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The design, fabrication and operation of a 4.5 MHz dual-beam probe capable of high velocity blood flow measurement with a good lateral resolution is described. Previous work on this form of flow measurement estimated the velocity of 600 pm diameter alumina particles at a focal length of 17 mm. This paper describes an aluminum lens which is used to generate two adjacent focal spots at a focal length in the range 30 mm to 80 mm, making it suitable for invivo blood velocity measurements. The l i m i t s on the maximum velocity detectable with this system are dependent on the separation of the two spots and the pulse repetition frequency (PRF). A comparison between simulated PSFs and the measured lens characteristics at different focal lengths are discussed.A flow phantom that uses 8 mm and 4 mm diameter distensible tubes passing through water was used to test the estimation technique at different velocities. A possible extension of the technique to three-dimensional velocity measurements will also be discussed.upper limit such as this is sufficient. However, for higher flowrates such as those that occur in turbulent jets in the region of a stenosis, the retumed Doppler signal will be aliased.The design of a dual beam lens for flow measurement has been discussed previously.3-5 As shown in Figure 1, the lens is designed so that the central rays from each transducer cross at the back focal plane. This then generates two focal spots a fixed distance apart. Each transducer is excited with a separate pulser and the scattered signals are amplitude detected and fed to two sample-and-hold (S/H) modules. If the scattering characteristics are unchanged between the foci, then the signals would be identical in shape but shifted by z (the transit time from one focus to the other). In practice the signals will not be identical, so a method of cross correlation is used. By correlating the "flow" signals retumed from each of the focal spots, an estimate of the time taken for an ensemble of scatterers to pass from one foci to another can be made.