Velocity distributions in blood vessels can be displayed using ultrasound scanners by making a Fourier transform of the received signal and then showing spectra in an M-mode display. It is desired to show a B-mode image for orientation, and data for this have to be acquired interleaved with the flow data. This either halves the effective pulse repetition frequency fprf or gaps appear in the spectrum from B-mode emissions. This paper presents a techniques for maintaining the highest possible fprf and at the same time show a B-mode image. The power spectrum can be calculated from the Fourier transform of the autocorrelation function, and it is shown that the autocorrelation function can be calculated for a sparse set of data where flow and B-mode emissions are inter-spaced. Both short deterministic sequences of emissions and full random sequences can be used. The dynamic range of the sparse sequence is reduced compared to a full sequence. Typically a reduction of 20 dB is found when using 66% of the data compared to using all data. The theory of the method and examples from simulations of flow in arteries are presented. The audio signal can also be generated from the spectrogram.
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