Abstract
A new vector velocity estimation scheme is developed, termed tapered vector Doppler (TVD), aiming to improve the accuracy of low velocity flow estimation. This is done by assessing the effects of singular value decomposition (SVD) and finite impulse response (FIR) filters and designing an estimator which accounts for signal loss due to filtering. Synthetic data created using a combination of in vivo recordings and flow simulations were used to investigate scenarios with low blood flow, in combination with true clutter motion. Using this approach, the accuracy and precision of TVD was investigated for a range of clutter-to-blood and signal-to-noise ratios. The results indicated that for the investigated carotid application and setup, the SVD filter performed as a frequency-based filter. For both SVD and FIR filters, suppression of the clutter signal resulted in large bias and variance in the estimated blood velocity magnitude and direction close to the vessel walls. Application of the proposed tapering technique yielded significant improvement in the accuracy and precision of near-wall vector velocity measurements, compared to non-TVD and weighted least squares approaches. In synthetic data, for a blood SNR of 5 dB, and in a near-wall region where the average blood velocity was 9 cm/s, the use of tapering reduced the average velocity magnitude bias from 26.3 to 1.4 cm/s. Complex flow in a carotid bifurcation was used to demonstrate the in vivo performance of TVD, and it was shown that tapering enables vector velocity estimation less affected by clutter and clutter filtering than what could be obtained by adaptive filter design only.
Highlights
V ECTOR velocity imaging (VVI) of blood flow has been a topic of interest for ultrasound research since the 1970s
The use of VVI based on vector Doppler, transverse oscillations, speckle tracking, or vector flow mapping has been investigated in applications ranging from the assessment of tissue elasticity [1], [2], visualization of complex flow fields in the hearts of newborns, children, and adults [3]–[6] as well as in the carotid arteries [7]–[9] and ascending aorta [10], mapping of vorticity and energy loss in the left ventricle [11], [12] and mapping of flow complexity in the carotid bulb [13]
This motivated a comparison between singular value decomposition (SVD) filters and adaptive finite impulse response (FIR) filters with regard to their impact on the frequency content of the signal in this application
Summary
V ECTOR velocity imaging (VVI) of blood flow has been a topic of interest for ultrasound research since the 1970s. The widespread research utilizing 2-D and 3-D velocity estimation techniques indicate that the different approaches have matured in recent years and have the potential to provide additional diagnostic information in cardiovascular applications. Though visualization of flow patterns already provides more information than the conventional color Doppler images, many applications would benefit from more quantitative measurements of blood velocities. Whereas quantitative blood flow measurements are currently performed using spectral Doppler, VVI techniques have the potential to improve diagnostic accuracy by providing measurements that are less dependent on the operator and ideally independent of the flow angle. The use of VVI for quantitative measurements is, still limited, because the underlying mean velocity estimates are associated with large bias and variance
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