Accurate Doppler angle estimation for vector flow measurements

Abstract

Traditional Doppler methods measure only the axial component of the velocity vector. The lack of information on the beam-flow angle creates an ambiguity that can lead to large errors in velocity magnitude estimates. Different triangulation techniques so far have been proposed, which basically perform multiple measurements of the Doppler frequency shift originating from the same region. In this work, an original approach is introduced, in which two ultrasound beams with known relative orientation are directed toward the same vessel, but only one of them is committed to perform a Doppler measurement; the second (reference) beam has the specific task of detecting the beam-flow angle. The latter goal is obtained by accurately identifying the achievement of the target 90 reference-beam-to-flow angle through the inspection of the backscattered Doppler signal spectrum. In transverse flow conditions, in fact, such spectrum is expected to be centered on the zero frequency, and even small deviations from the desired 90 orientation cause noticeable losses of spectral symmetry. Validation of the new method has been performed through experimental tests, which show that the beam-flow angle can be estimated with high accuracy (rms errors lower than 1 ), and repeatable velocity magnitude measurements are possible. A procedure for automatically tracking the desired orientation by the reference beam is also introduced and shown suitable for implementation in steerable linear array transducers.

Ultrasound Vector Flow Imaging: I: Sequential Systems

“…7(b) shows a spectrogram obtained from a transversely insonated common carotid artery [16]. The spectra are centered at 0 Hz and the bandwidth follows the change in velocity v.…”

“…For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. measures -one for the beam-to-flow angle θ and the other for the axial component v z [16], [106]. The approach uses two beams, each beam giving one spectrum, transverse and axial, respectively.…”

“…During operation, the left and right transducers would alternate as the transmitter and receiver in a sequence of pulse firing events, so that the two transmit-receive angle pairs are essentially trigonometric complements. Variations of this scheme exist [11], [15], and some of them involved hybrid use of Doppler spectral broadening principles to aid the flow vector determination process [16], [17]. The downside of this approach is that the effective PRF is inevitably halved, as each flow vector must be derived from two separate pulse firing events.…”

“…Characterization of the rotational flow in the ascending aorta of such patients may help distinguish those at risk for development of ascending aortic aneurysm. Vector flow imaging also allows one to estimate wall shear rates in the carotid [16], [179], [180]. Although wall shear rates are known to play an important role in plaque development, the clinical impact of arterial vector flow imaging, however, has not yet been demonstrated.…”

Recent advances in blood flow vector velocity imaging

“…A method for determining the flow angle was suggested by Tortoli et al [13], where a spectral range gate is placed with a beam direction at right angles to the vessel, and the direction adjusted so that the spectrum is always symmetric around zero frequency. A second beam is then used for finding the spectrum.…”