Synthetic aperture flow angle estimation on in-vivo data from the carotid artery

In conventional ultrasound velocity estimation sys- tems only the velocity projected onto the direction of the steered ultrasound beam is found. It has previously been shown how true blood flow velocity magnitudes can be found using synthetic transmit aperture imaging. The method is based on cross- correlation between lines beamformed along the flow direction. This method assumes the direction of flow is known. Jensen (2004) presented a method for estimating the direction of flow (1). The angle determination method is based on a search for the maximum normalized cross-correlation as a function of angle. It assumes the largest correlation is seen for the angle of flow. Previously, this method has only been validated using data from a circulating flow rig. This paper presents an In-Vivo investigation of the method. Real time data covering 2.2 seconds of the carotid artery of a healthy 30-year old male volunteer is acquired and then processed off-line using a large computer cluster. Data are acquired using our RASMUS experimental ul- trasound scanner and a 128 element 6.2 MHz linear array transducer. A 20 µs chirp was used during emission and virtual transmit sources were created behind the transducer using 11 transmitting elements. Data from 8 transmissions with each 64 receiving elements are beamformed and coherently summed to create high resolution lines at different angles for a set of points within the region of flow. The pulse repetition frequency was set to 10 kHz. The direction of flow is estimated using the above mentioned method. It is compared to the flow angle of 106 ◦ with respect to the axial direction, determined visually from the B-mode image. For a point in the center of the common carotid artery, 76 % of the flow angle estimates over the 2.2 seconds were within ±10 ◦ of the visually determined flow angle. The standard deviation of these estimates was below 2.7 ◦ .