velocities line by line over the region of interest. For each velocity estimate, multiple repetitions are needed. This sets a limit on the frame rate, which becomes increasingly severe when imaging deeper lying structures or when simultaneously acquiring spectrogram data for triplex imaging. This paper proposes a method for decreasing the data acquisition time by simultaneously sampling multiple lines at different spatial positions for the color flow map using narrow band signals with disjoint spectral support. The signals are separated in the receiver by filters matched to the emitted waveforms and the autocorrelation estimator is applied. Alternatively, one spectral band can be used for creating a color flow map, while data for a number of spectrograms are acquired simultaneously. Using three disjoint spectral bands, this will result in a multi-frequency quadroplex imaging mode featuring a color flow map and two spectrograms at the same frame rate as a normal color flow map. The method is presented, various side-effects are considered, and the method is tested on data from a re-circulating flow phantom where a constant parabolic flow with a peak of 0.1 m/s is generated with a flow angle of 60 degrees. A commercial linear array transducer is used and data are sampled using our RASMUS multi-channel sampling system. An in-vivo multi- frequency quadroplex movie of the common carotid artery of a healthy male volunteer was created. The flow phantom measurements gave a mean standard deviation across the flow profile of 3.1%, 2.5%, and 2.1% of the peak velocity for bands at 5 MHz, 7 MHz, and 9 MHz, respectively. The in-vivo multi-frequency quadroplex movie showed the color flow map, and the two independent spectrograms at different spatial positions. This enables studying the flow over an arterial stenosis by simultaneously acquiring spectrograms on both sides of the stenosis, while maintaining the color flow map. A fr- ame rate of 21.4 frames per second was achieved in this in-vivo experiment.