Transverse flow imaging using synthetic aperture directional beamforming

Current ultrasound scanners only determine the velocity along the ultrasound beam, since data is only focused along the emitted beam. Synthetic aperture ultrasound systems have the capability of focusing simultaneously in all directions. This is used here to focus along the flow direction and then cross-correlate these measurements to obtain the correct velocity magnitude.

The approach was investigated in a flow system with a laminar flow. ne flow profile was measured with a B-K Medical 8804 7.5 MHz linear array transducer. A plastic tube with an entrance length of 1 m and a diameter of 17 nm was used with an EcoWatt 1 pump generating a laminar, stationary flow. The velocity profile was measured for flow angles of 90 and 60 degrees.

The RASMUS research scanner was used for acquiring RF data from 128 elements of the array using 8 emissions with 11 elements in each emission. A 20μs chirp was used during emission. The RF data were subsequently beamformed off-line and stationary echo canceling was performed. The 60 degrees flow was determined using 16 groups of 8 emissions and the relative standard deviation was 0.36 % (0.65 mm/s).

Using the same setup for the purely transverse flow gave a std. of 1.2 % (2.1 mm/s). An in-vivo image of the carotid artery and jugular vein of a healthy 29 years old volunteer. A full color flow image using only 128 emissions could be made with a high velocity precision.